Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended					6 Months Ended		12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Numerator
Net income (loss)	$ 11,254	$ 246,975	$ 60,044	$ 77,241	$ 31,261	$ (33,668)	$ 108,502	$ 415,522	$ 263,483
Denominator
Weighted-average shares outstanding, basic (in shares)	68,251			69,370		68,237	69,359
Effect of dilutive stock awards (in shares)	59			191		0	217
Adjusted weighted-average shares outstanding, diluted (in shares)	68,310			69,561		68,237	69,576
Net income (loss) per share
Basic earnings per common share (in dollars per share)	$ 0.16	$ 3.59	$ 0.87	$ 1.11	$ 0.45	$ (0.49)	$ 1.56	$ 6.00	$ 3.75
Diluted earnings per common share (in dollars per share)	$ 0.16	$ 3.58	$ 0.86	$ 1.11	$ 0.45	$ (0.49)	$ 1.56	$ 5.99	$ 3.74
Anti-dilutive weighted-average shares (in shares)	248			17		264	52