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	3 Months Ended
	Mar. 31, 2018 USD ($) $ / Loan	Dec. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale securities	$ 1,895,688	$ 1,803,666	$ 1,803,733
Loans held-for-investment	67,962		28,548
MSRs	54,572	33,676	21,596	$ 19,103
Derivative assets	78,960	52,069	54,273
Impaired loans—collateral based	$ 103,131	105,088	87,522
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	$ 121,758	115,352	127,563
U.S. Government agencies
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale securities	149,336	156,915	177,829
Measured at fair value on a recurring basis:
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Loans held-for-investment	67,962	33,717	28,548
MSRs	54,572	33,676	21,596
Derivative assets	78,960	52,069	54,273
Measured at fair value on a recurring basis: | Municipal Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale securities	121,758	115,352	127,563
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	149,336	156,915	177,829
Measured at fair value on a non-recurring basis:
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Impaired loans—collateral based	68,684
Other real estate owned	36,598
Level 3 | Measured at fair value on a recurring basis:
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Loans held-for-investment	26,620	33,717	28,548
MSRs	54,572	33,676	21,596
Derivative assets	$ 4,605	2,157	3,582
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)	8.76%
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 discount	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 discount	11.00%
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	4.14%
Credit discount	1.88%
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	7.00%
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	17.00%
Constant prepayment rate (CPR)	87.00%
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	10.02%
Constant prepayment rate (CPR)	9.40%
Cost of servicing (in dollars per loan) | $ / Loan	78
Cost of servicing - delinquent (in dollars per loan) | $ / Loan	368
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	$ 84,592	77,181	79,745
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	3,616	$ 3,779	$ 4,283
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	68,684
Other real estate owned	$ 36,598
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	4.14%
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 discount	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 discount	11.00%
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 discount	1.88%
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)	8.76%
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)	8.76%
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	7.00%
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	17.00%
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	10.02%
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)	87.00%
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)	9.40%
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	70
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	368
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	32.00%
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	85.34%
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]
Appraisal adjustment - cost of sale	10.00%
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
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 | Weighted Average of Inputs
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Appraisal adjustment - cost of sale	10.00%