Income Taxes - Reconciliation Between Income Tax Expense and Amount Computed by Applying Federal Statutory Tax Rate from Continuing Operations (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
Income Tax Disclosure [Abstract]
Statutory federal income tax expense									$ 71,167	$ 60,053	$ 46,955
Tax-exempt interest income									(20,914)	(18,725)	(15,525)
Tax-exempt life insurance related income									(3,420)	(4,007)	(1,744)
Meals, entertainment and related expenses									924	1,647	1,547
State and local income taxes, net of federal tax benefits									5,835	4,273	2,767
Impacts related to the 2017 Tax Act											(4,974)
Equity-based compensation									(299)	(597)	(2,364)
Federal tax credits, net of amortization of LIHTC investments									(1,772)	(980)	(1,135)
Other									867	701	1,807
Total tax expense	$ 34,787	$ 10,293	$ 8,123	$ (815)	$ 10,753	$ 10,616	$ 10,466	$ 10,530	$ 52,388	$ 42,365	$ 27,334