Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Anti-dilutive securities excluded from earnings per share	0		0		715	0
Calculation of basic and diluted earnings per share
Net income (loss)	$ 50,388	$ (177,568)	$ 17,197	$ 317,401	$ (127,180)	$ 334,598
Basic weighted-average common shares outstanding	112,262		111,701		112,257	111,698
Dilutive effect of non-vested RSUs and contingent PSUs	670		1,929		0	1,569
Diluted weighted-average common shares outstanding	112,932		113,630		112,257	113,267
Basic net income (loss) per common share	$ 0.45		$ 0.15		$ (1.13)	$ 3.00
Diluted net income (loss) per common share	0.45		0.15		(1.13)	2.95
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		$ 40.50		$ 40.50	$ 40.50