Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Basic and diluted earnings per share computation
Net income available to common stockholders, basic and diluted	$ 22,702	$ 18,981	$ 43,470	$ 35,314
Average outstanding shares - basic	72,480,019	71,928,697	72,224,263	71,921,885
Add: dilutive stock options and awards	68,153	156	57,841	105
Average outstanding shares - diluted	72,548,172	71,928,853	72,282,104	71,921,990
Basic earnings per share	$ 0.31	$ 0.26	$ 0.60	$ 0.49
Diluted earnings per share	$ 0.31	$ 0.26	$ 0.60	$ 0.49
Earnings Per Share (Textual) [Abstract]
Options excluded from the diluted average outstanding share calculation			74,159	945,063