Condensed Financial Information - Parent Company Only - Schedule of Condensed Cash Flow Statement (Detail) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
	Dec. 06, 2017	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		$ 10,264	$ 11,196	$ 11,778	$ 10,856	$ 6,918	$ 58	$ 5,240	$ 7,289	$ 44,094	$ 19,505	$ 27,268
Adjustments to reconcile net income to net cash provided by operating activities:
Net gain on sales of investment securities										(48)	(518)	(1,265)
Bank owned life insurance income										(3,988)	(2,931)	(1,295)
Depreciation of premises and equipment										5,561	4,089	3,757
Stock-based compensation										3,166	2,084	1,421
Contributions to pension and other postretirement benefit plans										(2,295)	(2,261)	(2,271)
(Increase) decrease in other assets										(1,369)	1,956	3,055
Increase in accrued interest payable and other liabilities										215	2,160	1,442
Net cash provided by operating activities										68,660	33,306	39,343
Cash flows from investing activities:
Outlays for business acquisitions										0	(94,835)	(2,967)
Proceeds from bank owned life insurance										2,961	662	0
Net cash used in investing activities										(352,107)	(294,557)	(190,042)
Cash flows from financing activities:
Proceeds from issuance of subordinated notes										0	44,515	49,267
Purchases of treasury stock										(3,519)	(8,359)	(13,342)
Proceeds from public offering of common stock	$ 70,500									70,501	0	0
Stock issued under dividend reinvestment and employee stock purchase plans										2,413	2,472	2,434
Cash dividends paid										(21,299)	(17,024)	(15,011)
Net cash provided by financing activities										281,411	277,652	151,060
Net increase (decrease) in cash and due from financial institutions										(2,036)	16,401	361
Cash and due from banks at beginning of year					48,757				32,356	48,757	32,356	31,995
Cash and due from banks at end of period		46,721				48,757				46,721	48,757	32,356
Cash paid during the year for:
Interest										21,493	13,982	8,099
Income tax, net of refunds received										12,599	8,053	2,142
Parent Company [Member]
Cash flows from operating activities:
Net income										44,094	19,505	27,268
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in undistributed net (income) loss of subsidiaries										(23,541)	67,056	(3,137)
Net gain on sales of investment securities										(3)	(23)	(285)
Bank owned life insurance income										(343)	(182)	(5)
Depreciation of premises and equipment										387	339	275
Stock-based compensation										3,166	2,084	1,421
Contributions to pension and other postretirement benefit plans										(2,295)	(2,261)	(2,271)
(Increase) decrease in other assets										(3,384)	1,098	(4,268)
Increase in accrued interest payable and other liabilities										4,101	213	2,027
Net cash provided by operating activities										22,182	87,829	21,025
Cash flows from investing activities:
Investments in subsidiaries										0	(40,000)	(30,000)
Proceeds from sales of securities										3	38	708
Outlays for business acquisitions										0	(87,683)	0
Proceeds from bank owned life insurance										183	0	0
Other, net										(364)	(1,619)	(1,640)
Net cash used in investing activities										(178)	(129,264)	(30,932)
Cash flows from financing activities:
Net decrease in short-term borrowings										0	(253)	0
Proceeds from issuance of subordinated notes										0	44,515	49,267
Purchases of treasury stock										(3,519)	(8,359)	(13,342)
Proceeds from public offering of common stock										70,501	0	0
Stock issued under dividend reinvestment and employee stock purchase plans										2,413	2,472	2,434
Proceeds from exercise of stock options, including excess tax benefits										1,676	4,968	534
Cash dividends paid										(21,299)	(17,024)	(15,010)
Net cash provided by financing activities										49,772	26,319	23,883
Net increase (decrease) in cash and due from financial institutions										71,776	(15,116)	13,976
Cash and due from banks at beginning of year					$ 1,980				$ 17,096	1,980	17,096	3,120
Cash and due from banks at end of period		$ 73,756				$ 1,980				73,756	1,980	17,096
Cash paid during the year for:
Interest										4,800	3,956	1,275
Income tax, net of refunds received										$ 11,600	$ 6,675	$ 1,770