Foreign Exchange (Gain) Loss, Net (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Foreign Exchange Transactions [Line Items]
Unrealized foreign exchange (gain) loss	$ (291)	$ (140)	$ 687
Foreign exchange on settlements	24	(68)	358
Foreign Exchange (Gain) Loss, Net	(279)	(210)	1,082
Translation Of U.S. Dollar Debt Issued From Canada [Member]
Schedule Of Foreign Exchange Transactions [Line Items]
Unrealized foreign exchange (gain) loss	(243)	(130)	754
Foreign exchange on settlements	14	(73)	269
Translation Of U.S. Dollar Risk Management Contracts Issued From Canada [Member]
Schedule Of Foreign Exchange Transactions [Line Items]
Unrealized foreign exchange (gain) loss	(44)	4	(67)
Foreign exchange on settlements	(15)
Translation of Intercompany Notes [Member]
Schedule Of Foreign Exchange Transactions [Line Items]
Unrealized foreign exchange (gain) loss	(4)	(14)
Foreign exchange on settlements	10	5	89
Other Monetary Revaluations [Member]
Schedule Of Foreign Exchange Transactions [Line Items]
Foreign exchange on settlements	$ 3	$ (2)	$ 37