Pension and Other Postretirement Employee Benefit Plans - Gains and Losses (Details)	12 Months Ended
	Dec. 31, 2018
Defined benefit pension plans
Defined Benefit Plan Disclosure [Line Items]
Percent above which amortization of net gains and losses is included in annual net periodic benefit cost	10.00%
OPEB plans
Defined Benefit Plan Disclosure [Line Items]
Percent above which amortization of net gains and losses is included in annual net periodic benefit cost	10.00%
Average future service period	11 years
Period over which the firm uses a calculated value that recognizes changes in fair value to determine expected return on plan assets	5 years
Average remaining amortization period	1 year
U.S. | Defined benefit pension plans
Defined Benefit Plan Disclosure [Line Items]
Average future service period	8 years