Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Basic and diluted earnings per share computation
Net income available to common stockholders, basic and diluted	$ 25,628	$ 19,444	$ 69,098	$ 54,758
Average outstanding shares - basic	73,945,523	71,933,141	72,804,321	71,925,664
Add: dilutive stock options and awards	76,348	40,844	65,154	97
Average outstanding shares - diluted	74,021,871	71,973,985	72,869,475	71,925,761
Basic earnings per share	$ 0.35	$ 0.27	$ 0.95	$ 0.76
Diluted earnings per share	$ 0.35	$ 0.27	$ 0.95	$ 0.76
Earnings Per Share
Options excluded from the diluted average outstanding share calculation			49,932	903,945