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	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	$ 2,946,459		$ 3,106,214	$ 2,270,059
Equity securities with readily determinable fair value	54,398		50,840	46,086
Loans held-for-investment	253,515			116,022
MSRs	86,907	$ 77,203	85,638	75,585	$ 72,850	$ 75,183
U.S. Treasury
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	280,108		121,088	221,670
U.S. Government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	178,666		365,442	225,459
Municipal Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	154,195		145,318	146,104
Measured at fair value on a recurring basis:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account securities	1,720		1,068	3,204
Equity securities with readily determinable fair value	54,398		50,840	46,086
Mortgage loans held-for-sale, at fair value	959,671		377,313	464,727
Loans held-for-investment	253,515		132,718	116,022
MSRs	86,907		85,638	75,585
Nonqualified deferred compensation assets	14,197		14,213	13,750
Derivative assets	321,488		103,644	138,286
Total financial assets	4,638,355		3,871,648	3,127,719
Derivative liabilities	326,990		129,204	171,009
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	280,108		121,088	221,670
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	178,666		365,442	225,459
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	154,195		145,318	146,104
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	104,135		94,841	94,004
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	2,229,355		2,379,525	1,582,822
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	46,332		42,774	38,020
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	326,440		163,862	259,690
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	280,108		121,088	221,670
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	1,720		1,068	3,204
Equity securities with readily determinable fair value	8,066		8,066	8,066
Mortgage loans held-for-sale, at fair value	959,671		377,313	464,727
Loans held-for-investment	240,902		123,098	106,023
MSRs	0		0	0
Nonqualified deferred compensation assets	14,197		14,213	13,750
Derivative assets	254,601		101,013	135,307
Total financial assets	4,023,908		3,495,301	2,663,597
Derivative liabilities	326,990		129,204	171,009
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	176,537		362,796	222,648
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	34,724		33,368	33,046
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	104,135		94,841	94,004
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	2,229,355		2,379,525	1,582,822
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	12,613		9,620	9,999
MSRs	86,907		85,638	75,585
Nonqualified deferred compensation assets	0		0	0
Derivative assets	66,887		2,631	2,979
Total financial assets	288,007		212,485	204,432
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	2,129		2,646	2,811
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	119,471		111,950	113,058
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