Fair Values of Assets and Liabilities (Schedule of Valuation Techniques and Significant Unobservable Inputs Used to Measure Both Recurring and Non-Recurring) (Detail) $ in Thousands	Sep. 30, 2020 USD ($)	Sep. 30, 2020 $ / shares	Sep. 30, 2020	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities, at fair value	$ 2,946,459				$ 3,106,214	$ 2,270,059
Loans held-for-investment	253,515					116,022
MSRs	86,907			$ 77,203	85,638	75,585	$ 72,850	$ 75,183
Individually assessed loans - foreclosure probable and collateral-dependent	183,667					131,050
Municipal Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities, at fair value	154,195				145,318	146,104
U.S. Government agencies
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities, at fair value	178,666				365,442	225,459
Measured at fair value on a recurring basis:
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Loans held-for-investment	253,515				132,718	116,022
MSRs	86,907				85,638	75,585
Derivative assets	321,488				103,644	138,286
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	154,195				145,318	146,104
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	178,666				365,442	225,459
Measured at fair value on a non-recurring basis:
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other real estate owned	9,217
Individually assessed loans - foreclosure probable and collateral-dependent	144,151
Level 3 | Measured at fair value on a recurring basis:
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Loans held-for-investment	12,613				9,620	9,999
MSRs	86,907				85,638	75,585
Derivative assets	66,887				2,631	2,979
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	119,471				111,950	113,058
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	2,129				$ 2,646	$ 2,811
Level 3 | Measured at fair value on a non-recurring basis:
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other real estate owned	9,217
Individually assessed loans - foreclosure probable and collateral-dependent	$ 144,151
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			0.02
MSRs, measurement input			0.03
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			0.03
MSRs, measurement input			0.20
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			0.0250
MSRs, measurement input			0.1008
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			0.0114
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			0.1763
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			0.06
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			0.95
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			0.1763
MSRs, measurement input			0.1763
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 | $ / shares		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 | $ / shares		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		76	76
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]
MSRs, measurement input | $ / shares		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]
MSRs, measurement input | $ / shares		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		417	417
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
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			0.79
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