Condensed Financial Information - Parent Company Only - Schedule of Condensed Cash Flow Statement (Detail) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net income		$ 15,510	$ 17,662	$ 16,468	$ 16,079	$ 18,372	$ 14,964	$ 4,357	$ 12,850	$ 65,719	$ 50,543	$ 44,094
Adjustments to reconcile net income to net cash provided by operating activities:
Net gain on sales of investment securities	[1]									(54)	(10)	(48)
Bank owned life insurance income	[1]									(3,179)	(3,174)	(3,988)
Depreciation of premises and equipment										5,277	5,581	5,561
Stock-based compensation										2,348	2,557	3,166
Contributions to pension and other postretirement benefit plans										(266)	(3,264)	(2,295)
Decrease (increase) in other assets										(2,332)	4,547	(1,369)
Increase in accrued interest payable and other liabilities										1,409	6,540	215
Net cash provided by operating activities										73,148	86,006	68,660
Cash flows from investing activities:
Proceeds from bank owned life insurance										0	1,384	2,961
Net cash used in investing activities										(343,369)	(431,318)	(332,487)
Cash flows from financing activities:
Purchases of treasury stock										(2,045)	(5,984)	(3,519)
Proceeds from public offering of common stock										0	0	70,501
Stock issued under dividend reinvestment and employee stock purchase plans										2,233	2,295	2,413
Proceeds from exercise of stock options										1,203	1,131	1,676
Cash dividends paid										(23,435)	(23,495)	(21,299)
Net cash provided by financing activities										285,929	379,323	281,411
Cash and cash equivalents at beginning of year					109,420				75,409	109,420	75,409	57,825
Cash and cash equivalents at end of year		125,128				109,420				125,128	109,420	75,409
Cash paid during the year for:
Interest										44,479	30,875	21,493
Income tax, net of refunds received										17,171	2,022	12,599
Parent Company [Member]
Cash flows from operating activities:
Net income										65,719	50,543	44,094
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in undistributed net income of subsidiaries										(40,810)	(32,144)	(23,541)
Net gain on sales of investment securities										0	0	(3)
Bank owned life insurance income										(331)	109	(343)
Depreciation of premises and equipment										328	386	387
Stock-based compensation										2,348	2,557	3,166
Contributions to pension and other postretirement benefit plans										(266)	(3,264)	(2,295)
Decrease (increase) in other assets										554	14,205	(3,384)
Increase in accrued interest payable and other liabilities										(1,319)	(865)	4,101
Net cash provided by operating activities										26,223	31,527	22,182
Cash flows from investing activities:
Proceeds from sales of securities										0	0	3
Proceeds from bank owned life insurance										0	0	183
Other, net										(371)	(188)	(364)
Net cash used in investing activities										(371)	(188)	(178)
Cash flows from financing activities:
Purchases of treasury stock										(2,045)	(5,984)	(3,519)
Proceeds from public offering of common stock										0	0	70,501
Stock issued under dividend reinvestment and employee stock purchase plans										2,233	2,295	2,413
Proceeds from exercise of stock options										1,203	1,131	1,676
Cash dividends paid										(23,435)	(23,495)	(21,299)
Net cash provided by financing activities										(22,044)	(26,053)	49,772
Net increase in cash and due from financial institutions										3,808	5,286	71,776
Cash and cash equivalents at beginning of year					$ 79,042				$ 73,756	79,042	73,756	1,980
Cash and cash equivalents at end of year		$ 82,850				$ 79,042				82,850	79,042	73,756
Cash paid during the year for:
Interest										4,800	4,800	4,800
Income tax, net of refunds received										$ 16,460	$ 643	$ 11,600

[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.