Income Taxes (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Taxes [Line Items]
Effective tax rate of provision for income taxes	31.00%	36.00%	41.00%
Cash paid for income taxes	$ (6,000,000)	$ 28,000,000	$ 19,000,000
Net operating loss carryforward	445,000,000	323,000,000
Alternative minimum tax credit carryforward	125,000,000	131,000,000
Statutory depletion carryforward	13,000,000
Increase in valuation allowance	488,000,000
Windfalls included in net operating loss carryforwards but not reflected in deferred tax assets	149,000,000
Unrecognized tax benefits related to alternative minimum tax	37,000,000	44,000,000
Accrued liability of interest related to uncertain tax position	2,000,000
Exploration Program in Canada [Member]
Income Taxes [Line Items]
Net operating loss carryforward	51,000,000
State [Member]
Income Taxes [Line Items]
Cash paid for income taxes		14,000,000
Net operating loss carryforward	789,000,000
Federal [Member]
Income Taxes [Line Items]
Cash paid for income taxes	0	$ 14,000,000
Net operating loss carryforward	$ 1,278,000,000
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, 2035
Maximum [Member] | Exploration Program in Canada [Member]
Income Taxes [Line Items]
Net operating loss carryforwards expiration date	Dec. 31, 2035
Maximum [Member] | State [Member]
Income Taxes [Line Items]
Cash paid for income taxes	$ 1,000,000