Income Taxes (Parenthetical) (Detail) - CAD CAD in Millions			12 Months Ended
	Nov. 02, 2017	Jun. 30, 2015	Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Schedule Of Income Tax [Line Items]
Increase in the provincial tax rate	1.00%	2.00%	0.90%	[1]		16.10%	[1]
Cash outflow from income taxes, plus investment credits earned			CAD 322		CAD 172	CAD 202
Minimum
Schedule Of Income Tax [Line Items]
Provincial tax rate	11.00%	10.00%
Maximum
Schedule Of Income Tax [Line Items]
Provincial tax rate	12.00%	12.00%

[1] On November 2, 2017 the British Columbia government enacted a 1 percent increase in the provincial tax rate from 11 percent to 12 percent. On June 30, 2015 the Alberta government enacted a 2 percent increase in the provincial tax rate, from 10 percent to 12 percent.