Fair Values of Assets and Liabilities (Narrative) (Detail)	3 Months Ended	9 Months Ended
	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) $ / Loan	Dec. 31, 2016 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	$ 1,650,096,000	$ 1,665,903,000	$ 1,724,667,000
Loans held-for-investment	17,603,000	29,414,000
Derivative assets	88,828,000	48,198,000	56,394,000
Mortgage loans held-for-sale	559,634,000	370,282,000	418,374,000
Impaired loans—collateral based	90,540,000	$ 78,594,000	90,516,000
Other real estate owned, including covered other real estate owned
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Appraisal adjustment - cost of sale		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	1,650,096,000	$ 1,665,903,000	1,724,667,000
MSRs	13,901,000	29,414,000	19,103,000
Remaining contractual principal balance outstanding, mortgage loans held-for-sale	537,000,000	356,400,000	414,400,000
Mortgage loans held-for-sale	559,634,000	370,282,000	418,374,000
Portion at Other than Fair Value Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans—collateral based		21,100,000
Municipal Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	109,281,000	116,016,000	131,809,000
U.S. Government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	93,683,000	159,328,000	189,152,000
Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	50,782,000	36,352,000	35,248,000
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	17,603,000	29,704,000	22,137,000
MSRs	13,901,000	29,414,000	19,103,000
Derivative assets	88,828,000	48,198,000	56,394,000
Mortgage loans held-for-sale	559,634,000	370,282,000	418,374,000
Measured at fair value on a recurring basis: | Municipal Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	109,281,000	116,016,000	131,809,000
Measured at fair value on a recurring basis: | U.S. Government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	93,683,000	159,328,000	189,152,000
Measured at fair value on a recurring basis: | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	50,782,000	36,352,000	35,248,000
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		57,548,000
Other real estate owned, including covered other real estate owned		40,229,000
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	0	29,704,000	22,137,000
MSRs	13,901,000	29,414,000	19,103,000
Derivative assets	6,037,000	1,216,000	2,291,000
Mortgage loans held-for-sale	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)		9.62%
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		3.00%
Credit loss rate		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		9.00%
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		38.00%
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		4.00%
Credit loss rate		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		15.00%
Constant prepayment rate (CPR)		47.00%
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		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		3.65%
Credit loss rate		0.94%
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		9.97%
Constant prepayment rate (CPR)		10.50%
Range of Inputs (in dollars per loan) | $ / Loan		69
Weighted Average of Inputs (in dollars per loan) | $ / Loan		634
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		88.85%
Level 3 | Measured at fair value on a recurring basis: | Municipal Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	67,208,000	$ 68,383,000	79,626,000
Level 3 | Measured at fair value on a recurring basis: | U.S. Government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	0	3,943,000	0
Level 3 | Measured at fair value on a recurring basis: | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	0	0	0
Gain on transfer	2,400,000
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		57,548,000
Other real estate owned, including covered other real estate owned		40,229,000
Non-performing
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage loans held-for-sale	$ 0	$ 0	$ 0