Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Basic and Diluted Earnings Per Share
Net income available to common stockholders, basic and diluted	$ 30,957	$ 29,614	$ 90,090	$ 86,619
Average outstanding shares - basic	76,288,640	75,531,923	76,195,550	75,424,147
Add: dilutive stock options and awards	62,233	54,530	51,501	45,208
Average outstanding shares - diluted	76,350,873	75,586,453	76,247,051	75,469,355
Basic earnings per share	$ 0.40	$ 0.39	$ 1.18	$ 1.15
Diluted earnings per share	$ 0.40	$ 0.39	$ 1.18	$ 1.15
Earnings Per Share
Stock options and restricted stock awards excluded from the diluted average outstanding share calculation			10,677	3,181