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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Provision at statutory rate of 35%									$ 70,149	$ 57,013	$ 49,688
Tax-exempt interest									(14,356)	(13,111)	(11,460)
Section 291/265 interest disallowance									191	142	93
Company-owned life insurance income									(2,968)	(3,011)	(2,423)
Tax-exempt income									(17,133)	(15,980)	(13,790)
Reserve for unrecognized tax benefits									(1)	(5)	(1,076)
State income taxes									3,461	4,173	2,676
ONB Insurance Group, Inc. nondeductible goodwill									8,328
Effect of Illinois branch sale										1,835
State statutory rate change											904
Other, net									1,358	(859)	(105)
Income tax expense	$ 14,710	$ 10,969	$ 30,812	$ 9,671	$ 11,598	$ 16,395	$ 8,959	$ 9,225	$ 66,162	$ 46,177	$ 38,297
Effective tax rate									33.00%	28.30%	27.00%