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	9 Months Ended
	Sep. 30, 2018 USD ($) $ / Loan	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale securities	$ 2,164,985		$ 1,803,666	$ 1,665,903
Loans held-for-investment	77,883			29,414
MSRs	74,530	$ 63,194	33,676	29,414	$ 27,307	$ 19,103
Derivative assets	96,153		52,069	48,198
Impaired loans—collateral based	132,522		105,088	78,594
Measured at fair value on a recurring basis:
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Loans held-for-investment	77,883		33,717	29,704
MSRs	74,530		33,676	29,414
Derivative assets	96,153		52,069	48,198
Measured at fair value on a non-recurring basis:
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Impaired loans—collateral based	106,219
Other real estate owned	28,303
Level 3 | Measured at fair value on a recurring basis:
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Loans held-for-investment	13,456		33,717	29,704
MSRs	74,530		33,676	29,414
Derivative assets	$ 2,492		$ 2,157	$ 1,216
Level 3 | Measured at fair value on a recurring basis: | Mortgage servicing rights | Weighted Average of Inputs
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Cost of servicing (in dollars per loan) | $ / Loan	78
Cost of servicing - delinquent (in dollars per loan) | $ / Loan	274
Level 3 | Measured at fair value on a non-recurring basis:
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Impaired loans—collateral based	$ 106,219
Other real estate owned	$ 28,303
Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Loans held-for-investment Discount Rate | Minimum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	0.04
Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Loans held-for-investment Discount Rate | Maximum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	0.05
Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Loans held-for-investment Discount Rate | Weighted Average of Inputs
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	0.0457
Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Loans held-for-investment Credit Spread | Minimum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	0.00
Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Loans held-for-investment Credit Spread | Maximum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	0.07
Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Loans held-for-investment Credit Spread | Weighted Average of Inputs
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	0.0100
Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Mortgage Servicing Rights Discount Rate | Minimum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	0.07
Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Mortgage Servicing Rights Discount Rate | Maximum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	0.18
Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Mortgage Servicing Rights Discount Rate | Weighted Average of Inputs
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	0.1003
Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Mortgage Servicing Rights Prepayment Rate | Weighted Average of Inputs
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	0.1003
Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Mortgage Servicing Rights Cost of servicing | Minimum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Cost of servicing (in dollars per loan) | $ / Loan	15
Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Mortgage Servicing Rights Cost of servicing | Maximum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Cost of servicing (in dollars per loan) | $ / Loan	200
Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Mortgage Servicing Rights Cost of servicing | Weighted Average of Inputs
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Cost of servicing (in dollars per loan) | $ / Loan	78
Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Mortgage Servicing Rights Cost of servicing - delinquent | Minimum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Cost of servicing - delinquent (in dollars per loan) | $ / Loan	200
Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Mortgage Servicing Rights Cost of servicing - delinquent | Maximum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Cost of servicing - delinquent (in dollars per loan) | $ / Loan	1,000
Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Mortgage Servicing Rights Cost of servicing - delinquent | Weighted Average of Inputs
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Cost of servicing - delinquent (in dollars per loan) | $ / Loan	274
Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Derivatives | Minimum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Pull-through rate	20.00%
Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Derivatives | Maximum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Pull-through rate	100.00%
Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Derivatives | Weighted Average of Inputs
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Pull-through rate	91.12%
Appraisal value | Level 3 | Measured at fair value on a non-recurring basis: | Impaired loans—collateral based
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other Real Estate Owned, Measurement Input	0.10
Appraisal value | Level 3 | Measured at fair value on a non-recurring basis: | Impaired loans—collateral based | Weighted Average of Inputs
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other Real Estate Owned, Measurement Input	0.1000
Appraisal value | Level 3 | Measured at fair value on a non-recurring basis: | Other real estate owned
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other Real Estate Owned, Measurement Input	0.10
Appraisal value | Level 3 | Measured at fair value on a non-recurring basis: | Other real estate owned | Weighted Average of Inputs
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other Real Estate Owned, Measurement Input	0.1000
Measurement Input, Constant Prepayment Rate | Level 3 | Measured at fair value on a recurring basis: | Loans held-for- investment
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	0.1031
Measurement Input, Constant Prepayment Rate | Level 3 | Measured at fair value on a recurring basis: | Mortgage servicing rights | Minimum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	0.00
Measurement Input, Constant Prepayment Rate | Level 3 | Measured at fair value on a recurring basis: | Mortgage servicing rights | Maximum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	0.81
Measurement Input, Constant Prepayment Rate | Level 3 | Measured at fair value on a recurring basis: | Mortgage servicing rights | Weighted Average of Inputs
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	0.1003
Measurement Input, Constant Prepayment Rate | Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Loans held-for-investment Constant Prepayment Rate
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	0.1031
Measurement Input, Constant Prepayment Rate | Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Loans held-for-investment Constant Prepayment Rate | Weighted Average of Inputs
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	0.1031
Measurement Input, Constant Prepayment Rate | Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Mortgage Servicing Rights Prepayment Rate | Minimum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	0.00
Measurement Input, Constant Prepayment Rate | Discounted cash flows | Level 3 | Measured at fair value on a recurring basis: | Mortgage Servicing Rights Prepayment Rate | Maximum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	0.81