Fair Value of Assets and Liabilities - Summary Of The Balances Of Assets And Liabilities Measured At Fair Value On A Recurring Basis (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Assets
Available for sale securities	$ 4,848,781	$ 3,183,861
Government-sponsored residential mortgage-backed securities
Assets
Available for sale securities	3,478,529	2,148,800
Government-sponsored commercial mortgage-backed securities
Assets
Available for sale securities	181,795	17,081
U.S. Agency bonds
Assets
Available for sale securities	841,005	666,709
U.S. Treasury securities
Assets
Available for sale securities	69,331	70,369
Fair Value, Recurring
Assets
Rabbi trust investments	98,900	91,683
Loans held for sale	2,734	1,140
Customer-related positions	92,980	141,822
Risk participation agreements	432	722
Matched customer book	94	90
Foreign currency loan	48	9
Total	5,043,969	3,419,327
Liabilities
Customer-related positions	28,498	42,600
Risk participation agreements	675	1,230
Matched customer book	89	77
Foreign currency loan	0	69
Total	29,262	43,976
Fair Value, Recurring \| Government-sponsored residential mortgage-backed securities
Assets
Available for sale securities	3,478,529	2,148,800
Fair Value, Recurring \| Government-sponsored commercial mortgage-backed securities
Assets
Available for sale securities	181,795	17,081
Fair Value, Recurring \| U.S. Agency bonds
Assets
Available for sale securities	841,005	666,709
Fair Value, Recurring \| U.S. Treasury securities
Assets
Available for sale securities	69,331	70,369
Fair Value, Recurring \| State and municipal bonds and obligations
Assets
Available for sale securities	278,121	280,902
Fair Value, Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Rabbi trust investments	91,187	83,884
Loans held for sale	0	0
Customer-related positions	0	0
Risk participation agreements	0	0
Matched customer book	0	0
Foreign currency loan	0	0
Total	160,518	154,253
Liabilities
Customer-related positions	0	0
Risk participation agreements	0	0
Matched customer book	0	0
Foreign currency loan	0	0
Total	0	0
Fair Value, Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Government-sponsored residential mortgage-backed securities
Assets
Available for sale securities	0	0
Fair Value, Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Government-sponsored commercial mortgage-backed securities
Assets
Available for sale securities	0	0
Fair Value, Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| U.S. Agency bonds
Assets
Available for sale securities	0	0
Fair Value, Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| U.S. Treasury securities
Assets
Available for sale securities	69,331	70,369
Fair Value, Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| State and municipal bonds and obligations
Assets
Available for sale securities	0	0
Fair Value, Recurring \| Significant Other Observable Inputs (Level 2)
Assets
Rabbi trust investments	7,713	7,799
Loans held for sale	2,734	1,140
Customer-related positions	92,980	141,822
Risk participation agreements	432	722
Matched customer book	94	90
Foreign currency loan	48	9
Total	4,883,451	3,265,074
Liabilities
Customer-related positions	28,498	42,600
Risk participation agreements	675	1,230
Matched customer book	89	77
Foreign currency loan	0	69
Total	29,262	43,976
Fair Value, Recurring \| Significant Other Observable Inputs (Level 2) \| Government-sponsored residential mortgage-backed securities
Assets
Available for sale securities	3,478,529	2,148,800
Fair Value, Recurring \| Significant Other Observable Inputs (Level 2) \| Government-sponsored commercial mortgage-backed securities
Assets
Available for sale securities	181,795	17,081
Fair Value, Recurring \| Significant Other Observable Inputs (Level 2) \| U.S. Agency bonds
Assets
Available for sale securities	841,005	666,709
Fair Value, Recurring \| Significant Other Observable Inputs (Level 2) \| U.S. Treasury securities
Assets
Available for sale securities	0	0
Fair Value, Recurring \| Significant Other Observable Inputs (Level 2) \| State and municipal bonds and obligations
Assets
Available for sale securities	278,121	280,902
Fair Value, Recurring \| Significant Unobservable Inputs (Level 3)
Assets
Rabbi trust investments	0	0
Loans held for sale	0	0
Customer-related positions	0	0
Risk participation agreements	0	0
Matched customer book	0	0
Foreign currency loan	0	0
Total	0	0
Liabilities
Customer-related positions	0	0
Risk participation agreements	0	0
Matched customer book	0	0
Foreign currency loan	0	0
Total	0	0
Fair Value, Recurring \| Significant Unobservable Inputs (Level 3) \| Government-sponsored residential mortgage-backed securities
Assets
Available for sale securities	0	0
Fair Value, Recurring \| Significant Unobservable Inputs (Level 3) \| Government-sponsored commercial mortgage-backed securities
Assets
Available for sale securities	0	0
Fair Value, Recurring \| Significant Unobservable Inputs (Level 3) \| U.S. Agency bonds
Assets
Available for sale securities	0	0
Fair Value, Recurring \| Significant Unobservable Inputs (Level 3) \| U.S. Treasury securities
Assets
Available for sale securities	0	0
Fair Value, Recurring \| Significant Unobservable Inputs (Level 3) \| State and municipal bonds and obligations
Assets
Available for sale securities	$ 0	$ 0

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