Annual report pursuant to Section 13 and 15(d)

Postretirement Benefits Postretirement Benefits - Additional Information (Details)

v3.8.0.1
Postretirement Benefits Postretirement Benefits - Additional Information (Details)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plan, Assumed Health Care Cost Trend Rates [Abstract]      
Health Care Cost Trend Rate for Next Year 5.60% 5.70%  
Rate to Which the Cost Trend Rate is Assumed to Decline (the Ultimate Trend Rate) 4.50% 4.50%  
Year Rate Reaches the Ultimate Trend Rate 2037 2037  
Defined Benefit Plan, Actual Plan Asset Allocations [Abstract]      
Total 100.00% 100.00%  
Defined Benefit Plan, Assets, Target Allocations [Abstract]      
Total 100.00%    
Pension Plans [Member]      
Defined Benefit Plan, Weighted Average Assumptions Used to Determine Pension and Other Postretirement Benefit Obligations [Abstract]      
Discount Rate 3.63% 4.00%  
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]      
Discount Rate [1] 3.40% 3.58% [2] 3.95%
Long-Term Rate of Return on Plan Assets [3] 4.75% 5.25% [2] 6.00%
Other Postretirement Benefit Plans [Member]      
Defined Benefit Plan, Weighted Average Assumptions Used to Determine Pension and Other Postretirement Benefit Obligations [Abstract]      
Discount Rate 3.51% 3.93%  
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]      
Discount Rate [1] 3.17% 3.30% [2] 3.75%
Equity Securities [Member] | United States [Member]      
Defined Benefit Plan, Actual Plan Asset Allocations [Abstract]      
Total 4.00% 19.00%  
Defined Benefit Plan, Assets, Target Allocations [Abstract]      
Total 4.00%    
Equity Securities [Member] | International [Member]      
Defined Benefit Plan, Actual Plan Asset Allocations [Abstract]      
Total 1.00% 3.00%  
Defined Benefit Plan, Assets, Target Allocations [Abstract]      
Total 1.00%    
Fixed Income Securities [Member]      
Defined Benefit Plan, Actual Plan Asset Allocations [Abstract]      
Total 89.00% 75.00%  
Defined Benefit Plan, Assets, Target Allocations [Abstract]      
Total 95.00%    
Short-Term Investments [Member]      
Defined Benefit Plan, Actual Plan Asset Allocations [Abstract]      
Total 6.00% 3.00%  
Defined Benefit Plan, Assets, Target Allocations [Abstract]      
Total 0.00%    
[1] (A) The Company utilizes a yield curve analysis to calculate the discount rates used to determine pension and other postretirement benefit obligations. The yield curve analysis matches the cash flows of the Company's benefit obligations. The yield curve consisted of spot interest rates at half year increments for each of the next 30 years and was developed based on pricing and yield information for high quality corporate bonds rated Aa by either Moody's or Standard & Poor's, private placement bonds that are traded in reliance with Rule 144A and are at least two years from date of issuance, bonds with make-whole provisions and bonds issued by foreign corporations that are denominated in U.S. dollars, excluding callable bonds and bonds less than a minimum size and other filtering criteria. Additionally, the Company's yield curve methodology includes bonds having a yield that is greater than the regression mean yield curve as the Company believes this methodology represents an appropriate estimate of the rates at which the Company could effectively settle its pension obligations.
[2] (C) The Company evaluates its assumption regarding the estimated long-term rate of return on plan assets based on historical experience, future expectations of investment returns, asset allocations, investment strategies and views of investment professionals.
[3] (B) Pension expense in 2017 and 2016 includes the impact of a change in methodology used to calculate the interest cost component of pension expense. In 2015 and prior years, the Company used a single-weighted average discount rate to calculate pension and postretirement interest costs. Beginning in 2016, the Company is utilizing a "spot rate approach" in the calculation of pension and postretirement interest costs to provide a more accurate measurement of interest costs. The spot rate approach applies separate discount rates for each projected benefit payment in the calculation of pension and postretirement interest costs. This calculation change is considered to be a change in accounting estimate and is being applied prospectively starting in 2016.