Fair Values of Assets and Liabilities (Narrative) (Detail)	6 Months Ended
	Jun. 30, 2018 USD ($) $ / Loan	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	$ 1,940,787,000		$ 1,803,666,000	$ 1,649,636,000
Loans held-for-investment	67,181,000			30,173,000
MSRs	63,194,000	$ 54,572,000	33,676,000	27,307,000	$ 21,596,000	$ 19,103,000
Derivative assets	92,156,000		52,069,000	49,703,000
Mortgage loans held-for-sale, at fair value	455,712,000		313,592,000	382,837,000
Impaired loans—collateral based	$ 120,302,000		105,088,000	79,318,000
Other real estate owned
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Appraisal adjustment - cost of sale (as a percent)	10.00%
Estimate of Fair Value Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	$ 1,940,787,000		1,803,666,000	1,649,636,000
MSRs	63,194,000		33,676,000	27,307,000
Remaining contractual principal balance outstanding, mortgage loans held-for-sale	430,700,000		299,500,000	368,800,000
Mortgage loans held-for-sale, at fair value	455,712,000		313,592,000	382,837,000
Portion at Other than Fair Value Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans—collateral based	52,600,000
Non-performing
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage loans held-for-sale, at fair value	0		0	0
Measured at fair value on a recurring basis:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-investment	67,181,000		33,717,000	30,173,000
MSRs	63,194,000		33,676,000	27,307,000
Derivative assets	92,156,000		52,069,000	49,703,000
Mortgage loans held-for-sale, at fair value	455,712,000		313,592,000	382,837,000
Measured at fair value on a non-recurring basis:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans—collateral based	67,700,000
Other real estate owned	35,331,000
Level 3 | Measured at fair value on a recurring basis:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-investment	13,764,000		33,717,000	30,173,000
MSRs	63,194,000		33,676,000	27,307,000
Derivative assets	3,819,000		2,157,000	1,047,000
Mortgage loans held-for-sale, at fair value	$ 0		0	0
Level 3 | Measured at fair value on a recurring basis: | Loans held-for-investment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Constant prepayment rate (CPR)	9.59%
Level 3 | Measured at fair value on a recurring basis: | Minimum | Loans held-for-investment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rate input (as a percent)	4.00%
Credit discount	0.00%
Level 3 | Measured at fair value on a recurring basis: | Minimum | Mortgage servicing rights
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rate input (as a percent)	6.00%
Constant prepayment rate (CPR)	0.00%
Level 3 | Measured at fair value on a recurring basis: | Minimum | Derivatives | Discounted cash flows
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Pull-through rate (as a percent)	33.00%
Level 3 | Measured at fair value on a recurring basis: | Maximum | Loans held-for-investment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rate input (as a percent)	5.00%
Credit discount	3.00%
Level 3 | Measured at fair value on a recurring basis: | Maximum | Mortgage servicing rights
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rate input (as a percent)	16.00%
Constant prepayment rate (CPR)	88.00%
Level 3 | Measured at fair value on a recurring basis: | Maximum | Derivatives | Discounted cash flows
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Pull-through rate (as a percent)	100.00%
Level 3 | Measured at fair value on a recurring basis: | Weighted Average of Inputs | Loans held-for-investment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rate input (as a percent)	4.40%
Credit discount	1.21%
Level 3 | Measured at fair value on a recurring basis: | Weighted Average of Inputs | Mortgage servicing rights
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rate input (as a percent)	10.04%
Constant prepayment rate (CPR)	9.28%
Cost of servicing loan (in dollars per loan) | $ / Loan	78
Cost of servicing loan - delinquent (in dollars per loan) | $ / Loan	375
Level 3 | Measured at fair value on a recurring basis: | Weighted Average of Inputs | Derivatives | Discounted cash flows
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Pull-through rate (as a percent)	81.35%
Level 3 | Measured at fair value on a non-recurring basis:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans—collateral based	$ 67,700,000
Other real estate owned	35,331,000
Municipal Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	132,593,000		115,352,000	124,120,000
Municipal Securities | Measured at fair value on a recurring basis:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	132,593,000		115,352,000	124,120,000
Municipal Securities | Level 3 | Measured at fair value on a recurring basis:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	96,566,000		77,181,000	77,341,000
U.S. Government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	114,408,000		156,915,000	157,510,000
U.S. Government agencies | Measured at fair value on a recurring basis:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	114,408,000		156,915,000	157,510,000
U.S. Government agencies | Level 3 | Measured at fair value on a recurring basis:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	$ 3,482,000		$ 3,779,000	$ 4,110,000