Fair Values of Assets and Liabilities (Schedule of Valuation Techniques and Significant Unobservable Inputs Used to Measure Both Recurring and Non-Recurring) (Detail)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities, at fair value	$ 3,570,959,000	$ 3,106,214,000	$ 2,185,782,000
Loans held-for-investment	142,267,000		101,071,000
MSRs	73,504,000	85,638,000	71,022,000	$ 75,183,000
Individually assessed loans - foreclosure probable and collateral-dependent	188,700,000
Municipal Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities, at fair value	146,609,000	145,318,000	136,353,000
U.S. Government agencies
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities, at fair value	368,643,000	365,442,000	130,687,000
Measured at fair value on a recurring basis:
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Loans held-for-investment	142,267,000	132,718,000	101,071,000
MSRs	73,504,000	85,638,000	71,022,000
Derivative assets	301,368,000	103,644,000	75,504,000
Measured at fair value on a recurring basis: | Municipal Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities, at fair value	146,609,000	145,318,000	136,353,000
Measured at fair value on a recurring basis: | U.S. Government agencies
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities, at fair value	368,643,000	365,442,000	130,687,000
Measured at fair value on a non-recurring basis:
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Individually assessed loans - foreclosure probable and collateral-dependent	147,443,000
Other real estate owned	11,026,000
Level 3 | Measured at fair value on a recurring basis:
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Loans held-for-investment	9,568,000	9,620,000	11,249,000
MSRs	73,504,000	85,638,000	71,022,000
Derivative assets	39,816,000	2,631,000	3,089,000
Level 3 | Measured at fair value on a recurring basis: | Municipal Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities, at fair value	113,267,000	111,950,000	103,834,000
Level 3 | Measured at fair value on a recurring basis: | U.S. Government agencies
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities, at fair value	2,457,000	$ 2,646,000	$ 2,993,000
Level 3 | Measured at fair value on a non-recurring basis:
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Individually assessed loans - foreclosure probable and collateral-dependent	147,443,000
Other real estate owned	$ 11,026,000
Discount rate | Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Loans held-for-investment, measurement input	0.03
MSRs, measurement input	0.04
Discount rate | Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Loans held-for-investment, measurement input	0.04
MSRs, measurement input	0.21
Discount rate | Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Weighted Average of Inputs
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Loans held-for-investment, measurement input	0.0340
MSRs, measurement input	0.0998
Credit discount | Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Loans held-for-investment, measurement input	0.00
Credit discount | Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Loans held-for-investment, measurement input	0.06
Credit discount | Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Weighted Average of Inputs
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Loans held-for-investment, measurement input	0.0125
Constant prepayment rate (CPR) | Discounted cash flows | Level 3 | Measured at fair value on a recurring basis:
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Loans held-for-investment, measurement input	0.1569
Constant prepayment rate (CPR) | Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
MSRs, measurement input	0.00
Constant prepayment rate (CPR) | Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
MSRs, measurement input	0.94
Constant prepayment rate (CPR) | Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Weighted Average of Inputs
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Loans held-for-investment, measurement input	0.1569
MSRs, measurement input	0.1569
Cost of servicing | Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
MSRs, measurement input	70
Cost of servicing | Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
MSRs, measurement input	200
Cost of servicing | Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Weighted Average of Inputs
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
MSRs, measurement input	77
Cost of servicing - delinquent | Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Servicing Asset, Measurement Input, Cost	$ 200
Cost of servicing - delinquent | Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Servicing Asset, Measurement Input, Cost	$ 1,000
Cost of servicing - delinquent | Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Weighted Average of Inputs
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
MSRs, measurement input	386
Pull-through rate | Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivatives, measurement input	0.10
Pull-through rate | Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivatives, measurement input	1
Pull-through rate | Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Weighted Average of Inputs
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivatives, measurement input	0.86
Appraisal adjustment - cost of sale | Appraisal value | Level 3 | Measured at fair value on a non-recurring basis:
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Impaired loans—collateral based, measurement input	0.10
Other real estate owned, measurement input	0.10
Appraisal adjustment - cost of sale | Appraisal value | Level 3 | Measured at fair value on a non-recurring basis: | Weighted Average of Inputs
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Impaired loans—collateral based, measurement input	0.1000
Other real estate owned, measurement input	0.1000