Income Taxes - Income Tax (Benefit) Expense Attributable to Income from Continuing Operations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Current:
Current Income Tax Expense (Benefit)	$ 274	$ 283	$ 209
Deferred:
Total	(208)	58	29
Income tax expense (benefit)	66	341	238
Canadian
Current:
Canadian	45	47	25
Deferred:
Canadian	(67)	43	78
United States
Current:
U.S. Federal	125	122	95
U.S. state, net of federal income tax benefit	26	20	17
Deferred:
U.S. Federal	(82)	8	(65)
U.S. state, net of federal income tax benefit	(27)	0	13
Other Foreign
Current:
Canadian	78	94	72
Deferred:
Canadian	$ (32)	$ 7	$ 3