Fair Value Disclosures of Financial Assets and Liabilities - Schedule of Financial Instruments Carried at Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	$ 2,529,057	$ 1,944,914
Fair Value, Measurements, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other assets	14,448	4,884
Total assets measured at fair value	2,543,505	1,949,798
Other liabilities	32,257	3,918
Fair Value, Measurements, Nonrecurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total assets measured at fair value	210,143	156,523
Other real estate owned	3,061	2,605
Quoted Prices in Active Markets for Identical Asset (Level 1) | Fair Value, Measurements, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other assets	0	0
Total assets measured at fair value	0	0
Other liabilities	0	0
Quoted Prices in Active Markets for Identical Asset (Level 1) | Fair Value, Measurements, Nonrecurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total assets measured at fair value	0	0
Other real estate owned	0	0
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other assets	14,448	4,884
Total assets measured at fair value	2,543,505	1,949,798
Other liabilities	8,218	3,918
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Nonrecurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total assets measured at fair value	197,541	83,872
Other real estate owned	0	0
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other assets	0	0
Total assets measured at fair value	0	0
Other liabilities	24,039	0
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Nonrecurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total assets measured at fair value	12,602	72,651
Other real estate owned	3,061	2,605
CMO
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	471,325	340,230
CMO | Fair Value, Measurements, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	471,325	340,230
CMO | Quoted Prices in Active Markets for Identical Asset (Level 1) | Fair Value, Measurements, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	0	0
CMO | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	471,325	340,230
CMO | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	0	0
FNMA MBS
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	1,598,970	1,242,453
FNMA MBS | Fair Value, Measurements, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	1,598,970	1,242,453
FNMA MBS | Quoted Prices in Active Markets for Identical Asset (Level 1) | Fair Value, Measurements, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	0	0
FNMA MBS | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	1,598,970	1,242,453
FNMA MBS | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	0	0
FHLMC MBS
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	202,893	328,946
FHLMC MBS | Fair Value, Measurements, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	202,893	328,946
FHLMC MBS | Quoted Prices in Active Markets for Identical Asset (Level 1) | Fair Value, Measurements, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	0	0
FHLMC MBS | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	202,893	328,946
FHLMC MBS | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	0	0
GNMA MBS
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	23,762	33,285
GNMA MBS | Fair Value, Measurements, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	23,762	33,285
GNMA MBS | Quoted Prices in Active Markets for Identical Asset (Level 1) | Fair Value, Measurements, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	0	0
GNMA MBS | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	23,762	33,285
GNMA MBS | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	0	0
GSE agency notes
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	232,107
GSE agency notes | Fair Value, Measurements, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	232,107
GSE agency notes | Quoted Prices in Active Markets for Identical Asset (Level 1) | Fair Value, Measurements, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	0
GSE agency notes | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	232,107
GSE agency notes | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	0
Other equity investments | Fair Value, Measurements, Nonrecurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other investments	9,541	70,046
Other equity investments | Quoted Prices in Active Markets for Identical Asset (Level 1) | Fair Value, Measurements, Nonrecurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other investments	0	0
Other equity investments | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Nonrecurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other investments	0	0
Other equity investments | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Nonrecurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other investments	9,541	70,046
Loans held for sale | Fair Value, Measurements, Nonrecurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Loans, held for sale	197,541	83,872
Loans held for sale | Quoted Prices in Active Markets for Identical Asset (Level 1) | Fair Value, Measurements, Nonrecurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Loans, held for sale	0	0
Loans held for sale | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Nonrecurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Loans, held for sale	197,541	83,872
Loans held for sale | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Nonrecurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Loans, held for sale	$ 0	$ 0