Fair Values of Assets and Liabilities (Summary of Balances of Assets and Liabilities Measured at Fair Value On A Recurring Basis) (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	$ 1,940,787		$ 1,803,666	$ 1,649,636
Trading account securities	862		995	1,987
Equity securities with readily determinable fair value	37,839		0	0
Mortgage loans held-for-sale, at fair value	455,712		313,592	382,837
Loans held-for-investment	67,181			30,173
MSRs	63,194	$ 54,572	33,676	27,307	$ 21,596	$ 19,103
Derivative assets	92,156		52,069	49,703
Derivative liabilities	69,364		35,671	37,651
U.S. Treasury
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	30,216		143,822	119,081
U.S. Government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	114,408		156,915	157,510
Municipal Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	132,593		115,352	124,120
Mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,568,193		1,319,725	1,153,290
Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0		36,802	35,037
Measured at fair value on a recurring basis:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account securities	862		995	1,987
Equity securities with readily determinable fair value	37,839
Mortgage loans held-for-sale, at fair value	455,712		313,592	382,837
Loans held-for-investment	67,181		33,717	30,173
MSRs	63,194		33,676	27,307
Nonqualified deferred compensation assets	12,053		11,065	10,556
Derivative assets	92,156		52,069	49,703
Total	2,669,784		2,248,780	2,152,199
Derivative liabilities	69,364		35,671	37,651
Measured at fair value on a recurring basis: | U.S. Treasury
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	30,216		143,822	119,081
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	114,408		156,915	157,510
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	132,593		115,352	124,120
Measured at fair value on a recurring basis: | Corporate notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	95,377		31,050	60,598
Measured at fair value on a recurring basis: | Mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,568,193		1,319,725	1,153,290
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			36,802	35,037
Measured at fair value on a recurring basis: | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-investment			0
Measured at fair value on a recurring basis: | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account securities	862		995	1,987
Equity securities with readily determinable fair value	37,839
Mortgage loans held-for-sale, at fair value	455,712		313,592	382,837
Loans held-for-investment	53,417		0	0
MSRs	0		0	0
Nonqualified deferred compensation assets	12,053		11,065	10,556
Derivative assets	88,337		49,912	48,656
Total	2,488,959		2,098,270	2,012,221
Derivative liabilities	69,364		35,671	37,651
Measured at fair value on a recurring basis: | Level 2 | U.S. Treasury
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	30,216		143,822	119,081
Measured at fair value on a recurring basis: | Level 2 | U.S. Government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	110,926		153,136	153,400
Measured at fair value on a recurring basis: | Level 2 | Municipal Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	36,027		38,171	46,779
Measured at fair value on a recurring basis: | Level 2 | Corporate notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	95,377		31,050	60,598
Measured at fair value on a recurring basis: | Level 2 | Mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,568,193		1,319,725	1,153,290
Measured at fair value on a recurring basis: | Level 2 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities			36,802	35,037
Measured at fair value on a recurring basis: | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account securities	0		0	0
Equity securities with readily determinable fair value	0
Mortgage loans held-for-sale, at fair value	0		0	0
Loans held-for-investment	13,764		33,717	30,173
MSRs	63,194		33,676	27,307
Nonqualified deferred compensation assets	0		0	0
Derivative assets	3,819		2,157	1,047
Total	180,825		150,510	139,978
Derivative liabilities	0		0	0
Measured at fair value on a recurring basis: | Level 3 | U.S. Treasury
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0		0	0
Measured at fair value on a recurring basis: | Level 3 | U.S. Government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	3,482		3,779	4,110
Measured at fair value on a recurring basis: | Level 3 | Municipal Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	96,566		77,181	77,341
Measured at fair value on a recurring basis: | Level 3 | Corporate notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0		0	0
Measured at fair value on a recurring basis: | Level 3 | Mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	$ 0		0	0
Measured at fair value on a recurring basis: | Level 3 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities			$ 0	$ 0