Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	2,433	0	0	78
Calculation of basic and diluted earnings per share
Net income (loss)	$ (135,923)	$ (89,112)	$ 198,675	$ (134,585)
Basic weighted-average common shares outstanding	112,107	111,575	111,836	111,366
Dilutive effect of non-vested RSUs and contingent PSUs	0	0	1,764	0
Dilutive effect of Senior Convertible Notes	0	0	0	0
Diluted weighted-average common shares outstanding	112,107	111,575	113,600	111,366
Basic net income (loss) per common share	$ (1.21)	$ (0.80)	$ 1.78	$ (1.21)
Diluted net income (loss) per common share	(1.21)	$ (0.80)	1.75	$ (1.21)
1.50% Senior Convertible Notes Due 2021 [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Debt Instrument, Convertible, Conversion Price	$ 40.50		$ 40.50