Postretirement Benefit Obligations - Assumptions Used in Calculations (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Key actuarial assumptions used to determine benefit obligations at December 31,
Effective discount rate for benefit obligation	4.60%	3.90%	4.50%
Expected annual rate of compensation increase	2.80%	2.80%	2.80%
Key actuarial assumptions used to determine the net periodic benefit cost for the years ended December 31,
Effective discount rate for service cost	3.90%	4.50%	3.70%
Effective discount rate for interest cost	3.70%	4.00%	3.60%
Expected long-term rate of return	7.00%	5.80%	5.80%
Expected annual rate of compensation increase	2.80%	2.80%	3.80%