Income Taxes (Narrative) (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Effective tax rate			(10.00%)	1.00%	31.00%
Alternative minimum tax carryforward, expected refund			$ 68,000,000
Income tax refund received			5,000,000	$ 15,000,000	$ 6,000,000
Net operating loss carryforward			1,043,000,000	1,177,000,000
Statutory depletion carryforward			13,000,000
Net deferred tax assets			0
Deferred other tax expense			807,000,000
Reduction to equity			$ 63,000,000
Cumulative loss period			3 years
Unrecognized tax benefits			$ 12,000,000	17,000,000	$ 37,000,000
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]
Net operating loss carryforward			2,700,000,000
Federal [Member]
Income Taxes [Line Items]
Income tax refund received			4,200,000	15,000,000
Net operating loss carryforward			$ 4,100,000,000
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, 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, 2037
Maximum [Member] | State [Member]
Income Taxes [Line Items]
Cash paid for income taxes				$ 1,000,000
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
Scenario, Plan [Member]
Income Taxes [Line Items]
Corporate income tax rate		21.00%