Income Taxes - Summary of Differences in Taxes from Continuing Operations Computed at Statutory Rate (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 rate									21.00%
Provision at statutory rate									$ 53,667	$ 60,975	$ 43,823
Tax-exempt interest									(10,776)	(10,243)	(9,021)
Section 291/265 interest disallowance									189	435	321
Company-owned life insurance income									(2,290)	(2,423)	(2,223)
Tax-exempt income									(12,877)	(12,231)	(10,923)
State income taxes									4,840	6,720	5,621
Tax credit investments - federal									(15,159)	(4,411)	(21,576)
Other, net									(1,324)	1,097	905
Income tax expense	$ 4,293	$ 12,154	$ 9,761	$ 2,939	$ 11,433	$ 13,254	$ 14,359	$ 13,104	$ 29,147	$ 52,150	$ 17,850
Effective tax rate									11.40%	18.00%	8.60%