Retirement Plans and Other Benefits - Asset Allocation (Details)	Dec. 31, 2017
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Target allocation (as a percent)	100.00%
Actual asset allocation (as a percent)	100.00%
Pension Benefits | Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Target allocation (as a percent)	62.00%
Actual asset allocation (as a percent)	58.00%
Pension Benefits | Return-generating assets
Defined Benefit Plan Disclosure [Line Items]
Target allocation (as a percent)	38.00%
Actual asset allocation (as a percent)	42.00%
Pension Benefits | Equities in US and other developed markets
Defined Benefit Plan Disclosure [Line Items]
Target allocation (as a percent)	18.00%
Pension Benefits | Equities in emerging markets
Defined Benefit Plan Disclosure [Line Items]
Target allocation (as a percent)	6.00%
Pension Benefits | Alternative investments
Defined Benefit Plan Disclosure [Line Items]
Target allocation (as a percent)	14.00%
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation (as a percent)	100.00%
Other Benefits | Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation (as a percent)	67.00%
Other Benefits | Return-generating assets
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation (as a percent)	33.00%