Selected Quarterly Data (Unaudited) (Details) - CAD CAD / shares in Units, CAD in Millions	3 Months Ended	12 Months Ended
Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Total revenues	CAD 1,713	CAD 1,595	CAD 1,643	CAD 1,603	CAD 1,637	CAD 1,554	CAD 1,450	CAD 1,591	CAD 6,554	CAD 6,232	CAD 6,712
Operating income	753	690	679	671	717	657	551	653	2,793	2,578	2,688
Net income	CAD 984	CAD 510	CAD 480	CAD 431	CAD 384	CAD 347	CAD 328	CAD 540	CAD 2,405	CAD 1,599	CAD 1,352
Basic earnings per share	CAD 6.79	CAD 3.50	CAD 3.28	CAD 2.94	CAD 2.63	CAD 2.35	CAD 2.16	CAD 3.53	CAD 16.49	CAD 10.69	CAD 8.47
Diluted earnings per share	CAD 6.77	CAD 3.50	CAD 3.27	CAD 2.93	CAD 2.61	CAD 2.34	CAD 2.15	CAD 3.51	CAD 16.44	CAD 10.63	CAD 8.40

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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The net result for the period of deducting operating expenses from operating revenues.

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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