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Fair Value - Schedule of Fair Value Assets and Liabilities Measured on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity securities	$ 6,562	$ 5,582	$ 5,828
Investment securities available-for-sale	4,916,979	4,123,416	3,272,456
Derivative assets	70,346	29,005
Derivative liabilities	15,484	12,550
U.S. Treasury [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	5,494	5,301	5,237
U.S. Government-Sponsored Entities and Agencies [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	486,104	628,151	611,696
Mortgage-Backed Securities - Agency [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	3,136,180	2,209,295	1,508,692
States and Political Subdivisions [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	969,475	940,429	$ 792,666
Pooled Trust Preferred Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	7,695	8,495
Other Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	312,031	331,745
Fair Value on Recurring Basis [Member] \| Carrying Value [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity securities	6,562	5,582
Residential loans held for sale	58,285	14,911
Derivative assets	70,346	29,005
Derivative liabilities	15,484	12,550
Fair Value on Recurring Basis [Member] \| Carrying Value [Member] \| U.S. Treasury [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	5,494	5,301
Fair Value on Recurring Basis [Member] \| Carrying Value [Member] \| U.S. Government-Sponsored Entities and Agencies [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	486,104	628,151
Fair Value on Recurring Basis [Member] \| Carrying Value [Member] \| Mortgage-Backed Securities - Agency [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	3,136,180	2,209,295
Fair Value on Recurring Basis [Member] \| Carrying Value [Member] \| States and Political Subdivisions [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	969,475	940,429
Fair Value on Recurring Basis [Member] \| Carrying Value [Member] \| Pooled Trust Preferred Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	7,695	8,495
Fair Value on Recurring Basis [Member] \| Carrying Value [Member] \| Other Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	312,031	331,745
Fair Value on Recurring Basis [Member] \| Estimate of Fair Value Measurement [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity securities	6,562	5,582
Fair Value on Recurring Basis [Member] \| Estimate of Fair Value Measurement [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Residential loans held for sale	58,285	14,911
Derivative assets	70,346	29,005
Derivative liabilities	15,484	12,550
Fair Value on Recurring Basis [Member] \| Estimate of Fair Value Measurement [Member] \| U.S. Treasury [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	5,494	5,301
Fair Value on Recurring Basis [Member] \| Estimate of Fair Value Measurement [Member] \| U.S. Government-Sponsored Entities and Agencies [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	486,104	628,151
Fair Value on Recurring Basis [Member] \| Estimate of Fair Value Measurement [Member] \| Mortgage-Backed Securities - Agency [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	3,136,180	2,209,295
Fair Value on Recurring Basis [Member] \| Estimate of Fair Value Measurement [Member] \| States and Political Subdivisions [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	969,435	936,321
Fair Value on Recurring Basis [Member] \| Estimate of Fair Value Measurement [Member] \| States and Political Subdivisions [Member] \| Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	40	4,108
Fair Value on Recurring Basis [Member] \| Estimate of Fair Value Measurement [Member] \| Pooled Trust Preferred Securities [Member] \| Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	7,695	8,495
Fair Value on Recurring Basis [Member] \| Estimate of Fair Value Measurement [Member] \| Other Securities [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	31,317	30,259
Fair Value on Recurring Basis [Member] \| Estimate of Fair Value Measurement [Member] \| Other Securities [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	$ 280,714	$ 301,486