Retirement Benefits - Schedule of Pension Plan's Asset Allocations by Asset Category (Details)	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Pension plans weighted average asset allocation	100.00%	100.00%
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Pension plans weighted average asset allocation	6.00%	26.00%
Equity securities
Defined Benefit Plan Disclosure [Line Items]
Pension plans weighted average asset allocation	52.00%	43.00%
Fixed income
Defined Benefit Plan Disclosure [Line Items]
Pension plans weighted average asset allocation	16.00%	15.00%
Private assets
Defined Benefit Plan Disclosure [Line Items]
Pension plans weighted average asset allocation	10.00%	8.00%
Hedge funds
Defined Benefit Plan Disclosure [Line Items]
Pension plans weighted average asset allocation	16.00%	8.00%