Fair Values of Assets and Liabilities (Summary Of Carrying Amounts And Estimated Fair Values Of Financial Instruments) (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest bearing deposits with banks	$ 4,015,072	$ 2,164,560	$ 1,437,105
Available-for-sale securities	3,194,961	3,106,214	2,186,154
Held-to-maturity securities	744,286	1,138,396	1,198,478
Equity securities with readily determinable fair value	52,460	50,840	44,319
FHLB and FRB stock, at cost	135,571	100,739	92,026
Brokerage customer receivables	14,623	16,573	13,569
Mortgage loans held-for-sale, at fair value	833,163	377,313	394,975
Accrued interest receivable and other	1,437,832	1,061,141	1,023,896
FHLB advances	1,228,416	674,870	574,823
Other borrowings	508,535	418,174	418,057
Subordinated notes	436,298	436,095	436,021
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	345,057	286,476	300,992
Interest bearing deposits with banks	4,015,072	2,164,560	1,437,105
Available-for-sale securities	3,194,961	3,106,214	2,186,154
Held-to-maturity securities	728,465	1,134,400	1,191,634
Trading account securities	890	1,068	2,430
Equity securities with readily determinable fair value	52,460	50,840	44,319
FHLB and FRB stock, at cost	135,571	100,739	92,026
Brokerage customer receivables	14,623	16,573	13,569
Mortgage loans held-for-sale, at fair value	833,163	377,313	394,975
Loans held-for-investment, at fair value	254,614	132,718	106,081
Loans held-for-investment, at amortized cost	31,148,289	26,667,572	25,198,578
Nonqualified deferred compensation assets	13,576	14,213	13,672
Derivative assets	327,624	103,644	108,784
Accrued interest receivable and other	263,743	303,090	271,988
Total financial assets	41,328,108	34,459,420	31,362,307
Non-maturity deposits	30,814,487	24,483,867	22,013,192
Deposits with stated maturities	4,837,387	5,623,271	5,505,623
FHLB advances	1,228,416	674,870	574,823
Other borrowings	508,535	418,174	418,057
Subordinated notes	436,298	436,095	436,021
Junior subordinated debentures	253,566	253,566	253,566
Derivative liabilities	355,265	129,204	136,312
Accrued interest payable	17,200	19,940	17,503
Total financial liabilities	38,451,154	32,038,987	29,355,097
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	345,057	286,476	300,992
Interest bearing deposits with banks	4,015,072	2,164,560	1,437,105
Available-for-sale securities	3,194,961	3,106,214	2,186,154
Held-to-maturity securities	744,286	1,138,396	1,198,478
Trading account securities	890	1,068	2,430
Equity securities with readily determinable fair value	52,460	50,840	44,319
FHLB and FRB stock, at cost	135,571	100,739	92,026
Brokerage customer receivables	14,623	16,573	13,569
Mortgage loans held-for-sale, at fair value	833,163	377,313	394,975
Loans held-for-investment, at fair value	254,614	132,718	106,081
Loans held-for-investment, at amortized cost	31,004,403	26,659,903	25,086,371
Nonqualified deferred compensation assets	13,576	14,213	13,672
Derivative assets	327,624	103,644	108,784
Accrued interest receivable and other	263,743	303,090	271,988
Total financial assets	41,200,043	34,455,747	31,256,944
Non-maturity deposits	30,814,487	24,483,867	22,013,192
Deposits with stated maturities	4,838,831	5,635,475	5,526,715
FHLB advances	1,181,462	715,129	596,689
Other borrowings	508,535	418,174	418,057
Subordinated notes	473,324	458,796	451,874
Junior subordinated debentures	170,449	243,158	250,697
Derivative liabilities	355,265	129,204	136,312
Accrued interest payable	17,200	19,940	17,503
Total financial liabilities	$ 38,359,553	$ 32,103,743	$ 29,411,039