Income Taxes - Expected Income Tax Expense at Canadian Statutory Rates Reconciled to Income Tax Expense (Details) - CAD CAD in Millions	3 Months Ended		12 Months Ended
	Jun. 30, 2015	Sep. 30, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Statutory federal and provincial income tax rate (Canada)			26.47%	26.31%	26.32%
Expected income tax expense at Canadian enacted statutory tax rates			CAD 519	CAD 536	CAD 296
Increase (decrease) in taxes resulting from:
(Gains) / losses not subject to tax			28	(5)	(6)
Canadian tax rate differentials			1	(1)	(1)
Foreign tax rate differentials			39	36	(36)
Effect of tax rate increases			23	0	7
Other			(3)	(4)	(10)
Income tax expense	CAD 23	CAD 7	CAD 607	CAD 562	CAD 250