Condensed Financial Information of Parent Company (Details 1) (USD $) In Thousands, 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
Income from subsidiary:
Net interest income after provision									$ 40,385	$ 43,711	$ 40,071
Gain on sales of investment securities									2,635	43	0
Total income									111,523	106,535	96,217
Expenses [Abstract]
Salaries and employee benefits									62,563	56,573	51,368
Other expenses									1,080	1,849	1,987
Total expenses	19,823	19,929	20,240	20,341	19,347	18,913	18,635	18,134	80,333	75,029	68,284
Income tax expense	1,607	1,890	2,205	2,185	1,683	2,358	2,229	2,227	7,887	8,497	7,623
Net income	5,341	6,092	5,962	5,908	5,491	6,060	5,739	5,719	23,303	23,009	20,310
Parent Company [Member]
Income from subsidiary:
Interest									24	18	24
Management fees									1,955	1,794	1,849
Income from subsidiary									1,979	1,812	1,873
Information services revenue									66,417	60,688	54,183
Net interest income after provision									17,563	19,933	18,373
Gain on sales of investment securities									3,145	43	0
Other income									535	701	549
Total income									89,639	83,177	74,978
Expenses [Abstract]
Salaries and employee benefits									55,981	50,296	45,598
Other expenses									14,492	14,462	13,205
Total expenses									70,473	64,758	58,803
Income before income tax and equity in undistributed income of subsidiary									19,166	18,419	16,175
Income tax expense									2,914	3,156	2,955
Income before undistributed income of subsidiary									16,252	15,263	13,220
Equity in undistributed income of subsidiary									7,523	7,746	7,090
Intercompany elimination									(472)	0	0
Net income									$ 23,303	$ 23,009	$ 20,310