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Unrecognized Tax Benefits (Detail) (CAD) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Schedule of Unrecognized Tax Benefits [Line Items] |  |  |
January 1 balance | 151 us-gaap_UnrecognizedTaxBenefits | 143 us-gaap_UnrecognizedTaxBenefits | 134 us-gaap_UnrecognizedTaxBenefits
Additions based on current year's tax position | 4 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions | 10 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions | 4 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions
Additions for prior years' tax positions |  | 2 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions | 10 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions
Reductions for prior years' tax positions | (4)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions | (4)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions | (3)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions
Reductions due to lapse of the statute of limitations |  |  | (2)us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations
December 31 balance | 151 us-gaap_UnrecognizedTaxBenefits | 151 us-gaap_UnrecognizedTaxBenefits | 143 us-gaap_UnrecognizedTaxBenefits

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- Definition Amount of unrecognized tax benefits pertaining to uncertain tax positions taken in tax returns. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Unrecognized Tax Benefit -URI http://asc.fasb.org/extlink&oid=6527854 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319 + Details
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- Definition Amount of decrease in unrecognized tax benefits resulting from tax positions taken in prior period tax returns. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319 + Details
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- Definition Amount of increase in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319 + Details
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- Definition Amount of increase in unrecognized tax benefits resulting from tax positions taken in prior period tax returns. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319 + Details
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- Definition Amount of decrease in unrecognized tax benefits resulting from lapses of applicable statutes of limitations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(4) -URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319 + Details
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