Fair Values of Assets and Liabilities (Summary Of Carrying Amounts And Estimated Fair Values Of Financial Instruments) (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest bearing deposits with banks	$ 2,260,806	$ 1,099,594	$ 1,137,044
Available-for-sale securities	2,270,059	2,126,081	2,164,985
Held-to-maturity securities	1,104,903	1,036,096	911,597
Trading account securities	3,204	1,692	688
Equity securities with readily determinable fair value	46,086	34,717	36,414
FHLB and FRB stock, at cost	92,714	91,354	99,998
Brokerage customer receivables	14,943	12,609	15,649
Mortgage loans held-for-sale, at fair value	464,727	264,070	338,111
Accrued interest receivable and other	1,087,864	696,707	700,568
FHLB advances	574,847	426,326	615,000
Other borrowings	410,488	393,855	373,571
Subordinated notes	435,979	139,210	139,172
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	448,814	392,200	279,993
Interest bearing deposits with banks	2,260,806	1,099,594	1,137,044
Available-for-sale securities	2,270,059	2,126,081	2,164,985
Held-to-maturity securities	1,095,802	1,067,439	966,438
Trading account securities	3,204	1,692	688
Equity securities with readily determinable fair value	46,086	34,717	36,414
FHLB and FRB stock, at cost	92,714	91,354	99,998
Brokerage customer receivables	14,943	12,609	15,649
Mortgage loans held-for-sale, at fair value	464,727	264,070	338,111
Loans held-for-investment, at fair value	116,022	93,857	77,883
Loans held-for-investment, at amortized cost	25,594,149	23,726,834	23,046,068
Nonqualified deferred compensation assets	13,750	11,282	12,503
Derivative assets	138,286	73,172	96,153
Accrued interest receivable and other	272,072	260,281	254,879
Total financial assets	32,831,434	29,255,182	28,526,806
Non-maturity deposits	23,091,143	20,833,837	19,456,677
Deposits with stated maturities	5,619,236	5,260,841	5,460,038
FHLB advances	574,847	426,326	615,000
Other borrowings	410,488	393,855	373,571
Subordinated notes	435,979	139,210	139,172
Junior subordinated debentures	253,566	253,566	253,566
Derivative liabilities	171,009	68,088	71,381
Accrued interest payable	22,899	16,025	15,374
Total financial liabilities	30,579,167	27,391,748	26,384,779
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	448,814	392,200	279,993
Interest bearing deposits with banks	2,260,806	1,099,594	1,137,044
Available-for-sale securities	2,270,059	2,126,081	2,164,985
Held-to-maturity securities	1,104,903	1,036,096	911,597
Trading account securities	3,204	1,692	688
Equity securities with readily determinable fair value	46,086	34,717	36,414
FHLB and FRB stock, at cost	92,714	91,354	99,998
Brokerage customer receivables	14,943	12,609	15,649
Mortgage loans held-for-sale, at fair value	464,727	264,070	338,111
Loans held-for-investment, at fair value	116,022	93,857	77,883
Loans held-for-investment, at amortized cost	25,472,310	23,780,739	23,261,545
Nonqualified deferred compensation assets	13,750	11,282	12,503
Derivative assets	138,286	73,172	96,153
Accrued interest receivable and other	272,072	260,281	254,879
Total financial assets	32,718,696	29,277,744	28,687,442
Non-maturity deposits	23,091,143	20,833,837	19,456,677
Deposits with stated maturities	5,635,597	5,283,063	5,475,048
FHLB advances	620,136	429,830	615,342
Other borrowings	410,488	393,855	373,571
Subordinated notes	451,947	138,345	146,838
Junior subordinated debentures	254,426	263,846	258,488
Derivative liabilities	171,009	68,088	71,381
Accrued interest payable	22,899	16,025	15,374
Total financial liabilities	$ 30,657,645	$ 27,426,889	$ 26,412,719