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, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	$ 3,570,959	$ 3,106,214	$ 2,185,782
Equity securities with readily determinable fair value	47,310	50,840	47,653
Mortgage loans held-for-sale, at fair value	656,934	377,313	248,557
Loans held-for-investment	142,267		101,071
MSRs	73,504	85,638	71,022	$ 75,183
U.S. Treasury
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	121,670	121,088	126,718
U.S. Government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	368,643	365,442	130,687
Municipal Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	146,609	145,318	136,353
Measured at fair value on a recurring basis:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account securities	2,257	1,068	559
Equity securities with readily determinable fair value	47,310	50,840	47,653
Mortgage loans held-for-sale, at fair value	656,934	377,313	248,557
Loans held-for-investment	142,267	132,718	101,071
MSRs	73,504	85,638	71,022
Nonqualified deferred compensation assets	12,253	14,213	13,230
Derivative assets	301,368	103,644	75,504
Total financial assets	4,806,852	3,871,648	2,743,378
Derivative liabilities	367,620	129,204	73,868
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	121,670	121,088	126,718
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	368,643	365,442	130,687
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	146,609	145,318	136,353
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	101,018	94,841	93,333
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	2,833,019	2,379,525	1,698,691
Measured at fair value on a recurring basis: | Level 1
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	39,244	42,774	39,587
Mortgage loans held-for-sale, at fair value	0	0	0
Loans held-for-investment	0	0	0
MSRs	0	0	0
Nonqualified deferred compensation assets	0	0	0
Derivative assets	0	0	0
Total financial assets	160,914	163,862	166,305
Derivative liabilities	0	0	0
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	121,670	121,088	126,718
Measured at fair value on a recurring basis: | Level 1 | U.S. Government agencies
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 1 | Municipal Securities
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 1 | 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 1 | 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 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account securities	2,257	1,068	559
Equity securities with readily determinable fair value	8,066	8,066	8,066
Mortgage loans held-for-sale, at fair value	656,934	377,313	248,557
Loans held-for-investment	132,699	123,098	89,822
MSRs	0	0	0
Nonqualified deferred compensation assets	12,253	14,213	13,230
Derivative assets	261,552	101,013	72,415
Total financial assets	4,407,326	3,495,301	2,384,886
Derivative liabilities	367,620	129,204	73,868
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	0
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	366,186	362,796	127,694
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	33,342	33,368	32,519
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	101,018	94,841	93,333
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	2,833,019	2,379,525	1,698,691
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	9,568	9,620	11,249
MSRs	73,504	85,638	71,022
Nonqualified deferred compensation assets	0	0	0
Derivative assets	39,816	2,631	3,089
Total financial assets	238,612	212,485	192,187
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	2,457	2,646	2,993
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	113,267	111,950	103,834
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