Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended					12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Numerator
Net income	$ (44,922)	$ 246,975	$ 60,044	$ 77,241	$ 31,261	$ 415,522	$ 263,483
Denominator
Weighted-average shares outstanding, basic (in shares)	68,222				69,348
Effect of dilutive stock awards (in shares)	0				244
Adjusted weighted-average shares outstanding, diluted (in shares)	68,222				69,592
Net income per share
Basic earnings per common share (in dollars per share)	$ (0.66)	$ 3.59	$ 0.87	$ 1.11	$ 0.45	$ 6.00	$ 3.75
Diluted earnings per common share (in dollars per share)	$ (0.66)	$ 3.58	$ 0.86	$ 1.11	$ 0.45	$ 5.99	$ 3.74
Anti-dilutive weighted-average shares (in shares)	324				88