Fair Values of Assets and Liabilities (Summary Of Carrying Amounts And Estimated Fair Values Of Financial Instruments) (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest bearing deposits with banks	$ 1,943,743	$ 2,164,560	$ 1,609,852
Available-for-sale securities	3,570,959	3,106,214	2,185,782
Held-to-maturity securities	879,192	1,138,396	1,041,695
Equity securities with readily determinable fair value	47,310	50,840	47,653
FHLB and FRB stock, at cost	134,546	100,739	89,013
Brokerage customer receivables	16,293	16,573	14,219
Mortgage loans held-for-sale, at fair value	656,934	377,313	248,557
Accrued interest receivable and other	1,460,168	1,061,141	888,492
FHLB advances	1,174,894	674,870	576,353
Other borrowings	487,503	418,174	372,194
Subordinated notes	436,179	436,095	139,235
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	349,427	286,476	270,823
Interest bearing deposits with banks	1,943,743	2,164,560	1,609,852
Available-for-sale securities	3,570,959	3,106,214	2,185,782
Held-to-maturity securities	865,376	1,134,400	1,051,542
Trading account securities	2,257	1,068	559
Equity securities with readily determinable fair value	47,310	50,840	47,653
FHLB and FRB stock, at cost	134,546	100,739	89,013
Brokerage customer receivables	16,293	16,573	14,219
Mortgage loans held-for-sale, at fair value	656,934	377,313	248,557
Loans held-for-investment, at fair value	142,267	132,718	101,071
Loans held-for-investment, at amortized cost	27,665,054	26,667,572	24,113,558
Nonqualified deferred compensation assets	12,253	14,213	13,230
Derivative assets	301,368	103,644	75,504
Accrued interest receivable and other	275,499	303,090	275,464
Total financial assets	35,983,286	34,459,420	30,096,827
Non-maturity deposits	26,071,881	24,483,867	21,454,035
Deposits with stated maturities	5,389,779	5,623,271	5,350,707
FHLB advances	1,174,894	674,870	576,353
Other borrowings	487,503	418,174	372,194
Subordinated notes	436,179	436,095	139,235
Junior subordinated debentures	253,566	253,566	253,566
Derivative liabilities	367,620	129,204	73,868
Accrued interest payable	27,696	19,940	19,569
Total financial liabilities	34,209,118	32,038,987	28,239,527
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	349,427	286,476	270,823
Interest bearing deposits with banks	1,943,743	2,164,560	1,609,852
Available-for-sale securities	3,570,959	3,106,214	2,185,782
Held-to-maturity securities	879,192	1,138,396	1,041,695
Trading account securities	2,257	1,068	559
Equity securities with readily determinable fair value	47,310	50,840	47,653
FHLB and FRB stock, at cost	134,546	100,739	89,013
Brokerage customer receivables	16,293	16,573	14,219
Mortgage loans held-for-sale, at fair value	656,934	377,313	248,557
Loans held-for-investment, at fair value	142,267	132,718	101,071
Loans held-for-investment, at amortized cost	27,469,489	26,659,903	24,123,328
Nonqualified deferred compensation assets	12,253	14,213	13,230
Derivative assets	301,368	103,644	75,504
Accrued interest receivable and other	275,499	303,090	275,464
Total financial assets	35,801,537	34,455,747	30,096,750
Non-maturity deposits	26,071,881	24,483,867	21,454,035
Deposits with stated maturities	5,394,523	5,635,475	5,377,388
FHLB advances	1,130,327	715,129	604,976
Other borrowings	487,503	418,174	372,194
Subordinated notes	487,745	458,796	144,019
Junior subordinated debentures	224,523	243,158	252,451
Derivative liabilities	367,620	129,204	73,868
Accrued interest payable	27,696	19,940	19,569
Total financial liabilities	$ 34,191,818	$ 32,103,743	$ 28,298,500