Fair Values of Assets and Liabilities (Schedule of Valuation Techniques and Significant Unobservable Inputs Used to Measure Both Recurring and Non-Recurring) (Detail)	Jun. 30, 2020 USD ($)	Jun. 30, 2020 $ / shares	Jun. 30, 2020	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 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	$ 3,194,961,000				$ 3,106,214,000	$ 2,186,154,000
Loans held-for-investment	254,614,000					106,081,000
MSRs	77,203,000			$ 73,504,000	85,638,000	72,850,000	$ 71,022,000	$ 75,183,000
Individually assessed loans - foreclosure probable and collateral-dependent	183,400,000
Municipal Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities, at fair value	150,879,000				145,318,000	142,609,000
U.S. Government agencies
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities, at fair value	293,507,000				365,442,000	170,940,000
Measured at fair value on a recurring basis:
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Loans held-for-investment	254,614,000				132,718,000	106,081,000
MSRs	77,203,000				85,638,000	72,850,000
Derivative assets	327,624,000				103,644,000	108,784,000
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	150,879,000				145,318,000	142,609,000
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	293,507,000				365,442,000	170,940,000
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	140,473,000
Other real estate owned	10,197,000
Level 3 | Measured at fair value on a recurring basis:
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Loans held-for-investment	13,953,000				9,620,000	10,481,000
MSRs	77,203,000				85,638,000	72,850,000
Derivative assets	58,433,000				2,631,000	3,596,000
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	117,255,000				111,950,000	110,296,000
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	2,293,000				$ 2,646,000	$ 2,977,000
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	140,473,000
Other real estate owned	10,197,000
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			0.02
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			0.21
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			0.0280
MSRs, measurement input			0.1021
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			0.0124
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			0.1725
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			0.95
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			0.1725
MSRs, measurement input			0.1725
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		77	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		442	442
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