Fair Value Disclosures - Financial Assets and Financial Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	$ 4,118,863	$ 3,413,317	$ 3,259,492
Loans held-for-sale	66,370	28,228	22,305
U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	10,122	10,019	10,020
Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	1,950,279	1,288,983	1,157,214
Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	1,571,673	1,608,863	1,596,175
Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	582,220	500,744	491,389
IRLCs [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value	5,037	886	1,525
Forward Mortgage-Backed Securities Trades [Member]
Available-for-sale investment securities:
Forward mortgage-backed securities trades	1,099	152	254
Fair Value, Measurements, Recurring [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	4,118,863	3,413,317	3,259,492
Loans held-for-sale	63,293	23,076	18,981
Fair Value, Measurements, Recurring [Member] \| U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	10,122	10,019	10,020
Fair Value, Measurements, Recurring [Member] \| Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	1,950,279	1,288,983	1,157,214
Fair Value, Measurements, Recurring [Member] \| Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	1,571,673	1,608,863	1,596,175
Fair Value, Measurements, Recurring [Member] \| Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	582,220	500,744	491,389
Fair Value, Measurements, Recurring [Member] \| IRLCs [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value	5,037	886	1,525
Fair Value, Measurements, Recurring [Member] \| Forward Mortgage-Backed Securities Trades [Member]
Available-for-sale investment securities:
Forward mortgage-backed securities trades	(1,099)	(152)	(254)
Fair Value, Measurements, Recurring [Member] \| Corporate Bonds [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value		230	228
Fair Value, Measurements, Recurring [Member] \| Other Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	4,569	4,478	4,466
Fair Value, Measurements, Recurring [Member] \| Level 1 Inputs [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	14,691	14,497	14,486
Fair Value, Measurements, Recurring [Member] \| Level 1 Inputs [Member] \| U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	10,122	10,019	10,020
Fair Value, Measurements, Recurring [Member] \| Level 1 Inputs [Member] \| Forward Mortgage-Backed Securities Trades [Member]
Available-for-sale investment securities:
Forward mortgage-backed securities trades	(1,099)	(152)	(254)
Fair Value, Measurements, Recurring [Member] \| Level 1 Inputs [Member] \| Other Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	4,569	4,478	4,466
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	4,104,172	3,398,820	3,245,006
Loans held-for-sale	63,293	23,076	18,981
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member] \| Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	1,950,279	1,288,983	1,157,214
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member] \| Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	1,571,673	1,608,863	1,596,175
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member] \| Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	582,220	500,744	491,389
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member] \| IRLCs [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value	$ 5,037	886	1,525
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member] \| Corporate Bonds [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value		$ 230	$ 228

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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Amount, after valuation allowance, of financing receivable held for sale and not part of disposal group. Excludes loan covered under loss sharing agreement and loan classified as investment in debt security.

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