Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	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, 2011	Dec. 31, 2010	Dec. 31, 2009
Summarized quarterly financial data
Net interest income	$ 55,197	$ 55,980	$ 62,395	$ 57,109	$ 59,149	$ 62,615	$ 64,214	$ 73,339	$ 234,681	$ 259,317	$ 222,264
Provision for credit losses				7,068	12,700	25,300	11,000	12,200	7,068	61,200	80,500
Net earnings	$ 21,712	$ 22,383	$ 21,038	$ 16,600	$ 9,874	$ 17,927	$ 19,015	$ 16,119	$ 81,733	$ 62,935	$ 65,419
Basic earnings per common share	$ 0.21	$ 0.21	$ 0.20	$ 0.16	$ 0.09	$ 0.17	$ 0.18	$ 0.15	$ 0.77	$ 0.59	$ 0.56
Diluted earnings per common share	$ 0.21	$ 0.21	$ 0.20	$ 0.16	$ 0.09	$ 0.17	$ 0.18	$ 0.15	$ 0.77	$ 0.59	$ 0.56