Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive securities included in earnings per share	0	1,183
Anti-dilutive securities excluded from earnings per share	781	0
Calculation of basic and diluted earnings per share
Net income (loss)	$ (177,568)	$ 317,401
Basic weighted-average common shares outstanding	112,252	111,696
Dilutive effect of non-vested RSUs and contingent PSUs	0	1,183
Dilutive effect of Senior Convertible Notes	0	0
Diluted weighted-average common shares outstanding	112,252	112,879
Basic net income (loss) per common share	$ (1.58)	$ 2.84
Diluted net income (loss) per common share	(1.58)	$ 2.81
1.50% Senior Convertible Notes Due 2021 [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Senior Convertible Notes, conversion price	$ 40.50