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Employee Future Benefits - Assumptions (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017 age	Dec. 31, 2016
Other, through 2027
Assumed health care cost trend rate
Health care cost escalation	4.50%	4.50%
Maximum age disabled and retired members eligible for benefits	65	65
Percentage of reasonably possible decrease in actuarial assumption	0.30%	0.30%
Other, through 2026
Assumed health care cost trend rate
Health care cost escalation	5.00%	5.00%	5.00%
Maximum age disabled and retired members eligible for benefits	65
Percentage of reasonably possible decrease in actuarial assumption	0.30%	0.30%	0.30%
Other, through 2026 | US plans
Assumed health care cost trend rate
Health care cost escalation	4.50%	4.50%	4.50%
Other, through 2024
Assumed health care cost trend rate
Maximum age disabled and retired members eligible for benefits	65
Other, through 2024 | US plans
Assumed health care cost trend rate
Health care cost escalation			5.00%
Other, through 2024 | Canadian plans
Assumed health care cost trend rate
Health care cost escalation			5.00%
Percentage of reasonably possible decrease in actuarial assumption			0.35%
Accrued Benefit Obligation | Registered
Disclosure of defined benefit plans [line items]
Discount rate	330000.00%	330000.00%	370000.00%
Rate of compensation increase	290000.00%	290000.00%	290000.00%
Assumed health care cost trend rate
Health care cost escalation	0.00%	0.00%	0.00%
Dental care cost escalation	0.00%	0.00%	0.00%
Accrued Benefit Obligation | Supplemental
Disclosure of defined benefit plans [line items]
Discount rate	330000.00%	330000.00%	360000.00%
Rate of compensation increase	300000.00%	300000.00%	300000.00%
Assumed health care cost trend rate
Health care cost escalation	0.00%	0.00%	0.00%
Dental care cost escalation	0.00%	0.00%	0.00%
Accrued Benefit Obligation | Other, through 2027
Disclosure of defined benefit plans [line items]
Discount rate	340000.00%	340000.00%	370000.00%
Rate of compensation increase	0.00%	0.00%	0.00%
Assumed health care cost trend rate
Health care cost escalation	0.00%	0.00%	0.00%
Dental care cost escalation	400000.00%	400000.00%	400000.00%
Other, through 2027 | Registered
Disclosure of defined benefit plans [line items]
Discount rate	370000.00%	370000.00%	380000.00%
Rate of compensation increase	260000.00%	260000.00%	300000.00%
Assumed health care cost trend rate
Health care cost escalation	0.00%	0.00%	0.00%
Dental care cost escalation	0.00%	0.00%	0.00%
Provincial health care premium escalation	0.00%	0.00%	0.00%
Other, through 2027 | Supplemental
Disclosure of defined benefit plans [line items]
Discount rate	360000.00%	360000.00%	380000.00%
Rate of compensation increase	300000.00%	300000.00%	300000.00%
Assumed health care cost trend rate
Health care cost escalation	0.00%	0.00%	0.00%
Dental care cost escalation	0.00%	0.00%	0.00%
Provincial health care premium escalation	0.00%	0.00%	0.00%
Other, through 2027 | Other, through 2027
Disclosure of defined benefit plans [line items]
Discount rate	370000.00%	370000.00%	380000.00%
Rate of compensation increase	0.00%	0.00%	0.00%
Assumed health care cost trend rate
Health care cost escalation	0.00%	0.00%	0.00%
Dental care cost escalation	400000.00%	400000.00%	400000.00%
Provincial health care premium escalation	0.00%	0.00%	500000.00%

X

- Definition

The discount rate used as the significant actuarial assumption to determine the present value of a defined benefit obligation. [Refer: Defined benefit obligation, at present value; Actuarial assumptions [member]]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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- Definition

The expected rate of inflation that is used as a significant actuarial assumption to determine the present value of a defined benefit obligation. [Refer: Defined benefit plans [member]; Defined benefit obligation, at present value; Actuarial assumptions [member]]

+ References

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The expected rate of salary increases used as a significant actuarial assumption to determine the present value of a defined benefit obligation. [Refer: Defined benefit plans [member]; Defined benefit obligation, at present value; Actuarial assumptions [member]]

+ References

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Medical cost trend rate used as significant actuarial assumption to determine the present value of a defined benefit obligation. [Refer: Defined benefit obligation, at present value; Actuarial assumptions [member]]

+ References

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Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table.

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The reasonably possible percentage of the decrease in the actuarial assumption used to determine the present value of defined benefit obligation. [Refer: Actuarial assumptions [member]]

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Actuarial Assumption Of Dental Cost Trend Rates

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Actuarial Assumptions [Abstract]

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Defined Benefit Plan, Maximum Age Disabled and Retired Member Eligible For Benefits

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