Pensions and Other Benefits - Summary of Defined Benefit Pension Plan Assets at Fair Value (Detail) - CAD	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	CAD 12,196,000,000	CAD 12,300,000,000
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 132,000,000	140,000,000
Government bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	1,357,000,000	1,276,000,000
Corporate Debt Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	1,186,000,000	1,228,000,000
Mortgages [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	71,000,000	81,000,000
Public equity [Member] | Canada [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	1,537,000,000	1,495,000,000
Public equity [Member] | U.S. and International [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	5,021,000,000	5,203,000,000
Real Estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	625,000,000	451,000,000
Unfunded commitments	81,000,000	278,000,000
Derivative Financial Instruments, Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	7,000,000	0
Derivative, Notional Amount	937,000,000
Absolute Return [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	1,975,000,000	2,164,000,000
Unfunded commitments	0	0
Absolute Return [Member] | Hedge Funds, Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	668,000,000	781,000,000
Absolute Return [Member] | Hedge Funds, Multi-strategy [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	502,000,000	517,000,000
Absolute Return [Member] | Credit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	505,000,000	555,000,000
Absolute Return [Member] | Equity Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	300,000,000	311,000,000
Infrastructure [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	285,000,000	262,000,000
Unfunded commitments	CAD 0	0
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 6,586,000,000	6,747,000,000
Fair Value, Inputs, Level 1 [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	121,000,000	129,000,000
Fair Value, Inputs, Level 1 [Member] | Public equity [Member] | Canada [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	1,480,000,000	1,449,000,000
Fair Value, Inputs, Level 1 [Member] | Public equity [Member] | U.S. and International [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	4,985,000,000	5,169,000,000
Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	2,725,000,000	2,676,000,000
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	11,000,000
Fair Value, Inputs, Level 2 [Member] | Government bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	1,357,000,000	1,276,000,000
Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	1,186,000,000	1,228,000,000
Fair Value, Inputs, Level 2 [Member] | Mortgages [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	71,000,000	81,000,000
Fair Value, Inputs, Level 2 [Member] | Public equity [Member] | Canada [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	57,000,000	46,000,000
Fair Value, Inputs, Level 2 [Member] | Public equity [Member] | U.S. and International [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	36,000,000	34,000,000
Fair Value, Inputs, Level 2 [Member] | Derivative Financial Instruments, Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	7,000,000	0
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	437,000,000	451,000,000
Fair Value, Inputs, Level 3 [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	0
Fair Value, Inputs, Level 3 [Member] | Real Estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	437,000,000	451,000,000
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	2,448,000,000	2,426,000,000
Portion at Other than Fair Value Measurement [Member] | Real Estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	188,000,000
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	668,000,000	781,000,000
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	502,000,000	517,000,000
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	505,000,000	555,000,000
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	300,000,000	311,000,000
Portion at Other than Fair Value Measurement [Member] | Infrastructure [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans at fair values	CAD 285,000,000	CAD 262,000,000