Fair Value - Schedule of Fair Value Assets and Liabilities Measured on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity securities	$ 6,235	$ 5,582	$ 5,569
Investment securities available-for-sale	4,569,772	4,123,416	3,219,863
Derivative assets	39,952	29,005
Derivative liabilities	11,546	12,550
U.S. Treasury [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	9,777	5,301	9,295
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	698,514	628,151	572,689
Mortgage-Backed Securities - Agency [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	2,560,703	2,209,295	1,477,896
States and Political Subdivisions [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	965,436	940,429	$ 843,488
Pooled Trust Preferred Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	8,123	8,495
Other Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	327,219	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	6,235	5,582
Residential loans held for sale	14,082	14,911
Derivative assets	39,952	29,005
Derivative liabilities	11,546	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	9,777	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	698,514	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	2,560,703	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	965,436	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	8,123	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	327,219	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	6,235	5,582
Residential loans held for sale	0	0
Derivative assets	0	0
Derivative liabilities	0	0
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]
Equity securities	0	0
Residential loans held for sale	14,082	14,911
Derivative assets	39,952	29,005
Derivative liabilities	11,546	12,550
Fair Value on Recurring Basis [Member] | Estimate of Fair Value Measurement [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity securities	0	0
Residential loans held for sale	0	0
Derivative assets	0	0
Derivative liabilities	0	0
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	9,777	5,301
Fair Value on Recurring Basis [Member] | Estimate of Fair Value Measurement [Member] | U.S. Treasury [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	0	0
Fair Value on Recurring Basis [Member] | Estimate of Fair Value Measurement [Member] | U.S. Treasury [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	0	0
Fair Value on Recurring Basis [Member] | Estimate of Fair Value Measurement [Member] | U.S. Government-Sponsored Entities and Agencies [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	0	0
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	698,514	628,151
Fair Value on Recurring Basis [Member] | Estimate of Fair Value Measurement [Member] | U.S. Government-Sponsored Entities and Agencies [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	0	0
Fair Value on Recurring Basis [Member] | Estimate of Fair Value Measurement [Member] | Mortgage-Backed Securities - Agency [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	0	0
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	2,560,703	2,209,295
Fair Value on Recurring Basis [Member] | Estimate of Fair Value Measurement [Member] | Mortgage-Backed Securities - Agency [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available-for-sale	0	0
Fair Value on Recurring Basis [Member] | Estimate of Fair Value Measurement [Member] | States and Political Subdivisions [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	0	0
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	965,396	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] | 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	0	0
Fair Value on Recurring Basis [Member] | Estimate of Fair Value Measurement [Member] | Pooled Trust Preferred 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	0	0
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	8,123	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	30,640	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	296,579	301,486
Fair Value on Recurring Basis [Member] | Estimate of Fair Value Measurement [Member] | Other 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	$ 0	$ 0