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, 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
Income Tax Disclosure [Abstract]
Provision at statutory rate									$ 60,975	$ 43,823	$ 59,032
Tax-exempt interest									(10,243)	(9,021)	(15,026)
Section 291/265 interest disallowance									435	321	289
Company-owned life insurance income									(2,423)	(2,223)	(3,029)
Tax-exempt income									(12,231)	(10,923)	(17,766)
State income taxes									6,720	5,621	998
Tax credit investments - federal									(4,411)	(21,576)	(8,500)
Revaluation of deferred tax assets											39,300
Other, net									1,097	905	(125)
Income tax expense	$ 11,433	$ 13,254	$ 14,359	$ 13,104	$ 3,223	$ 5,325	$ 4,345	$ 4,957	$ 52,150	$ 17,850	$ 72,939
Effective tax rate									18.00%	8.60%	43.30%