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, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	$ 2,186,154		$ 2,126,081	$ 1,940,787
Trading account securities	2,430		1,692	862
Equity securities with readily determinable fair value	44,319		34,717	37,839
Mortgage loans held-for-sale, at fair value	394,975		264,070	455,712
Loans held-for-investment	106,081			67,181
MSRs	72,850	$ 71,022	75,183	63,194	$ 54,572	$ 33,676
U.S. Treasury
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	132,269		126,404	30,216
U.S. Government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	170,940		140,307	114,408
Municipal Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	142,609		138,490	132,593
Measured at fair value on a recurring basis:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account securities	2,430		1,692	862
Equity securities with readily determinable fair value	44,319		34,717	37,839
Mortgage loans held-for-sale, at fair value	394,975		264,070	455,712
Loans held-for-investment	106,081		93,857	67,181
MSRs	72,850		75,183	63,194
Nonqualified deferred compensation assets	13,672		11,282	12,053
Derivative assets	108,784		73,172	92,156
Total financial assets	2,929,265		2,680,054	2,669,784
Derivative liabilities	136,312		68,088	69,364
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	132,269		126,404	30,216
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	170,940		140,307	114,408
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	142,609		138,490	132,593
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	90,913		91,045	95,377
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	1,649,423		1,629,835	1,568,193
Measured at fair value on a recurring basis: | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities with readily determinable fair value	36,253
Loans held-for-investment			0
Total financial assets	168,522		126,404
Measured at fair value on a recurring basis: | Level 1 | U.S. Treasury
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	132,269		126,404
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	2,430		1,692	862
Equity securities with readily determinable fair value	8,066		34,717	37,839
Mortgage loans held-for-sale, at fair value	394,975		264,070	455,712
Loans held-for-investment	95,600		82,510	53,417
MSRs	0		0	0
Nonqualified deferred compensation assets	13,672		11,282	12,053
Derivative assets	105,188		70,715	88,337
Total financial assets	2,560,543		2,352,587	2,488,959
Derivative liabilities	136,312		68,088	69,364
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	0		0	30,216
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	167,963		137,157	110,926
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	32,313		29,564	36,027
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	90,913		91,045	95,377
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	1,649,423		1,629,835	1,568,193
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		0	0
Mortgage loans held-for-sale, at fair value	0		0	0
Loans held-for-investment	10,481		11,347	13,764
MSRs	72,850		75,183	63,194
Nonqualified deferred compensation assets	0		0	0
Derivative assets	3,596		2,457	3,819
Total financial assets	200,200		201,063	180,825
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	2,977		3,150	3,482
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	110,296		108,926	96,566
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