Summary of Significant Accounting Policies ASU Adjustment (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018	Jan. 01, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings		$ (1,165,842)	[1]	$ (665,657)
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent		34,875		65,890
Exploration		55,166		54,713	$ 64,970
General and administrative		116,504		117,283	124,828
Total operating expenses		1,230,735		1,293,097	2,274,494
Income (loss) from operations		836,337		(163,721)	(1,057,044)
Other non-operating income (expense), net		3,086		(800)	(1,909)
Cumulative effect of accounting change	[2]	0		44,732
ASU 2016-09 Cumulative Effect Adjustment for Timing of Recognition of Excess Tax Benefits [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings							$ (44,300)
ASU 2016-09 Cumulative Effect of Actual Forfeiture Rate [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings							700
Additional Paid in Capital							1,100
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent							$ (400)
Accounting Standards Update 2016-15, 2016-18 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Restricted Cash					3,000
Accounting Standards Update 2018-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting change	[3]	$ 2,969		0	0	$ (2,969)
Previously Reported [Member] | Accounting Standards Update 2017-07 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Exploration				56,179	65,641
General and administrative				120,585	126,428
Total operating expenses				1,297,865	2,276,765
Income (loss) from operations				(168,489)	(1,059,315)
Other non-operating income (expense), net				$ 3,968	$ 362

[1] (1) The Company reclassified $3.0 million of tax effects stranded in accumulated other comprehensive loss to retained earnings as of January 1, 2018. Please refer to Note 1 – Summary of Significant Accounting Policies for further detail.

[2] (1) Refer to Recently Issued Accounting Standards in Note 1 – Summary of Significant Accounting Policies for additional information.

[3] (1) Refer to Recently Issued Accounting Standards in Note 1 – Summary of Significant Accounting Policies and Statements of Stockholders’ Equity for additional information.