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Fair Value - Schedule of Fair Value Assets and Liabilities Measured on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	$ 5,828	$ 5,584	$ 5,351
Investment securities available-for-sale	3,272,456	3,196,207	2,775,837
Derivative assets	16,530	14,118
Derivative liabilities	24,304	16,292
U.S. Treasury [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	5,237	5,551	5,615
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	611,696	664,286	576,436
Mortgage-Backed Securities - Agency [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	1,508,692	1,667,682	1,450,736
States and Political Subdivisions [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	792,666	530,193	$ 414,673
Pooled Trust Preferred Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	9,072	8,448
Other Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	345,093	320,047
Fair Value on Recurring Basis [Member] \| Carrying Value [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	5,828	5,584
Residential loans held for sale	21,384	17,930
Derivative assets	16,530	14,118
Derivative liabilities	24,304	16,292
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	5,237	5,551
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	611,696	664,286
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	1,508,692	1,667,682
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	792,666	530,193
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	9,072	8,448
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	345,093	320,047
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]
Trading securities	5,828	5,584
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	21,384	17,930
Derivative assets	16,530	14,118
Derivative liabilities	24,304	16,292
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	5,237	5,551
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	611,696	664,286
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	1,508,692	1,667,682
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	788,687	530,193
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	3,979
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	9,072	8,448
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	30,026	30,965
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	$ 315,067	$ 289,082