Unaudited Quarterly Financial Data (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, 2012	Dec. 31, 2011	Dec. 31, 2010
Summarized unaudited quarterly financial data
Interest income	$ 59,666	$ 62,015	$ 64,192	$ 67,884	$ 68,741	$ 71,433	$ 71,562	$ 68,373	$ 253,757	$ 280,109	$ 288,402
Interest expense	8,165	8,907	9,044	9,598	10,197	11,297	11,331	11,669	35,714	44,494	53,634
Net Interest Income	51,501	53,108	55,148	58,286	58,544	60,136	60,231	56,704	218,043	235,615	234,768
Provision for loan losses	2,275	2,700	7,925	8,625	8,675	17,175	19,150	19,500	21,525	64,500	84,693
Net interest income after provision for loan losses	49,226	50,408	47,223	49,661	49,869	42,961	41,081	37,204	196,518	171,115	150,075
Noninterest Income	25,393	23,974	21,791	20,338	22,017	20,936	17,851	17,395	91,496	78,199	87,546
Noninterest Expense	48,008	50,178	46,190	49,045	55,130	88,298	46,220	42,476	193,421	232,124	187,948
Income Before Income Taxes	26,611	24,204	22,824	20,954	16,756	(24,401)	12,712	12,123	94,593	17,190	49,673
Income Tax Expense (Benefit)	5,853	4,760	3,843	4,621	2,408	(5,353)	826	1,838	19,077	(281)	7,343
Net Income	$ 20,758	$ 19,444	$ 18,981	$ 16,333	$ 14,348	$ (19,048)	$ 11,886	$ 10,285	$ 75,516	$ 17,471	$ 42,330
Basic earnings per share	$ 0.29	$ 0.27	$ 0.26	$ 0.23	$ 0.20	$ (0.27)	$ 0.17	$ 0.14	$ 1.05	$ 0.24	$ 0.61
Diluted earnings per share	$ 0.29	$ 0.27	$ 0.26	$ 0.23	$ 0.20	$ (0.27)	$ 0.17	$ 0.14	$ 1.05	$ 0.24	$ 0.61