Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 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	0	44	0	59
Calculation of basic and diluted earnings per share
Net income (loss)	$ 17,197	$ (119,907)	$ 334,598	$ (45,473)
Basic weighted-average common shares outstanding	111,701	111,277	111,698	111,274
Dilutive effect of non-vested RSUs and contingent PSUs	1,929	0	1,569	0
Dilutive effect of Senior Convertible Notes	0	0	0	0
Diluted weighted-average common shares outstanding	113,630	111,277	113,267	111,274
Basic net income (loss) per common share	$ 0.15	$ (1.08)	$ 3.00	$ (0.41)
Diluted net income (loss) per common share	0.15	$ (1.08)	2.95	$ (0.41)
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