Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Basic and Diluted Earnings Per Share
Net income available to common stockholders, basic and diluted	$ 29,614	$ 29,294	$ 86,619	$ 84,701
Average outstanding shares - basic	75,531,923	74,631,317	75,424,147	74,512,806
Add: dilutive stock options and awards	54,530	44,807	45,208	41,457
Average outstanding shares - diluted	75,586,453	74,676,124	75,469,355	74,554,263
Basic earnings per share	$ 0.39	$ 0.40	$ 1.15	$ 1.14
Diluted earnings per share	$ 0.39	$ 0.40	$ 1.15	$ 1.14
Earnings Per Share
Stock options and restricted stock awards excluded from the diluted average outstanding share calculation			3,181	0