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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Provision at statutory rate									$ 43,823	$ 59,032	$ 70,149
Tax-exempt interest									(9,021)	(15,026)	(14,356)
Section 291/265 interest disallowance									321	289	191
Company-owned life insurance income									(2,223)	(3,029)	(2,968)
Tax-exempt income									(10,923)	(17,766)	(17,133)
State income taxes									5,621	998	3,461
Tax credit investments - federal									(21,576)	(8,500)	(321)
Revaluation of deferred tax assets					$ 39,300					39,300
ONB Insurance Group, Inc. nondeductible goodwill											8,328
Other, net									905	(125)	1,678
Income tax expense	$ 3,223	$ 5,325	$ 4,345	$ 4,957	$ 40,405	$ 11,459	$ 10,584	$ 10,491	$ 17,850	$ 72,939	$ 66,162
Effective tax rate									8.60%	43.30%	33.00%