Pension Benefits - Weighted Average Asset Allocation for Pension Plans (Details) - Pension Plans, Defined Benefit [Member]	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Weighted-average asset allocations	100.00%	100.00%
Cash and Short-term Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Weighted-average asset allocations	2.00%	2.00%
Corporate Bonds and Notes [Member]
Defined Benefit Plan Disclosure [Line Items]
Weighted-average asset allocations	31.00%	31.00%
Common and Preferred Stocks [Member]
Defined Benefit Plan Disclosure [Line Items]
Weighted-average asset allocations	50.00%	48.00%
Exchange Traded Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Weighted-average asset allocations	1.00%	1.00%
Other Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Weighted-average asset allocations	16.00%	18.00%