Income Taxes (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Taxes [Line Items]
Effective tax rate	1.00%	31.00%	36.00%
Income tax benefit	$ 29	$ 2,005	$ (525)
Cash paid for income taxes	(15)	(6)	28
Net operating loss carryforward	1,177	445
Alternative minimum tax credit carryforward	100	125
Statutory depletion carryforward	13
Increase in valuation allowance	983
Windfalls included in net operating loss carryforwards but not reflected in deferred tax assets	149
Unrecognized tax benefits related to alternative minimum tax	17	37	$ 44
Exploration Program in Canada [Member]
Income Taxes [Line Items]
Net operating loss carryforward	35
State [Member]
Income Taxes [Line Items]
Net operating loss carryforward	2,200
Federal [Member]
Income Taxes [Line Items]
Income tax refund received	15
Net operating loss carryforward	$ 3,200
Minimum [Member]
Income Taxes [Line Items]
Net operating loss carryforwards expiration date	Jan. 01, 2029
Minimum [Member] | Exploration Program in Canada [Member]
Income Taxes [Line Items]
Net operating loss carryforwards expiration date	Jan. 01, 2030
Maximum [Member]
Income Taxes [Line Items]
Net operating loss carryforwards expiration date	Dec. 31, 2036
Accrued liability of interest related to uncertain tax position	$ 1
Maximum [Member] | Exploration Program in Canada [Member]
Income Taxes [Line Items]
Net operating loss carryforwards expiration date	Dec. 31, 2036
Maximum [Member] | State [Member]
Income Taxes [Line Items]
Cash paid for income taxes	$ 1	$ 1