Fair Values of Assets and Liabilities (Narrative) (Detail)	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, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	$ 2,164,985,000		$ 1,803,666,000	$ 1,665,903,000
Loans held-for-investment	77,883,000			29,414,000
MSRs	74,530,000	$ 63,194,000	33,676,000	29,414,000	$ 27,307,000	$ 19,103,000
Derivative assets	96,153,000		52,069,000	48,198,000
Mortgage loans held-for-sale, at fair value	338,111,000		313,592,000	370,282,000
Impaired loans—collateral based	132,522,000		105,088,000	78,594,000
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	2,164,985,000		1,803,666,000	1,665,903,000
MSRs	74,530,000		33,676,000	29,414,000
Remaining contractual principal balance outstanding, mortgage loans held-for-sale	317,400,000		299,500,000	356,400,000
Mortgage loans held-for-sale, at fair value	338,111,000		313,592,000	370,282,000
Portion at Other than Fair Value Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans—collateral based	26,300,000
Non-performing
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage loans held-for-sale, at fair value	622,000		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	77,883,000		33,717,000	29,704,000
MSRs	74,530,000		33,676,000	29,414,000
Derivative assets	96,153,000		52,069,000	48,198,000
Mortgage loans held-for-sale, at fair value	338,111,000		313,592,000	370,282,000
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	106,219,000
Other real estate owned	28,303,000
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	13,456,000		33,717,000	29,704,000
MSRs	74,530,000		33,676,000	29,414,000
Derivative assets	2,492,000		2,157,000	1,216,000
Mortgage loans held-for-sale, at fair value	$ 0		$ 0	$ 0
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)	20.00%
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 | Mortgage servicing rights
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cost of servicing loan (in dollars per loan) | $ / Loan	78
Cost of servicing loan - delinquent (in dollars per loan) | $ / Loan	274
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)	91.12%
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	$ 106,219,000
Other real estate owned	$ 28,303,000
Measurement Input, Discount Rate | 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)	0.04
Measurement Input, Discount Rate | 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)	0.07
Measurement Input, Discount Rate | 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)	0.05
Measurement Input, Discount Rate | 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)	0.18
Measurement Input, Discount Rate | 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)	0.0457
Measurement Input, Discount Rate | 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)	0.1003
Measurement Input, Entity Credit Risk | 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)	0.00
Measurement Input, Entity Credit Risk | 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)	0.07
Measurement Input, Entity Credit Risk | 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)	0.0100
Measurement Input, Constant Prepayment Rate | 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]
Discount rate input (as a percent)	0.1031
Measurement Input, Constant Prepayment Rate | 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)	0.00
Measurement Input, Constant Prepayment Rate | 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)	0.81
Measurement Input, Constant Prepayment Rate | 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)	0.1003
Measurement Input, Discount for Lack of Marketability | 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)	0.10