Condensed Financial Information of Parent Company (Schedule of Statements of Cash Flows) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities:
Net income	$ 6,424	$ 6,222	$ 5,869	$ 5,833	$ 6,089	$ 5,880	$ 5,548	$ 5,539	$ 24,348	$ 23,056	$ 24,033
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Net cash provided by (used in) operating activities									35,189	33,493	34,843
Cash flows from investing activities:
Net increase in securities									(96,290)	(161,279)	(54,054)
Net decrease in loans									(5,771)	10,882	(16,954)
Purchases of premises and equipment, net									(4,684)	(5,747)	(6,291)
Net cash used in investing activities									(41,551)	(3,636)	(100,949)
Cash flows from financing activities:
Net increase (decrease) in accounts and drafts payable									65,028	(78,169)	111,475
Cash dividends paid									(9,979)	(9,697)	(9,337)
Purchase of common shares of treasury									(9,215)	(10,951)	(1,848)
Other financing activities									(1,378)	(488)	(814)
Net cash provided by (used in) financing activities									19,933	(71,020)	135,179
Net increase (decrease) in cash and cash equivalents									13,571	(41,163)	69,073
Cash and cash equivalents at beginning of year				253,172				294,335	253,172	294,335	225,262
Cash and cash equivalents at end of year	266,743				253,172				266,743	253,172	294,335
Parent Company [Member]
Cash flows from operating activities:
Net income									24,348	23,056	24,033
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Equity in undistributed income of subsidiaries									(10,059)	(7,889)	(7,164)
Net change in other assets									(7,085)	16,100	(44,879)
Net change in other liabilities									6,683	(2,779)	534
Amortization of stock-based awards									1,677	1,504	1,250
Other, net									7,558	10,389	13,487
Net cash provided by (used in) operating activities									23,122	40,381	(12,739)
Cash flows from investing activities:
Net increase in securities									(33,025)	(23,472)	(35,128)
Net decrease in loans									40,431	28,343	9,358
Purchases of premises and equipment, net									(4,557)	(5,708)	(8,941)
Net cash used in investing activities									2,849	(837)	(34,711)
Cash flows from financing activities:
Net increase (decrease) in accounts and drafts payable									64,026	(78,439)	111,405
Cash dividends paid									(9,979)	(9,697)	(9,337)
Purchase of common shares of treasury									(9,215)	(10,951)	(1,848)
Other financing activities									1,705	66	(21)
Net cash provided by (used in) financing activities									46,537	(99,021)	100,199
Net increase (decrease) in cash and cash equivalents									72,508	(59,477)	52,749
Cash and cash equivalents at beginning of year				$ 80,854				$ 140,331	80,854	140,331	87,582
Cash and cash equivalents at end of year	$ 153,362				$ 80,854				$ 153,362	$ 80,854	$ 140,331