Net Income per Share (Details) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Earnings Per Share [Abstract]
Net income	$ 49,991	$ 35,628
Less distributed and undistributed earnings allocated to participating securities	585	781
Net Income (Loss) Available to Common Stockholders, Basic	$ 49,406	$ 34,847
Weighted average common shares outstanding basic (in shares)	41,224	41,130
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	493	202
Weighted Average Number of Shares Outstanding, Diluted	41,717	41,332