Fair Values of Assets and Liabilities (Schedule of Valuation Techniques and Significant Unobservable Inputs Used to Measure Both Recurring and Non-Recurring) (Detail)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities, at fair value	$ 2,270,059,000		$ 2,126,081,000	$ 2,164,985,000
Loans held-for-investment	116,022,000			77,883,000
MSRs	75,585,000	$ 72,850,000	75,183,000	74,530,000	$ 63,194,000	$ 33,676,000
Impaired loans—collateral based	130,823,000		127,269,000	132,522,000
Municipal Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities, at fair value	146,104,000		138,490,000	133,373,000
U.S. Government agencies
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities, at fair value	225,459,000		140,307,000	111,020,000
Measured at fair value on a recurring basis:
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Loans held-for-investment	116,022,000		93,857,000	77,883,000
MSRs	75,585,000		75,183,000	74,530,000
Derivative assets	138,286,000		73,172,000	96,153,000
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	146,104,000		138,490,000	133,373,000
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	225,459,000		140,307,000	111,020,000
Measured at fair value on a non-recurring basis:
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Impaired loans—collateral based	91,103,000
Other real estate owned	17,482,000
Level 3 | Measured at fair value on a recurring basis:
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Loans held-for-investment	9,999,000		11,347,000	13,456,000
MSRs	75,585,000		75,183,000	74,530,000
Derivative assets	2,979,000		2,457,000	2,492,000
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	113,058,000		108,926,000	97,586,000
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	2,811,000		$ 3,150,000	$ 3,315,000
Level 3 | Measured at fair value on a non-recurring basis:
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Impaired loans—collateral based	91,103,000
Other real estate owned	$ 17,482,000
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	0.09
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	0.19
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	0.0336
MSRs, measurement input	0.0996
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	0.07
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	0.0135
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	0.1503
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	0.93
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	0.1503
MSRs, measurement input	0.1503
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	399
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	0.75
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