Fair Values of Assets and Liabilities (Narrative) (Detail)	3 Months Ended
	Mar. 31, 2018 USD ($) $ / Loan	Dec. 31, 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	$ 1,895,688,000	$ 1,803,666,000	$ 1,803,733,000
Loans held-for-investment	67,962,000		28,548,000
MSRs	54,572,000	33,676,000	21,596,000	$ 19,103,000
Derivative assets	78,960,000	52,069,000	54,273,000
Mortgage loans held-for-sale, at fair value	411,505,000	313,592,000	288,964,000
Impaired loans—collateral based	$ 103,131,000	105,088,000	87,522,000
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	$ 1,895,688,000	1,803,666,000	1,803,733,000
MSRs	54,572,000	33,676,000	21,596,000
Remaining contractual principal balance outstanding, mortgage loans held-for-sale	396,900,000	299,500,000	277,300,000
Mortgage loans held-for-sale, at fair value	411,505,000	313,592,000	288,964,000
Portion at Other than Fair Value Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans—collateral based	34,400,000
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	67,962,000	33,717,000	28,548,000
MSRs	54,572,000	33,676,000	21,596,000
Derivative assets	78,960,000	52,069,000	54,273,000
Mortgage loans held-for-sale, at fair value	411,505,000	313,592,000	288,964,000
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	68,684,000
Other real estate owned	36,598,000
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	26,620,000	33,717,000	28,548,000
MSRs	54,572,000	33,676,000	21,596,000
Derivative assets	4,605,000	2,157,000	3,582,000
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)	8.76%
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)	3.00%
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)	7.00%
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)	32.00%
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)	4.00%
Credit discount	11.00%
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)	17.00%
Constant prepayment rate (CPR)	87.00%
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)	4.14%
Credit discount	1.88%
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)	10.02%
Constant prepayment rate (CPR)	9.40%
Cost of servicing loan (in dollars per loan) | $ / Loan	78
Cost of servicing loan - delinquent (in dollars per loan) | $ / Loan	368
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)	85.34%
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	$ 68,684,000
Other real estate owned	36,598,000
Municipal Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	121,758,000	115,352,000	127,563,000
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	121,758,000	115,352,000	127,563,000
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	84,592,000	77,181,000	79,745,000
U.S. Government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	149,336,000	156,915,000	177,829,000
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	149,336,000	156,915,000	177,829,000
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	$ 3,616,000	$ 3,779,000	$ 4,283,000