Income Taxes (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Taxes [Line Items]
Effective tax rate of provision for income taxes	36.00%	41.00%	39.00%
Cash paid for income taxes	$ 28	$ 19	$ 1
Operating Loss Carryforwards	318	412
Alternative minimum tax credit carryforward	131	77
Statutory depletion carryforward	13
Windfalls included in net operating loss carryforwards but not reflected in deferred tax assets	152
Deferred tax assests, valuation allowance	5
Net long-term deferred tax liabilities	1,951	1,527
Net current deferred tax liabilities	109	24
Accrued liability of interest related to uncertain tax position	1
Unrecognized tax benefits related to alternative minimum tax	44
Exploration Program in Canada [Member]
Income Taxes [Line Items]
Net operating loss carryforward	26
Federal [Member]
Income Taxes [Line Items]
Cash paid for income taxes	14	16
Operating Loss Carryforwards	988
State [Member]
Income Taxes [Line Items]
Cash paid for income taxes	14	3
Operating Loss Carryforwards	$ 556
Minimum [Member]
Income Taxes [Line Items]
Net operating loss carryforwards expiration date	Jan. 01, 2028
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, 2034
Maximum [Member] | Exploration Program in Canada [Member]
Income Taxes [Line Items]
Net operating loss carryforwards expiration date	Dec. 31, 2034