Condensed Financial Information - Parent Company Only - Schedule of Condensed Cash Flow Statement (Detail) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net income		$ 25,874	$ 18,119	$ 2,085	$ 838	$ 15,510	$ 17,662	$ 16,468	$ 16,079	$ 46,916	$ 65,719	$ 50,543
Adjustments to reconcile net income to net cash provided by operating activities:
Bank owned life insurance income	[1]									(2,940)	(3,179)	(3,174)
Depreciation of premises and equipment										4,780	5,277	5,581
Stock-based compensation										1,480	2,348	2,557
Contributions to pension and other postretirement benefit plans										(270)	(266)	(3,264)
Decrease in other assets										(2,835)	(2,332)	4,547
Increase in accrued interest payable and other liabilities										8,608	1,409	6,540
Net cash provided by operating activities										51,179	73,148	86,006
Cash flows from investing activities:
Net cash used in investing activities										(867,392)	(343,369)	(431,318)
Cash flows from financing activities:
Proceeds from issuance of subordinated notes										100,000	0	0
Purchases of treasury stock										(4,452)	(2,045)	(5,984)
Stock issued under dividend reinvestment and employee stock purchase plans										2,369	2,233	2,295
Proceeds from exercise of stock options										384	1,203	1,131
Cash dividends paid										(17,536)	(23,435)	(23,495)
Net cash provided by financing activities										910,943	285,929	379,323
Cash and cash equivalents at beginning of year					125,128				109,420	125,128	109,420	75,409
Cash and cash equivalents at end of year		219,858				125,128				219,858	125,128	109,420
Cash paid during the year for:
Interest										29,233	44,479	30,875
Income tax, net of refunds received										17,316	17,171	2,022
Parent Company [Member]
Cash flows from operating activities:
Net income										46,916	65,719	50,543
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in undistributed net income of subsidiaries										(43,426)	(40,810)	(32,144)
Bank owned life insurance income										(108)	(331)	109
Depreciation of premises and equipment										320	328	386
Stock-based compensation										1,480	2,348	2,557
Contributions to pension and other postretirement benefit plans										(270)	(266)	(3,264)
Decrease in other assets										2,321	554	14,205
Increase in accrued interest payable and other liabilities										(4,631)	(1,319)	(865)
Net cash provided by operating activities										2,602	26,223	31,527
Cash flows from investing activities:
Other, net										(32)	(371)	(188)
Net cash used in investing activities										(32)	(371)	(188)
Cash flows from financing activities:
Proceeds from issuance of subordinated notes										98,448	0	0
Repayments of Subordinated Debt										(10,000)	0	0
Purchases of treasury stock										(4,452)	(2,045)	(5,984)
Stock issued under dividend reinvestment and employee stock purchase plans										2,369	2,233	2,295
Proceeds from exercise of stock options										384	1,203	1,131
Cash dividends paid										(17,536)	(23,435)	(23,495)
Net cash provided by financing activities										69,213	(22,044)	(26,053)
Net increase in cash and cash equivalents										71,783	3,808	5,286
Cash and cash equivalents at beginning of year					$ 82,850				$ 79,042	82,850	79,042	73,756
Cash and cash equivalents at end of year		$ 154,633				$ 82,850				154,633	82,850	79,042
Cash paid during the year for:
Interest										5,121	4,800	4,800
Income tax, net of refunds received										$ 16,172	$ 16,460	$ 643

[1] Net interest income as well as many other revenues for financial assets and liabilities including loans, leases, securities, and derivatives are excluded from the scope of FASB ASC 606 "Revenue from Contracts with Customers" (FASB ASC 606). Noninterest income streams that are out of scope of FASB ASC 606 include bank owned life insurance income, sales of investment securities and mortgage banking activities.