Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Basic earnings per share [Abstract]
Net income	$ 82,151	$ 78,409	$ 76,425
Weighted average shares, basic (in shares)	43,575	43,244	43,836
Basic EPS (in dollars per share)	$ 1.89	$ 1.81	$ 1.74
Diluted earnings per share [Abstract]
Net income	$ 82,151	$ 78,409	$ 76,425
Dilutive effect of stock based compensation (in shares)	330	378	553
Weighted average shares, diluted (in shares)	43,905	43,622	44,389
Diluted EPS (in dollars per share)	$ 1.87	$ 1.80	$ 1.72