Condensed Financial Statements of Parent Company, Condensed Statements of Cash Flows (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Cash Flows from Operating Expenses
Net income	$ 17,747	$ 17,638	$ 15,569
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization of premiums and accretion of discounts, net	(224)	(217)	(300)
Depreciation expense	764	799	886
Net change in other assets	(44)	501	(858)
Net change in other liabilities	(175)	(4,135)	(280)
Net cash provided by operating activities	22,712	19,456	18,565
Cash Flows from Investing Activities
Purchases of securities available for sale	(171,601)	(64,567)	(93,862)
Maturities and calls of securities available for sale	151,051	74,961	68,565
Net cash used in investing activities	(39,739)	(45,321)	(47,763)
Cash Flows from Financing Activities
Exercise of stock options	119	92	0
Net cash provided by financing activities	19,913	27,904	26,162
Net change in cash and due from banks	2,886	2,039	(3,036)
Cash and due from banks at beginning of year	11,897	9,858	12,894
Cash and due from banks at end of year	14,783	11,897	9,858
NBI [Member]
Cash Flows from Operating Expenses
Net income	17,747	17,638	15,569
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in undistributed net income of subsidiaries	(10,512)	(10,757)	(9,640)
Amortization of premiums and accretion of discounts, net	1	1	2
Depreciation expense	12	9	9
Net change in refundable income taxes due from subsidiaries	77	0	56
Net change in other assets	(199)	147	(24)
Net change in other liabilities	(124)	(364)	244
Net cash provided by operating activities	7,002	6,674	6,216
Cash Flows from Investing Activities
Purchases of securities available for sale	(815)	(1,973)	(1,868)
Maturities and calls of securities available for sale	1,352	2,250	1,950
Net cash used in investing activities	537	277	82
Cash Flows from Financing Activities
Cash dividends paid	(7,639)	(6,938)	(6,309)
Exercise of stock options	119	92	0
Capital distribution to subsidiary	0	(105)	0
Net cash provided by financing activities	(7,520)	(6,951)	(6,309)
Net change in cash and due from banks	19	0	(11)
Cash and due from banks at beginning of year	2	2	13
Cash and due from banks at end of year	$ 21	$ 2	$ 2