Condensed Financial Information of Parent Company (Details 2) (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
Cash flows from operating activities:
Net income	$ 5,341	$ 6,092	$ 5,962	$ 5,908	$ 5,491	$ 6,060	$ 5,739	$ 5,719	$ 23,303	$ 23,009	$ 20,310
Cash flows from investing activities:
Net increase in securities									114,646	61,768	49,944
Net decrease (increase) in loans									19,165	(35,981)	67,169
Purchases of premises and equipment, net									3,099	1,925	1,190
Net cash provided by (used in) investing activities									(65,207)	(31,041)	(124,505)
Cash flows from financing activities:
Net (decrease) increase in accounts and drafts payable									(72,440)	79,094	85,856
Cash dividends paid									7,361	6,279	5,448
Purchase of common shares of treasury									0	0	(467)
Other financing activities									(534)	(161)	(291)
Net (decrease) increase in cash and cash equivalents									(94,874)	97,033	59,635
Cash and cash equivalents at beginning of year				235,962				138,929	235,962	138,929	79,294
Cash and cash equivalents at end of year	141,088				235,962				141,088	235,962	138,929
Parent Company [Member]
Cash flows from operating activities:
Net income									23,303	23,009	20,310
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Equity in undistributed income of subsidiary									7,523	7,746	7,090
Net change in other assets									(3,338)	(19,669)	(4,463)
Net change in other liabilities									5,603	1,655	(4,930)
Amortization of stock-based awards									1,201	1,390	1,472
Other, net									(2,673)	(7,497)	0
Net cash provided by (used in) operating activities									16,573	(8,858)	5,299
Cash flows from investing activities:
Net increase in securities									(7,697)	(37,329)	(45,173)
Net decrease (increase) in loans									16,319	19,068	(19,911)
Purchases of premises and equipment, net									(3,555)	(2,107)	(1,166)
Net cash provided by (used in) investing activities									5,067	(20,368)	(66,250)
Cash flows from financing activities:
Net (decrease) increase in accounts and drafts payable									72,440	(79,094)	(85,856)
Cash dividends paid									(7,361)	(6,279)	(5,448)
Purchase of common shares of treasury									0	0	(467)
Other financing activities									(2,454)	(1,177)	(275)
Net cash (used in) provided by financing activities									(82,255)	71,638	79,666
Net (decrease) increase in cash and cash equivalents									(60,615)	42,412	18,715
Cash and cash equivalents at beginning of year				122,029				79,617	122,029	79,617	60,902
Cash and cash equivalents at end of year	$ 61,414				$ 122,029				$ 61,414	$ 122,029	$ 79,617