Defined benefit plan (Details 3) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011
Weighted-Average Assumptions used to calculate benefit obligations
Discount rate (as a percent)	4.00%	4.75%
Weighted-Average Assumptions used to calculate benefit expense
Discount rate (as a percent)	4.00%	4.75%
Rate of return on plan assets (as a percent)	5.50%	5.50%
Percentage of plan assets	100.00%	100.00%
Period over which corporate AA rated yield curve extrapolated to derive CIA/Natcan curve	10 years
Maturity period of corporate bonds rated AA	10 years
Expected future benefit payments
2013	$ 252
2014	293
2015	319
2016	362
2017	412
2018-2022	$ 3,003
Minimum
Weighted-Average Assumptions used to calculate benefit obligations
Rate of compensation increase (as a percent)	3.00%	3.00%
Weighted-Average Assumptions used to calculate benefit expense
Rate of compensation increase (as a percent)	3.00%	3.00%
Maximum
Weighted-Average Assumptions used to calculate benefit obligations
Rate of compensation increase (as a percent)	4.00%	4.00%
Weighted-Average Assumptions used to calculate benefit expense
Rate of compensation increase (as a percent)	4.00%	4.00%
Canadian equity
Weighted-Average Assumptions used to calculate benefit expense
Percentage of plan assets	30.00%	30.00%
U.S. equity
Weighted-Average Assumptions used to calculate benefit expense
Percentage of plan assets	13.00%	14.00%
International equity
Weighted-Average Assumptions used to calculate benefit expense
Percentage of plan assets	14.00%	13.00%
Canadian fixed income
Weighted-Average Assumptions used to calculate benefit expense
Percentage of plan assets	40.00%	40.00%
International fixed income
Weighted-Average Assumptions used to calculate benefit expense
Percentage of plan assets	3.00%	3.00%

Represents the maturity period of corporate bonds rated AA.

No definition available.

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- Definition

Represents the period over which corporate AA rated yield curve extrapolated to derive CIA/Natcan curve.

+ References

No definition available.

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- Definition

The interest rate used to adjust for the time value of money for the plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 5

-Subparagraph j

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (k)(1)

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- Definition

Expected rate of compensation increases (for pay-related plans).

+ References

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- Definition

The interest rate used to adjust for the time value of money.

+ References

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- Definition

An assumption as to the rate of return on plan assets reflecting the average rate of earnings expected on the funds invested or to be invested to provide for the benefits included in the benefit obligation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (k)(3)

-URI http://asc.fasb.org/extlink&oid=21915506&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 87

-Paragraph 264

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 5

-Subparagraph j

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 30

-Glossary Expected Long-Term Rate of Return on Plan Assets

-URI http://asc.fasb.org/extlink&oid=6512013

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- Definition

Expected rate of compensation increases (for pay-related plans).

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- Definition

Amount of benefits expected to be paid in the five fiscal years after the fifth fiscal year following the latest fiscal year from a defined benefit plan.

+ References

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- Definition

Amount of benefits expected to be paid in the next fiscal year following the latest fiscal year from a defined benefit plan.

+ References

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Amount of benefits expected to be paid in the fifth fiscal year following the latest fiscal year from a defined benefit plan.

+ References

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- Definition

Amount of benefits expected to be paid in the fourth fiscal year following the latest fiscal year from a defined benefit plan.

+ References

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- Definition

Amount of benefits expected to be paid in the third fiscal year following the latest fiscal year from a defined benefit plan.

+ References

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- Definition

Amount of benefits expected to be paid in the second fiscal year following the latest fiscal year from a defined benefit plan.

+ References

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- Definition

Percentage of fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of plan assets held.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 5

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=21915506&loc=d3e2446-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (d)(1)

-URI http://asc.fasb.org/extlink&oid=21915506&loc=d3e1928-114920

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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

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