Employee Retirement Benefits - Assumptions Used for Periodic Income (Details) - Pension Benefits [Member]	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.09%	4.35%	3.95%
Salary increase	4.10%	4.10%	4.10%
Expected long-term rate of return			7.35%
Expected long-term rate of return, net of management fees	7.35%	7.35%