Pension and Other Post-Retirement Benefits - Assumed Health Care Cost Trend Rates (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Health Care Cost Trend Rate Assumed for Next Fiscal Year, Post Age 65	4.90%	5.80%
Medical Claims [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Health Care Cost Trend Rate Assumed for Next Fiscal Year, Pre Age 65	6.70%	6.60%
Review of historical returns
Rate to which the cost trend is expected to decline (the ultimate trend rate)	4.50%	4.50%
Year that rate reaches the ultimate trend rate	2025	2022
Prescription Drugs Claims [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Health Care Cost Trend Rate Assumed for Next Fiscal Year, Pre Age 65	11.10%	9.30%
Defined Benefit Plan, Health Care Cost Trend Rate Assumed for Next Fiscal Year, Post Age 65	11.10%	9.80%
Review of historical returns
Rate to which the cost trend is expected to decline (the ultimate trend rate)	4.50%	4.50%
Year that rate reaches the ultimate trend rate	2025	2024