Condensed Financial Information - Parent Company Only - Schedule of Condensed Cash Flow Statement (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash flows from operating activities:
Net income	$ 7,158	$ 7,528	$ 6,466	$ 6,116	$ 5,190	$ 6,235	$ 5,080	$ 5,726	$ 27,268	$ 22,231	$ 21,189
Adjustments to reconcile net income to net cash provided by operating activities:
Net gain on sales of investment securities									(1,265)	(635)	(3,389)
Loss on termination of interest rate swap									0	0	1,866
Bank owned life insurance income									(1,295)	(1,628)	(2,968)
Depreciation of premises and equipment									3,757	3,243	2,927
Stock-based compensation									1,421	1,141	978
Contributions to pension and other postretirement benefit plans									(2,271)	(254)	(2,243)
(Increase) decrease in other assets									(663)	(1,093)	(2,873)
Increase (decrease) in accrued interest payable and other liabilities									1,442	(587)	1,101
Net cash provided by operating activities									35,625	31,867	32,806
Cash flows from investing activities:
Proceeds from bank owned life insurance									0	0	3,540
Net cash (used in) provided by investing activities									(186,324)	(38,439)	21,595
Cash flows from financing activities:
Proceeds from issuance of subordinated notes									49,267	0	0
Payment for repurchase of trust preferred securities									0	0	(20,619)
Purchases of treasury stock									(13,342)	(4,605)	(12,012)
Stock issued under dividend reinvestment and employee stock purchase plans and other employee benefit programs									2,434	2,462	2,422
Cash dividends paid									(15,011)	(12,996)	(10,029)
Net cash provided by (used in) financing activities									151,060	5,921	(120,154)
Net increase (decrease) in cash and due from financial institutions									361	(651)	(65,753)
Cash and due from banks at beginning of year				31,995				32,646	31,995	32,646	98,399
Cash and due from banks at end of period	32,356				31,995				32,356	31,995	32,646
Cash paid during the year for:
Interest									8,099	4,118	5,997
Income tax, net of refunds received									2,142	5,899	5,352
Parent Company [Member]
Cash flows from operating activities:
Net income									27,268	22,231	21,189
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in undistributed net (income) loss of subsidiaries									(3,137)	(8,913)	(3,923)
Net gain on sales of investment securities									(285)	(306)	(644)
Loss on termination of interest rate swap									0	0	1,866
Bank owned life insurance income									(5)	(238)	(1,070)
Depreciation of premises and equipment									275	364	344
Stock-based compensation									1,421	1,141	978
Contributions to pension and other postretirement benefit plans									(2,271)	(254)	(2,243)
(Increase) decrease in other assets									(4,268)	714	671
Increase (decrease) in accrued interest payable and other liabilities									2,027	(639)	(2,426)
Net cash provided by operating activities									21,025	14,100	14,742
Cash flows from investing activities:
Investments in subsidiaries									(30,000)	0	0
Proceeds from sales of securities									708	1,131	1,244
Liquidation of subsidiary, net of cash acquired									0	0	15,011
Proceeds from bank owned life insurance									0	0	772
Other, net									(1,640)	(281)	(713)
Net cash (used in) provided by investing activities									(30,932)	850	16,314
Cash flows from financing activities:
Proceeds from issuance of subordinated notes									49,267	0	0
Repayment of long-term debt									0	0	(375)
Payment for repurchase of trust preferred securities									0	0	(20,619)
Purchases of treasury stock									(13,342)	(4,605)	(12,012)
Stock issued under dividend reinvestment and employee stock purchase plans and other employee benefit programs									2,434	2,462	2,422
Proceeds from exercise of stock options									534	310	0
Cash dividends paid									(15,010)	(12,996)	(10,029)
Net cash provided by (used in) financing activities									23,883	(14,829)	(40,613)
Net increase (decrease) in cash and due from financial institutions									13,976	121	(9,557)
Cash and due from banks at beginning of year				$ 3,120				$ 2,999	3,120	2,999	12,556
Cash and due from banks at end of period	$ 17,096				$ 3,120				17,096	3,120	2,999
Cash paid during the year for:
Interest									1,275	2	754
Income tax, net of refunds received									$ 1,770	$ 5,300	$ 5,017