Fair Values of Assets and Liabilities (Summary Of Carrying Amounts And Estimated Fair Values Of Financial Instruments) (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest-bearing deposits with banks	$ 3,825,823	$ 2,164,560	$ 2,260,806
Available-for-sale securities	2,946,459	3,106,214	2,270,059
Held-to-maturity securities	575,947	1,138,396	1,104,903
Equity securities with readily determinable fair value	54,398	50,840	46,086
FHLB and FRB stock, at cost	135,568	100,739	92,714
Brokerage customer receivables	16,818	16,573	14,943
Mortgage loans held-for-sale, at fair value	959,671	377,313	464,727
Loans held-for-investment	32,135,555	26,800,290	25,710,171
Accrued interest receivable and other	1,424,728	1,061,141	1,087,864
FHLB advances	1,228,422	674,870	574,847
Other borrowings	507,395	418,174	410,488
Subordinated notes	436,385	436,095	435,979
Junior subordinated debentures	253,566	253,566	253,566
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	308,695	286,476	448,814
Interest-bearing deposits with banks	3,825,823	2,164,560	2,260,806
Available-for-sale securities	2,946,459	3,106,214	2,270,059
Held-to-maturity securities	560,267	1,134,400	1,095,802
Trading account securities	1,720	1,068	3,204
Equity securities with readily determinable fair value	54,398	50,840	46,086
FHLB and FRB stock, at cost	135,568	100,739	92,714
Brokerage customer receivables	16,818	16,573	14,943
Mortgage loans held-for-sale, at fair value	959,671	377,313	464,727
Nonqualified deferred compensation assets	14,197	14,213	13,750
Derivative assets	321,488	103,644	138,286
Accrued interest receivable and other	272,940	303,090	272,072
Total financial assets	41,553,599	34,459,420	32,831,434
Non-maturity deposits	30,778,127	24,483,867	23,091,143
Deposits with stated maturities	5,066,295	5,623,271	5,619,236
FHLB advances	1,228,422	674,870	574,847
Other borrowings	507,395	418,174	410,488
Subordinated notes	436,385	436,095	435,979
Junior subordinated debentures	253,566	253,566	253,566
Derivative liabilities	326,990	129,204	171,009
Accrued interest payable	21,310	19,940	22,899
Total financial liabilities	38,618,490	32,038,987	30,579,167
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	308,695	286,476	448,814
Interest-bearing deposits with banks	3,825,823	2,164,560	2,260,806
Available-for-sale securities	2,946,459	3,106,214	2,270,059
Held-to-maturity securities	575,947	1,138,396	1,104,903
Trading account securities	1,720	1,068	3,204
Equity securities with readily determinable fair value	54,398	50,840	46,086
FHLB and FRB stock, at cost	135,568	100,739	92,714
Brokerage customer receivables	16,818	16,573	14,943
Mortgage loans held-for-sale, at fair value	959,671	377,313	464,727
Nonqualified deferred compensation assets	14,197	14,213	13,750
Derivative assets	321,488	103,644	138,286
Accrued interest receivable and other	272,940	303,090	272,072
Total financial assets	41,410,798	34,455,747	32,718,696
Non-maturity deposits	30,778,127	24,483,867	23,091,143
Deposits with stated maturities	5,063,732	5,635,475	5,635,597
FHLB advances	1,207,203	715,129	620,136
Other borrowings	507,395	418,174	410,488
Subordinated notes	473,825	458,796	451,947
Junior subordinated debentures	208,333	243,158	254,426
Derivative liabilities	326,990	129,204	171,009
Accrued interest payable	21,310	19,940	22,899
Total financial liabilities	38,586,915	32,103,743	30,657,645
Loans held-for-investment, at fair value | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans held-for-investment	253,515	132,718	116,022
Loans held-for-investment, at fair value | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans held-for-investment	253,515	132,718	116,022
Loans held-for-investment, at amortized cost | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans held-for-investment	31,882,040	26,667,572	25,594,149
Loans held-for-investment, at amortized cost | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans held-for-investment	$ 31,723,559	$ 26,659,903	$ 25,472,310