Pensions and Other Benefits - Summary of Defined Benefit Pension Plan Assets at Fair Value (Detail) - CAD	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	CAD 12,300,000,000	CAD 11,376,000,000
Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Notice period	60 days
Hedge Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Notice period	95 days
Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	CAD 140,000,000	189,000,000
Government bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	1,276,000,000	1,180,000,000
Corporate Debt Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	1,228,000,000	1,229,000,000
Mortgages [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	81,000,000	77,000,000
Equity Securities, Other [Member] | Canada [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	1,495,000,000	1,496,000,000
Equity Securities, Other [Member] | U.S. and International [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	5,203,000,000	4,481,000,000
Real Estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	451,000,000	654,000,000
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	278,000,000	0
Derivative Financial Instruments, Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	0	1,000,000
Absolute Return [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	2,164,000,000	1,861,000,000
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	0	0
Absolute Return [Member] | Hedge Funds, Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	781,000,000	652,000,000
Absolute Return [Member] | Hedge Funds, Multi-strategy [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	517,000,000	473,000,000
Absolute Return [Member] | Credit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	555,000,000	490,000,000
Absolute Return [Member] | Equity Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	311,000,000	246,000,000
Infrastructure [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	CAD 262,000,000	208,000,000
Hedge Funds, Multi-strategy [Member]
Defined Benefit Plan Disclosure [Line Items]
Notice period	60 days
Equity Funds, Quarterly Redemption [Member]
Defined Benefit Plan Disclosure [Line Items]
Notice period	60 days
Equity Funds, Tri-annual Redemption [Member]
Defined Benefit Plan Disclosure [Line Items]
Notice period	45 days
Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	CAD 6,747,000,000	6,008,000,000
Fair Value, Inputs, Level 1 [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	129,000,000	106,000,000
Fair Value, Inputs, Level 1 [Member] | Equity Securities, Other [Member] | Canada [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	1,449,000,000	1,448,000,000
Fair Value, Inputs, Level 1 [Member] | Equity Securities, Other [Member] | U.S. and International [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	5,169,000,000	4,454,000,000
Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	2,676,000,000	2,645,000,000
Fair Value, Inputs, Level 2 [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	11,000,000	83,000,000
Fair Value, Inputs, Level 2 [Member] | Government bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	1,276,000,000	1,180,000,000
Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	1,228,000,000	1,229,000,000
Fair Value, Inputs, Level 2 [Member] | Mortgages [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	81,000,000	77,000,000
Fair Value, Inputs, Level 2 [Member] | Equity Securities, Other [Member] | Canada [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	46,000,000	48,000,000
Fair Value, Inputs, Level 2 [Member] | Equity Securities, Other [Member] | U.S. and International [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	34,000,000	27,000,000
Fair Value, Inputs, Level 2 [Member] | Derivative Financial Instruments, Assets [Member] | U.S. and International [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values		1,000,000
Fair Value, Inputs, Level 2 [Member] | Absolute Return [Member] | Equity Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	0
Fair Value, Inputs, Level 2 [Member] | Infrastructure [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	0	0
Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	451,000,000	654,000,000
Fair Value, Inputs, Level 3 [Member] | Real Estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	451,000,000	654,000,000
Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	0	0
Fair Value, Inputs, Level 3 [Member] | Absolute Return [Member] | Hedge Funds, Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	0	0
Fair Value, Inputs, Level 3 [Member] | Absolute Return [Member] | Hedge Funds, Multi-strategy [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	0	0
Fair Value, Inputs, Level 3 [Member] | Absolute Return [Member] | Credit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	0	0
Fair Value, Inputs, Level 3 [Member] | Absolute Return [Member] | Equity Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	0	0
Portion at Other than Fair Value Measurement [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	2,426,000,000	2,069,000,000
Portion at Other than Fair Value Measurement [Member] | Absolute Return [Member] | Hedge Funds, Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	781,000,000	652,000,000
Portion at Other than Fair Value Measurement [Member] | Absolute Return [Member] | Hedge Funds, Multi-strategy [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	517,000,000	473,000,000
Portion at Other than Fair Value Measurement [Member] | Absolute Return [Member] | Credit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	555,000,000	490,000,000
Portion at Other than Fair Value Measurement [Member] | Absolute Return [Member] | Equity Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	311,000,000	246,000,000
Portion at Other than Fair Value Measurement [Member] | Infrastructure [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	CAD 262,000,000	CAD 208,000,000