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	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	$ 2,185,782	$ 2,126,081	$ 1,895,688
Trading account securities	559	1,692	1,682
Equity securities with readily determinable fair value	47,653	34,717	37,832
Mortgage loans held-for-sale, at fair value	248,557	264,070	411,505
Loans held-for-investment	101,071		67,962
MSRs	71,022	75,183	54,572	$ 33,676
U.S. Treasury
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	126,718	126,404	24,727
U.S. Government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	130,687	140,307	149,336
Municipal Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	136,353	138,490	121,758
Measured at fair value on a recurring basis:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account securities	559	1,692	1,682
Equity securities with readily determinable fair value	47,653	34,717	37,832
Mortgage loans held-for-sale, at fair value	248,557	264,070	411,505
Loans held-for-investment	101,071	93,857	67,962
MSRs	71,022	75,183	54,572
Nonqualified deferred compensation assets	13,230	11,282	11,724
Derivative assets	75,504	73,172	78,960
Total financial assets	2,743,378	2,680,054	2,559,925
Derivative liabilities	73,868	68,088	55,699
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	126,718	126,404	24,727
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	130,687	140,307	149,336
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	136,353	138,490	121,758
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	93,333	91,045	99,714
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	1,698,691	1,629,835	1,500,153
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	39,587
Loans held-for-investment		0
Total financial assets	166,305	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	126,718	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	559	1,692	1,682
Equity securities with readily determinable fair value	8,066	34,717	37,832
Mortgage loans held-for-sale, at fair value	248,557	264,070	411,505
Loans held-for-investment	89,822	82,510	41,342
MSRs	0	0	0
Nonqualified deferred compensation assets	13,230	11,282	11,724
Derivative assets	72,415	70,715	74,355
Total financial assets	2,384,886	2,352,587	2,385,920
Derivative liabilities	73,868	68,088	55,699
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	24,727
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	127,694	137,157	145,720
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	32,519	29,564	37,166
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	93,333	91,045	99,714
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	1,698,691	1,629,835	1,500,153
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	11,249	11,347	26,620
MSRs	71,022	75,183	54,572
Nonqualified deferred compensation assets	0	0	0
Derivative assets	3,089	2,457	4,605
Total financial assets	192,187	201,063	174,005
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	2,993	3,150	3,616
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	103,834	108,926	84,592
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