Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Basic earnings per share [Abstract]
Net income	$ 78,409	$ 76,425	$ 75,074
Weighted average shares, basic (in shares)	43,244	43,836	43,877
Basic EPS (in dollars per share)	$ 1.81	$ 1.74	$ 1.71
Diluted earnings per share [Abstract]
Net income available to common shareholders	$ 78,409	$ 76,425	$ 75,074
Dilutive effect of stock based compensation (in shares)	378	553	518
Weighted average shares, diluted (in shares)	43,622	44,389	44,395
Diluted EPS (in dollars per share)	$ 1.80	$ 1.72	$ 1.69