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, 2021 USD ($)	Sep. 30, 2021	Sep. 30, 2021 $ / shares	Jun. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities, at fair value	$ 2,373,478				$ 3,055,839	$ 2,946,459
Loans held-for-investment	30,979					253,515
MSRs	133,552			$ 127,604	92,081	86,907	$ 77,203	$ 85,638
Individually assessed loans - foreclosure probable and collateral-dependent	109,011					183,667
Municipal Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities, at fair value	174,600				146,910	154,195
Measured at fair value on a recurring basis:
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Loans held-for-investment	30,979				55,134	253,515
MSRs	133,552				92,081	86,907
Derivative assets	191,612				278,423	321,488
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	174,600				146,910	154,195
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	78,653
Other real estate owned	13,845
Level 3 | Measured at fair value on a recurring basis:
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Loans held-for-investment	12,132				10,280	12,613
MSRs	133,552				92,081	86,907
Derivative assets	14,612				48,091	66,887
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	117,724				$ 109,876	$ 119,471
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	78,653
Other real estate owned	$ 13,845
Discount rate | 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		2.88
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]
MSRs, measurement input		0.07
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]
MSRs, measurement input		0.19
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		0.0288
MSRs, measurement input		0.0982
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
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		0.03
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		0.0051
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		0.1277
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		0.92
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		0.1277
MSRs, measurement input		0.1277
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		75	75
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		287	287
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		0.01
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		0.8628
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