Income Taxes (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred income taxes	$ 666	$ (598)	$ (2,811)
Effect of legislative changes	299		$ (11)
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals	$ 28	$ 48
Effective Income Tax Rate, Continuing Operations	42.20%	41.70%	35.50%
Canadian Statutory rate	27.00%	27.00%	26.40%
Effective Income Tax Rate Reconciliation, Foreign Income Tax Rate Differential, Percent	22.90%
Valuation Allowance	$ 187	$ 133
Undistributed Earnings from Foreign Subsidiaries	3,200
Unrecognized Tax Benefit, interest expense	12	1	$ 2
Unrecognized Tax Benefit, Liability for accrued interest	$ 16	$ 4
United States [Member]
Foreign Federal Corporate Income Tax Rate, Percent	21.00%	35.00%
Deferred income taxes	$ 611	$ (435)	$ (2,495)
Effect of legislative changes	327
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals	28
United States [Member] | Non-capital Losses Carried Foward [Member]
Valuation Allowance	28	0
United States [Member] | Tax Legislation [Member]
Valuation Allowance, Increase (Decrease), Amount	26
United States [Member] | Foreign Tax Credits [Member]
Valuation Allowance	156	129
United States [Member] | Charitable Donations [Member]
Valuation Allowance	$ 3	$ 4