Selected Quarterly Information (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended												12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Interest Income (Expense), Net	$ 21,145	$ 19,100	$ 19,008	$ 19,147	$ 16,972	$ 14,559	$ 15,033	$ 12,731	$ 10,895	$ 8,400	$ 7,932	$ 7,197	$ 78,401		$ 59,295		$ 34,424
Interest Income (Expense), after Provision for Loan Loss	21,123	18,990	19,008	19,087	16,851	14,259	14,833	12,481	10,285	8,190	7,782	6,788	78,209		58,424		33,045
Basic Net Income Per Common Share	$ 0.34	$ 0.34	$ 0.35	$ 0.33	$ 0.36	$ 0.04	$ 0.34	$ 0.31	$ 0.41	$ 0.35	$ 0.41	$ 0.32	$ 1.35		$ 1.03		$ 1.50
Diluted Net Income Per Common Share	$ 0.34	$ 0.34	$ 0.35	$ 0.33	$ 0.36	$ 0.04	$ 0.34	$ 0.31	$ 0.40	$ 0.33	$ 0.38	$ 0.30	$ 1.35	[1]	$ 1.03	[1]	$ 1.41
Dividends declared per common share	$ 0.29	$ 0.29	$ 0.29	$ 0.28	$ 0.28	$ 0.27	$ 0.27	$ 0.27	$ 0.27	$ 0.25	$ 0.23	$ 0.23	$ 1.15		$ 1.09		$ 0.98
Net Income (Loss) Available to Common Stockholders, Basic	$ 18,330	$ 18,353	$ 18,847	$ 16,476	$ 14,406	$ 1,532	$ 13,594	$ 10,280	$ 9,647	$ 5,966	$ 6,264	$ 4,534	$ 72,006		$ 39,812		$ 26,411

[1] For the years ended December 31, 2012, December 31, 2011 and December 31, 2010, the calculation of diluted net income per common share excludes the effect of 15,000 unexercised stock option awards because their inclusion would have been anti-dilutive.