Income Taxes (Narrative) (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Effective tax rate		0.00%	(10.00%)	1.00%
Corporate income tax rate		21.00%
Alternative minimum tax carryforward, expected refund		$ 68,000,000
Cash paid for income taxes		6,000,000	$ (5,000,000)	$ (15,000,000)
Net operating loss carryforward		777,000,000	1,043,000,000
Statutory depletion carryforward		13,000,000
Net deferred tax assets		0
Reduction to valuation allowance as a component of income tax expense		(121,000,000)
Reduction to equity		2,000,000
Reduction to valuation allowance		$ 123,000,000
Cumulative loss period		3 years
Unrecognized tax benefits		$ 7,000,000	12,000,000	$ 17,000,000
Share-based Payments [Member]
Income Taxes [Line Items]
Unrecognized tax benefits		0
Exploration Program in Canada [Member]
Income Taxes [Line Items]
Net operating loss carryforward		29,000,000
State [Member]
Income Taxes [Line Items]
Cash paid for income taxes		6,300,000
Net operating loss carryforward		2,100,000,000
Federal [Member]
Income Taxes [Line Items]
Income tax refund received			4,200,000
Net operating loss carryforward		$ 3,000,000,000
Minimum [Member]
Income Taxes [Line Items]
Fully refunded year		2019
Net operating loss carryforwards expiration date		Dec. 31, 2035
Minimum [Member] | Exploration Program in Canada [Member]
Income Taxes [Line Items]
Net operating loss carryforwards expiration date		Dec. 31, 2030
Maximum [Member]
Income Taxes [Line Items]
Fully refunded year		2021
Net operating loss carryforwards expiration date		Dec. 31, 2037
Accrued liability of interest related to uncertain tax position		$ 1,000,000
Maximum [Member] | Exploration Program in Canada [Member]
Income Taxes [Line Items]
Net operating loss carryforwards expiration date		Dec. 31, 2038
Maximum [Member] | State [Member]
Income Taxes [Line Items]
Income tax refund received			$ 1,000,000
Accounting Standards Update 2016-09 [Member]
Income Taxes [Line Items]
Unrecognized windfall tax benefits, cumulative-effect adjustment	$ 149,000,000
Net cumulative-effect adjustment	$ 59,000,000