Net Income per Share (Details) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share [Abstract]
Net income	$ 36,207	$ 19,202	$ 110,758	$ 68,848
Less distributed and undistributed earnings allocated to participating securities	503	337	1,546	1,233
Net Income (Loss) Available to Common Stockholders, Basic	$ 35,704	$ 18,865	$ 109,212	$ 67,615
Weighted average common shares outstanding basic (in shares)	41,330	40,946	41,311	41,110
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	218	296	283	238
Weighted Average Number of Shares Outstanding, Diluted	41,548	41,242	41,594	41,348

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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of share based payment arrangements using the treasury stock method.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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