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Income Taxes (Components) (Details) - CAD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Components of Income Tax Expense (Benefit), Continuing Operations [Abstract] |  |
Deferred income taxes | $ 469 | $ (67)
Income tax expense (recovery) | 520 | (22)
Supplemental Tax Information [Abstract] |  |
Income before provision for income taxes | 819 | 244
Income taxes, at statutory income tax rates | 254 | 76
Canada [Member] |  |
Components of Income Tax Expense (Benefit), Continuing Operations [Abstract] |  |
Current income taxes | 24 | 13
Deferred income taxes | 3 | (113)
Operating loss carry forwards | (40) | (2)
Supplemental Tax Information [Abstract] |  |
Income before provision for income taxes | 88 | 71
United States [Member] |  |
Components of Income Tax Expense (Benefit), Continuing Operations [Abstract] |  |
Current income taxes | 24 | 18
Deferred income taxes | 384 | 151
Operating loss carry forwards | (194) | (104)
Supplemental Tax Information [Abstract] |  |
Income before provision for income taxes | 693 | 44
Other [Member] |  |
Components of Income Tax Expense (Benefit), Continuing Operations [Abstract] |  |
Current income taxes | 3 | 15
Deferred income taxes | (1) | 0
Supplemental Tax Information [Abstract] |  |
Income before provision for income taxes | $ 38 | $ 129

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- Definition Amount of current income tax expense (benefit) pertaining to taxable income (loss) from continuing operations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32639-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08(h)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 6.I.7) -URI http://asc.fasb.org/extlink&oid=34349781&loc=d3e330036-122817 + Details
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- Definition Amount of operating loss carryforward, before tax effects, available to reduce future taxable income under enacted tax laws. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32559-109319 + Details
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