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, 2017 USD ($) $ / Loan	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale securities	$ 1,665,903	$ 1,724,667	$ 1,650,096
Loans held-for-investment	29,414		17,603
Derivative assets	48,198	56,394	88,828
Impaired loans—collateral based	$ 78,594	90,516	90,540
Other real estate owned, including covered other real estate owned
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Appraisal adjustment - cost of sale	10.00%
Municipal Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale securities	$ 116,016	131,809	109,281
U.S. Government agencies
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale securities	159,328	189,152	93,683
Measured at fair value on a recurring basis:
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Loans held-for-investment	29,704	22,137	17,603
MSRs	29,414	19,103	13,901
Derivative assets	48,198	56,394	88,828
Measured at fair value on a recurring basis: | Municipal Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale securities	116,016	131,809	109,281
Measured at fair value on a recurring basis: | U.S. Government agencies
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale securities	159,328	189,152	93,683
Measured at fair value on a non-recurring basis:
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Impaired loans—collateral based	57,548
Other real estate owned, including covered other real estate owned	40,229
Level 3 | Measured at fair value on a recurring basis:
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Loans held-for-investment	29,704	22,137	0
MSRs	29,414	19,103	13,901
Derivative assets	$ 1,216	2,291	6,037
Level 3 | Measured at fair value on a recurring basis: | Loans held-for- investment
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Constant prepayment rate (CPR)	9.62%
Level 3 | Measured at fair value on a recurring basis: | Loans held-for- investment | Minimum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	3.00%
Credit loss rate	0.00%
Level 3 | Measured at fair value on a recurring basis: | Loans held-for- investment | Maximum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	4.00%
Credit loss rate	3.00%
Level 3 | Measured at fair value on a recurring basis: | Loans held-for- investment | Weighted Average of Inputs
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	3.65%
Credit loss rate	0.94%
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	9.00%
Constant prepayment rate (CPR)	0.00%
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	15.00%
Constant prepayment rate (CPR)	47.00%
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	9.97%
Constant prepayment rate (CPR)	10.50%
Cost of servicing (in dollars per loan) | $ / Loan	69
Cost of servicing - delinquent (in dollars per loan) | $ / Loan	634
Level 3 | Measured at fair value on a recurring basis: | Municipal Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale securities	$ 68,383	79,626	67,208
Level 3 | Measured at fair value on a recurring basis: | U.S. Government agencies
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale securities	3,943	$ 0	$ 0
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	57,548
Other real estate owned, including covered other real estate owned	$ 40,229
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	3.00%
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	4.00%
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	3.65%
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]
Credit loss 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]
Credit loss rate	3.00%
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]
Credit loss rate	0.94%
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]
Constant prepayment rate (CPR)	9.62%
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]
Constant prepayment rate (CPR)	9.62%
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	9.00%
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	15.00%
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	9.97%
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]
Constant prepayment rate (CPR)	0.00%
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]
Constant prepayment rate (CPR)	47.00%
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]
Constant prepayment rate (CPR)	10.50%
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	65
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	69
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	634
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	38.00%
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	88.85%
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]
Appraisal adjustment - cost of sale	10.00%
Appraisal value | Level 3 | Measured at fair value on a non-recurring basis: | Other real estate owned, including covered other real estate owned | Weighted Average of Inputs
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Appraisal adjustment - cost of sale	10.00%