New Accounting Standards (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2019	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Undiscounted lease payment liability	$ 9,400								$ 9,400
Retained earnings	1,205,946				$ 1,400,296				1,205,946	$ 1,400,296
Increase (decrease) in results from operations	$ (98,826)	$ (14,084)	$ (3,551)	$ (6,277)	$ (5,107)	$ (62,045)	$ (1,114)	$ (870)	(122,738)	$ (69,136)	$ 112,859
Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase (decrease) in results from operations									$ (1,600)
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings													$ 1,700
Scenario, Forecast | Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Right-of-use asset												$ 7,100
Operating lease, liability												$ 7,100