Fair Values of Assets and Liabilities (Narrative) (Detail) $ in Thousands	Mar. 31, 2021 USD ($)	Mar. 31, 2021	Mar. 31, 2021 $ / shares	Dec. 31, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	$ 2,430,749			$ 3,055,839	$ 3,570,959
Loans held-for-investment	51,916				142,267
MSRs	124,316			92,081	73,504	$ 85,638
Individually assessed loans - foreclosure probable and collateral-dependent	124,404				188,740
Estimate of Fair Value Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	2,430,749			3,055,839	3,570,959
Remaining contractual principal balance outstanding, mortgage loans held-for-sale	1,200,000			1,200,000	618,700
Portion at Other than Fair Value Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Individually assessed loans - foreclosure probable and collateral-dependent	36,600
Non-performing
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage loans held-for-sale, at fair value	115,200			134,100	358
Appraisal adjustment - cost of sale | Other real estate owned
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other real estate owned, measurement input		0.10
Measured at fair value on a recurring basis:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-investment	51,916			55,134	142,267
MSRs	124,316			92,081	73,504
Mortgage loans held-for-sale, at fair value	1,260,193			1,272,090	656,934
Measured at fair value on a non-recurring basis:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Individually assessed loans - foreclosure probable and collateral-dependent	87,757
Other real estate owned	15,813
Level 3 | Measured at fair value on a recurring basis:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-investment	6,408			10,280	9,568
MSRs	124,316			92,081	73,504
Derivative assets	25,600
Mortgage loans held-for-sale, at fair value	0			0	0
Level 3 | Measured at fair value on a recurring basis: | Discount rate | Weighted Average of Inputs | Discounted cash flows
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
MSRs, measurement input		0.0988
Level 3 | Measured at fair value on a recurring basis: | Discount rate | Weighted Average of Inputs | Loans held-for-investment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rate		0.0238
Level 3 | Measured at fair value on a recurring basis: | Discount rate | Weighted Average of Inputs | Mortgage servicing rights
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rate		0.0988
Level 3 | Measured at fair value on a recurring basis: | Discount rate | Minimum | Discounted cash flows
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
MSRs, measurement input		0.03
Level 3 | Measured at fair value on a recurring basis: | Discount rate | Minimum | Loans held-for-investment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rate		0.02
Level 3 | Measured at fair value on a recurring basis: | Discount rate | Minimum | Mortgage servicing rights
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rate		0.03
Level 3 | Measured at fair value on a recurring basis: | Discount rate | Maximum | Discounted cash flows
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
MSRs, measurement input		0.19
Level 3 | Measured at fair value on a recurring basis: | Discount rate | Maximum | Loans held-for-investment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rate		0.03
Level 3 | Measured at fair value on a recurring basis: | Discount rate | Maximum | Mortgage servicing rights
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rate		0.19
Level 3 | Measured at fair value on a recurring basis: | Credit discount | Weighted Average of Inputs | Loans held-for-investment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rate		0.0026
Level 3 | Measured at fair value on a recurring basis: | Credit discount | Minimum | Loans held-for-investment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rate		0
Level 3 | Measured at fair value on a recurring basis: | Credit discount | Maximum | Loans held-for-investment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rate		0.03
Level 3 | Measured at fair value on a recurring basis: | Constant prepayment rate (CPR) | Loans held-for-investment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rate		0.1299
Level 3 | Measured at fair value on a recurring basis: | Constant prepayment rate (CPR) | Weighted Average of Inputs | Discounted cash flows
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
MSRs, measurement input		0.1299
Level 3 | Measured at fair value on a recurring basis: | Constant prepayment rate (CPR) | Weighted Average of Inputs | Mortgage servicing rights
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rate		0.1299
Level 3 | Measured at fair value on a recurring basis: | Constant prepayment rate (CPR) | Minimum | Discounted cash flows
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
MSRs, measurement input		0.06
Level 3 | Measured at fair value on a recurring basis: | Constant prepayment rate (CPR) | Minimum | Mortgage servicing rights
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rate		0.06
Level 3 | Measured at fair value on a recurring basis: | Constant prepayment rate (CPR) | Maximum | Discounted cash flows
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
MSRs, measurement input		0.92
Level 3 | Measured at fair value on a recurring basis: | Constant prepayment rate (CPR) | Maximum | Mortgage servicing rights
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rate		0.92
Level 3 | Measured at fair value on a recurring basis: | Cost of servicing | Weighted Average of Inputs | Discounted cash flows
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
MSRs, measurement input		76	76
Level 3 | Measured at fair value on a recurring basis: | Cost of servicing | Minimum | Discounted cash flows
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
MSRs, measurement input | $ / shares			70
Level 3 | Measured at fair value on a recurring basis: | Cost of servicing | Maximum | Discounted cash flows
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
MSRs, measurement input | $ / shares			200
Level 3 | Measured at fair value on a recurring basis: | Cost of servicing - delinquent | Weighted Average of Inputs | Discounted cash flows
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
MSRs, measurement input		354	354
Level 3 | Measured at fair value on a recurring basis: | Cost of servicing - delinquent | Minimum | Discounted cash flows
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
MSRs, measurement input | $ / shares			200
Level 3 | Measured at fair value on a recurring basis: | Cost of servicing - delinquent | Maximum | Discounted cash flows
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
MSRs, measurement input | $ / shares			1,000
Level 3 | Measured at fair value on a recurring basis: | Pull-through rate | Weighted Average of Inputs | Discounted cash flows
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives, measurement input		0.8639
Level 3 | Measured at fair value on a recurring basis: | Pull-through rate | Minimum | Discounted cash flows
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives, measurement input		0.14
Level 3 | Measured at fair value on a recurring basis: | Pull-through rate | Maximum | Discounted cash flows
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives, measurement input		1
Level 3 | Measured at fair value on a non-recurring basis:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Individually assessed loans - foreclosure probable and collateral-dependent	87,757
Other real estate owned	15,813
Municipal Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	148,300			146,910	146,609
Municipal Securities | Measured at fair value on a recurring basis:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	148,300			146,910	146,609
Municipal Securities | Level 3 | Measured at fair value on a recurring basis:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	101,748			109,876	113,267
U.S. Government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	81,526			84,513	368,643
U.S. Government agencies | Measured at fair value on a recurring basis:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	81,526			84,513	368,643
U.S. Government agencies | Level 3 | Measured at fair value on a recurring basis:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	$ 1,784			$ 1,966	$ 2,457