Basic and Diluted Net Income Per Common 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 Net Income Per Common Share
Net income available to common shareholders	$ 12,970	$ 10,884	$ 22,784	$ 18,622
Weighted Average Shares Outstanding
Basic (in shares)	53,799,261	56,639,330	53,710,300	56,575,779
Effect of Dilutive Securities (in shares)	747,928	551,027	665,235	581,629
Diluted (in shares)	54,547,189	57,190,357	54,375,535	57,157,408
Net income per common share Basic (in dollars per share)	$ 0.24	$ 0.19	$ 0.42	$ 0.33
Net income per common share Diluted (in dollars per share)	$ 0.24	$ 0.19	$ 0.42	$ 0.33
Antidilutive options excluded from calculation of net income per common share (in shares)	48,288	602,711	381,396	355,338