Employee Benefit Plans (Changes in Benefit Obligation and Plan Assets and the Funded Status) (Details) - CAD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Defined Benefit Plan, Fair Value of Plan Assets, Beginning Balance	$ 2,208
Defined Benefit Plan, Fair Value of Plan Assets, Ending Balance	2,408	$ 2,208
Pension Plan [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Defined Benefit Plan, Benefit Obligation, Beginning Balance	2,607	1,520
Addition of Teco	0	1,035
Defined Benefit Plan, Service Cost	49	35
Defined Benefit Plan, Contributions by Plan Participants	8	8
Defined Benefit Plan, Interest Cost	99	79
Defined Benefit Plan, Plan Amendments	0	0
Benefits Paid	(129)	(94)
Defined Benefit Plan, Actuarial Gain (Loss)	171	(2)
Special Termination	(35)	0
Defined Benefit Plan, Foreign Currency Exchange Rate Gain (Loss)	(87)	26
Defined Benefit Plan, Benefit Obligation, Ending Balance	2,683	2,607
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Defined Benefit Plan, Fair Value of Plan Assets, Beginning Balance	2,208	1,300
Addition of Teco	0	830
Defined Benefit Plan, Contributions by Employer	109	49
Defined Benefit Plan, Contributions by Plan Participants	8	8
Benefits Paid	(129)	(94)
Defined Benefit Plan, Actual Return on Plan Assets	313	93
Defined Benefit Plan, Foreign Currency Exchange Rate Changes, Plan Assets	(67)	22
Special Termination	(34)	0
Defined Benefit Plan, Fair Value of Plan Assets, Ending Balance	2,408	2,208
Defined Benefit Plan, Funded Status of Plan [Abstract]
Defined Benefit Plan, Funded Status of Plan	(275)	(399)
Non-pension Benefit Plans [Member
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Defined Benefit Plan, Benefit Obligation, Beginning Balance	358	88
Addition of Teco	0	277
Defined Benefit Plan, Service Cost	5	4
Defined Benefit Plan, Contributions by Plan Participants	4	0
Defined Benefit Plan, Interest Cost	14	9
Defined Benefit Plan, Plan Amendments	0	2
Benefits Paid	(27)	(16)
Defined Benefit Plan, Actuarial Gain (Loss)	25	(12)
Special Termination	0	0
Defined Benefit Plan, Foreign Currency Exchange Rate Gain (Loss)	(23)	6
Defined Benefit Plan, Benefit Obligation, Ending Balance	356	358
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Defined Benefit Plan, Fair Value of Plan Assets, Beginning Balance	39	6
Addition of Teco	0	29
Defined Benefit Plan, Contributions by Employer	27	17
Defined Benefit Plan, Contributions by Plan Participants	4	0
Benefits Paid	(27)	(16)
Defined Benefit Plan, Actual Return on Plan Assets	5	2
Defined Benefit Plan, Foreign Currency Exchange Rate Changes, Plan Assets	(3)	1
Special Termination	0	0
Defined Benefit Plan, Fair Value of Plan Assets, Ending Balance	45	39
Defined Benefit Plan, Funded Status of Plan [Abstract]
Defined Benefit Plan, Funded Status of Plan	$ (311)	$ (319)

Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses.

Name:	us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of gain (loss) from change in actuarial assumptions which (increases) decreases benefit obligation of defined benefit plan. Assumptions include, but are not limited to, interest, mortality, employee turnover, salary, and temporary deviation from substantive plan.