Net Income Per Share (Details) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income	$ 15,554	$ 19,202	$ 14,018	$ 35,628	$ 9,520	$ 14,065	$ 9,713	$ 32,693	$ 84,402	$ 65,991	$ 39,390
Less distributed and undistributed earnings allocated to participating securities									1,517	1,496	981
Net income allocated to common shares									$ 82,885	$ 64,495	$ 38,409
Weighted average common shares outstanding									41,067	41,668	48,616
Incremental shares from assumed conversions of common stock options and LTIP awards									204	94	14
Adjusted weighted average common shares outstanding									41,271	41,762	48,630
Potentially dilutive securities not included in weighted average share calculation due to anti-dilutive effect									0	0	0