Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Basic and Diluted Earnings Per Share
Net income available to common stockholders, basic and diluted	$ 29,335	$ 28,677	$ 57,005	$ 55,407
Average outstanding shares - basic	75,530,591	74,467,576	75,369,366	74,452,568
Add: dilutive stock options and awards	35,064	32,084	38,255	38,891
Average outstanding shares - diluted	75,565,655	74,499,660	75,407,621	74,491,459
Basic earnings per share	$ 0.39	$ 0.38	$ 0.76	$ 0.74
Diluted earnings per share	$ 0.39	$ 0.38	$ 0.76	$ 0.74
Earnings Per Share
Stock options and restricted stock awards excluded from the diluted average outstanding share calculation			0	0