Investment Securities - Additional Information (Detail)	3 Months Ended	12 Months Ended
	Dec. 31, 2019 USD ($) Security	Dec. 31, 2019 USD ($) Security	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Summary Of Investment Holdings [Line Items]
Reclassification of securities	$ 382,000,000.0	$ 382,000,000.0
Increase in capital due to reclassification of held-to-maturity portfolio to available-for-sale portfolio	19,400,000
Unrealized holding gains, net of tax	13,000,000.0
Unrealized holding gains (losses) on available-for-sale debt securities		93,479,000	$ 5,792,000	$ 3,455,000
Securities pledged to secure public and other funds, carrying value	$ 2,104,000,000	2,104,000,000	2,404,000,000
Held-to-Maturity, Unrecognized Losses			17,211,000
OTTI losses on securities		$ 0	0	0
Number of securities in security portfolio | Security	1,892	1,892
Number of securities in unrealized loss position | Security	249	249
Total investment securities - available-for-sale	$ 5,385,091,000	$ 5,385,091,000	4,123,416,000
Unrealized losses	21,660,000	21,660,000	70,573,000
Equity securities recorded at fair value	6,842,000	6,842,000	5,582,000
Gains on equity securities		700,000	100,000	$ 700,000
Impairments on equity securities without readily determinable fair value		0	0
Downward adjustments on equity securities without readily determinable fair value		0	0
Other Asstes [Member]
Summary Of Investment Holdings [Line Items]
Equity securities without readily determinable fair value	$ 91,400,000	$ 91,400,000	79,200,000
Pooled Trust Preferred Securities [Member]
Summary Of Investment Holdings [Line Items]
Number of pooled trust preferred securities | Security	2	2
Total investment securities - available-for-sale	$ 8,222,000	$ 8,222,000	8,495,000
Unrealized losses	5,589,000	5,589,000	5,366,000
OTTI losses on securities		0	0
Held-to-maturity Securities [Member]
Summary Of Investment Holdings [Line Items]
Held-to-Maturity, Unrecognized Losses	0	0	$ 10,700,000
Eastern Indiana [Member]
Summary Of Investment Holdings [Line Items]
Investment securities issued by states and political subdivisions, market value	$ 400,200,000	$ 400,200,000
State and political subdivision investment, equity percentage	14.00%	14.00%
Percentage of municipal bonds rated A or better	99.00%	99.00%
Percentage of non rated local interest bonds	1.00%	1.00%
Texas [Member]
Summary Of Investment Holdings [Line Items]
Investment securities issued by states and political subdivisions, market value	$ 165,700,000	$ 165,700,000
State and political subdivision investment, equity percentage	5.80%	5.80%
Municipal Bond [Member]
Summary Of Investment Holdings [Line Items]
Number of securities sold | Security	6	6
Investment securities held-to-maturity sold	$ 9,700,000
Gain on investment securities held-to-maturity sold	$ 300,000