Condensed Financial Information of Parent Company (Schedule of Statements of Cash Flows) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net income	$ 5,331	$ 6,863	$ 6,513	$ 6,307	$ 6,424	$ 6,222	$ 5,869	$ 5,833	$ 25,014	$ 24,348	$ 23,056
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Net cash (used in) provided by operating activities									38,890	35,189	33,493
Cash flows from investing activities:
Net increase in securities									(124,777)	(96,290)	(161,279)
Net decrease in loans									(21,335)	(5,771)	10,882
Purchases of premises and equipment, net									(4,127)	(4,684)	(5,747)
Net cash (used in) provided by investing activities									(139,839)	(41,551)	(3,636)
Cash flows from financing activities:
Net increase (decrease) in accounts and drafts payable									19,601	65,028	(78,169)
Cash dividends paid									(10,675)	(9,979)	(9,697)
Purchase of common shares of treasury									(2,270)	(9,215)	(10,951)
Other financing activities									(467)	(1,378)	(488)
Net cash provided by (used in) financing activities									62,316	19,933	(71,020)
Net increase (decrease) in cash and cash equivalents									(38,633)	13,571	(41,163)
Cash and cash equivalents at beginning of year				266,743				253,172	266,743	253,172	294,335
Cash and cash equivalents at end of year	228,110				266,743				228,110	266,743	253,172
Parent Company [Member]
Cash flows from operating activities:
Net income									25,014	24,348	23,056
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Equity in undistributed income of subsidiaries									(11,145)	(10,059)	(7,889)
Net change in other assets									(41,013)	(7,085)	16,100
Net change in other liabilities									10,118	6,683	(2,779)
Amortization of stock-based awards									1,743	1,677	1,504
Other, net									9,219	7,558	10,389
Net cash (used in) provided by operating activities									(6,064)	23,122	40,381
Cash flows from investing activities:
Net increase in securities									(80,621)	(33,025)	(23,472)
Net decrease in loans									34,944	40,431	28,343
Purchases of premises and equipment, net									(4,020)	(4,557)	(5,708)
Net cash (used in) provided by investing activities									(49,697)	2,849	(837)
Cash flows from financing activities:
Net increase (decrease) in accounts and drafts payable									20,397	64,026	(78,439)
Cash dividends paid									(10,675)	(9,979)	(9,697)
Purchase of common shares of treasury									(2,270)	(9,215)	(10,951)
Other financing activities									(267)	1,705	66
Net cash provided by (used in) financing activities									7,185	46,537	(99,021)
Net increase (decrease) in cash and cash equivalents									(48,576)	72,508	(59,477)
Cash and cash equivalents at beginning of year				$ 153,362				$ 80,854	153,362	80,854	140,331
Cash and cash equivalents at end of year	$ 104,786				$ 153,362				$ 104,786	$ 153,362	$ 80,854