Income Taxes (Income Taxes Reconciliation) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Reconciliation [Line Items]
Total Net Earnings (Loss) Before Income Tax	$ 147	$ 522	$ 327	$ 434	$ (251)	$ 379	$ (1,068)	$ (680)	$ 1,430	$ (1,620)	$ (8,010)
Canadian Statutory rate									27.00%	27.00%	26.40%
Expected Income Tax									$ 386	$ (437)	$ (2,115)
Income tax related to foreign operations									(73)	(266)	(776)
Effect of legislative changes									299		(11)
Non-taxable capital (gains) losses									(39)	(29)	132
Tax differences on divestitures and transactions									77	9	(8)
Partnership tax allocation in excess of funding									(54)	(17)	(21)
Adjustment in respect of prior periods									(49)	(11)	(8)
Change in Valuation allowance									54	121
Other									2	(46)	(38)
Income Tax Expense (Recovery), Total	$ 376	$ 228	$ (4)	$ 3	$ 30	$ 62	$ (467)	$ (301)	$ 603	$ (676)	$ (2,845)
Effective Tax Rate									42.20%	41.70%	35.50%
Canada [Member]
Income Tax Reconciliation [Line Items]
Total Net Earnings (Loss) Before Income Tax									$ 512	$ (627)	$ (2,014)
United States [Member]
Income Tax Reconciliation [Line Items]
Total Net Earnings (Loss) Before Income Tax									476	(1,522)	(6,963)
Effect of legislative changes									327
Other Countries [Member]
Income Tax Reconciliation [Line Items]
Total Net Earnings (Loss) Before Income Tax									$ 442	$ 529	$ 967