Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended					9 Months Ended		12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Numerator
Net income	$ 97,480	$ 246,975	$ 60,044	$ 77,241	$ 31,261	$ 63,812	$ 168,546	$ 415,522	$ 263,483
Denominator
Weighted-average shares outstanding, basic (in shares)	68,254		69,370			68,243	69,363
Effect of dilutive stock awards (in shares)	249		88			103	174
Adjusted weighted-average shares outstanding, diluted (in shares)	68,503		69,458			68,346	69,537
Net income per share
Basic earnings per common share (in dollars per share)	$ 1.43	$ 3.59	$ 0.87	$ 1.11	$ 0.45	$ 0.94	$ 2.43	$ 6.00	$ 3.75
Diluted earnings per common share (in dollars per share)	$ 1.42	$ 3.58	$ 0.86	$ 1.11	$ 0.45	$ 0.93	$ 2.42	$ 5.99	$ 3.74
Anti-dilutive weighted-average shares (in shares)	5		124			192	76