Significant Accounting Policies - Revisions to Statements of Cash Flow (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net income	$ (70,664)	$ (6,420)	$ 144,660	$ (5,363)	$ (30,486)	$ 31,266	$ 42,516	$ 55,759	$ 62,213	$ 99,055	$ 141,241
Deferred income taxes									15,874	(38,931)	7,975
Payroll and accrued liabilities									(1,258)	(61,800)	8,378
Increase (Decrease) in Other Operating Assets and Liabilities, Net									$ (17,917)	67,282	7,937
Adjustment [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net income		$ 9,178								(5,534)	2,668
Deferred income taxes										1,790	(3,101)
Payroll and accrued liabilities										3,846	(501)
Increase (Decrease) in Other Operating Assets and Liabilities, Net										$ (102)	$ 934