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, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	$ 3,194,961		$ 3,106,214	$ 2,186,154
Equity securities with readily determinable fair value	52,460		50,840	44,319
Mortgage loans held-for-sale, at fair value	833,163		377,313	394,975
Loans held-for-investment	254,614			106,081
MSRs	77,203	$ 73,504	85,638	72,850	$ 71,022	$ 75,183
U.S. Treasury
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	60,539		121,088	132,269
U.S. Government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	293,507		365,442	170,940
Municipal Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	150,879		145,318	142,609
Measured at fair value on a recurring basis:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account securities	890		1,068	2,430
Equity securities with readily determinable fair value	52,460		50,840	44,319
Mortgage loans held-for-sale, at fair value	833,163		377,313	394,975
Loans held-for-investment	254,614		132,718	106,081
MSRs	77,203		85,638	72,850
Nonqualified deferred compensation assets	13,576		14,213	13,672
Derivative assets	327,624		103,644	108,784
Total financial assets	4,754,491		3,871,648	2,929,265
Derivative liabilities	355,265		129,204	136,312
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	60,539		121,088	132,269
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	293,507		365,442	170,940
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	150,879		145,318	142,609
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	102,879		94,841	90,913
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	2,587,157		2,379,525	1,649,423
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	44,394		42,774	36,253
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	104,933		163,862	168,522
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	60,539		121,088	132,269
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	890		1,068	2,430
Equity securities with readily determinable fair value	8,066		8,066	8,066
Mortgage loans held-for-sale, at fair value	833,163		377,313	394,975
Loans held-for-investment	240,661		123,098	95,600
MSRs	0		0	0
Nonqualified deferred compensation assets	13,576		14,213	13,672
Derivative assets	269,191		101,013	105,188
Total financial assets	4,380,421		3,495,301	2,560,543
Derivative liabilities	355,265		129,204	136,312
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	291,214		362,796	167,963
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	33,624		33,368	32,313
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	102,879		94,841	90,913
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	2,587,157		2,379,525	1,649,423
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	13,953		9,620	10,481
MSRs	77,203		85,638	72,850
Nonqualified deferred compensation assets	0		0	0
Derivative assets	58,433		2,631	3,596
Total financial assets	269,137		212,485	200,200
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	2,293		2,646	2,977
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	117,255		111,950	110,296
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