Income Taxes (Deferred Income Taxes Liability) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Abstract]
Deferred Tax Assets, Property, Plant and Equipment	$ 281	$ 256
Deferred Tax Assets, Risk Management	34	81
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits	99	100
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals	28	48
Deferred Tax Assets, Unrealized Currency Losses		20
Deferred Income Tax Assets, Non-capital and net capital losses carried forward	1,014	1,149
Deferred Income Tax Assets, Alternative minimum tax and foreign tax credits	198	198
Deferred Tax Assets, Other	53	82
Valuation Allowance	(187)	(133)
Deferred Income Tax Liabilities , Property, plant and equipment	(386)	(155)
Deferred Income Tax Liabilities , Risk management	(97)
Deferred Income Tax Liabilities , Unrealized foreign exchange gains	(18)
Deferred Income Tax Liabilities , Other	(10)	(19)
Deferred Tax Assets, Net of Liabilities	$ 1,009	$ 1,627