Net Income per Share (Details) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share [Abstract]
Net income	$ 24,560	$ 14,018	$ 74,551	$ 49,646
Less distributed and undistributed earnings allocated to participating securities	346	248	1,064	909
Net Income (Loss) Available to Common Stockholders, Basic	$ 24,214	$ 13,770	$ 73,487	$ 48,737
Weighted average common shares outstanding basic (in shares)	41,377	41,258	41,301	41,194
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	156	217	324	210
Weighted Average Number of Shares Outstanding, Diluted	41,533	41,475	41,625	41,404