Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Earnings Per Share [Abstract]
Net Income	$ 40,310	$ 36,641
Average common shares outstanding - basic	28,633	27,448
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	90	112
Average common shares outstanding - diluted	28,723	27,560
Earnings per share of common stock - basic	$ 1.41	$ 1.33
Earnings per share of common stock - diluted	$ 1.40	$ 1.33
Antidilutive shares excluded from computation of earnings per share amount	22	22