Changes in Accumulated Other Comprehensive Loss ("AOCL") by Component - Changes in AOCL by Component (Details) - CAD CAD in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Changes in Accumulated Other Comprehensive Loss by Component [Roll Forward]
Beginning balance			CAD 4,796	CAD 5,610
Ending balance	CAD 4,691	CAD 5,251	4,691	5,251
Total [Member]
Changes in Accumulated Other Comprehensive Loss by Component [Roll Forward]
Beginning balance	(1,481)	(2,207)	(1,477)	(2,219)
Other comprehensive income (loss) before reclassifications	(26)	26	(66)	(11)
Amounts reclassified from accumulated other comprehensive loss	36	48	72	97
Net current-period other comprehensive (loss) income	10	74	6	86
Ending balance	(1,471)	(2,133)	(1,471)	(2,133)
Foreign currency net of hedging activities [Member]
Changes in Accumulated Other Comprehensive Loss by Component [Roll Forward]
Beginning balance	125	125	129	115
Other comprehensive income (loss) before reclassifications	(1)	0	(5)	10
Amounts reclassified from accumulated other comprehensive loss	0	0	0	0
Net current-period other comprehensive (loss) income	(1)	0	(5)	10
Ending balance	124	125	124	125
Derivatives and other [Member]
Changes in Accumulated Other Comprehensive Loss by Component [Roll Forward]
Beginning balance	(136)	(103)	(102)	(52)
Other comprehensive income (loss) before reclassifications	(23)	26	(59)	(26)
Amounts reclassified from accumulated other comprehensive loss	2	0	4	1
Net current-period other comprehensive (loss) income	(21)	26	(55)	(25)
Ending balance	(157)	(77)	(157)	(77)
Pension and post-retirement defined benefit plans [Member]
Changes in Accumulated Other Comprehensive Loss by Component [Roll Forward]
Beginning balance	(1,470)	(2,229)	(1,504)	(2,282)
Other comprehensive income (loss) before reclassifications	(2)	0	(2)	5
Amounts reclassified from accumulated other comprehensive loss	34	48	68	96
Net current-period other comprehensive (loss) income	32	48	66	101
Ending balance	CAD (1,438)	CAD (2,181)	CAD (1,438)	CAD (2,181)

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

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Amount after tax, before reclassification adjustments of other comprehensive income (loss).

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Amount after tax and reclassification adjustments of other comprehensive income (loss).

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Amount after tax of reclassification adjustments of other comprehensive income (loss).

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Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.

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