Fair Values of Assets and Liabilities (Summary Of Carrying Amounts And Estimated Fair Values Of Financial Instruments) (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest bearing deposits with banks	$ 1,437,105	$ 1,099,594	$ 1,221,407
Available-for-sale securities	2,186,154	2,126,081	1,940,787
Held-to-maturity securities	1,198,478	1,036,096	851,842
Trading account securities	2,430	1,692	862
Equity securities with readily determinable fair value	44,319	34,717	37,839
FHLB and FRB stock, at cost	92,026	91,354	96,699
Brokerage customer receivables	13,569	12,609	16,649
Mortgage loans held-for-sale, at fair value	394,975	264,070	455,712
Accrued interest receivable and other	1,023,896	696,707	666,673
FHLB advances	574,823	426,326	667,000
Other borrowings	418,057	393,855	255,701
Subordinated notes	436,021	139,210	139,148
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	300,992	392,200	304,642
Interest bearing deposits with banks	1,437,105	1,099,594	1,221,407
Available-for-sale securities	2,186,154	2,126,081	1,940,787
Held-to-maturity securities	1,191,634	1,067,439	890,834
Trading account securities	2,430	1,692	862
Equity securities with readily determinable fair value	44,319	34,717	37,839
FHLB and FRB stock, at cost	92,026	91,354	96,699
Brokerage customer receivables	13,569	12,609	16,649
Mortgage loans held-for-sale, at fair value	394,975	264,070	455,712
Loans held-for-investment, at fair value	106,081	93,857	67,181
Loans held-for-investment, at amortized cost	25,198,578	23,726,834	22,543,379
Nonqualified deferred compensation assets	13,672	11,282	12,053
Derivative assets	108,784	73,172	92,156
Accrued interest receivable and other	271,988	260,281	240,790
Total financial assets	31,362,307	29,255,182	27,920,990
Non-maturity deposits	22,013,192	20,833,837	19,465,510
Deposits with stated maturities	5,505,623	5,260,841	4,899,969
FHLB advances	574,823	426,326	667,000
Other borrowings	418,057	393,855	255,701
Subordinated notes	436,021	139,210	139,148
Junior subordinated debentures	253,566	253,566	253,566
Derivative liabilities	136,312	68,088	69,364
Accrued interest payable	17,503	16,025	9,802
Total financial liabilities	29,355,097	27,391,748	25,760,060
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	300,992	392,200	304,642
Interest bearing deposits with banks	1,437,105	1,099,594	1,221,407
Available-for-sale securities	2,186,154	2,126,081	1,940,787
Held-to-maturity securities	1,198,478	1,036,096	851,842
Trading account securities	2,430	1,692	862
Equity securities with readily determinable fair value	44,319	34,717	37,839
FHLB and FRB stock, at cost	92,026	91,354	96,699
Brokerage customer receivables	13,569	12,609	16,649
Mortgage loans held-for-sale, at fair value	394,975	264,070	455,712
Loans held-for-investment, at fair value	106,081	93,857	67,181
Loans held-for-investment, at amortized cost	25,086,371	23,780,739	22,767,838
Nonqualified deferred compensation assets	13,672	11,282	12,053
Derivative assets	108,784	73,172	92,156
Accrued interest receivable and other	271,988	260,281	240,790
Total financial assets	31,256,944	29,277,744	28,106,457
Non-maturity deposits	22,013,192	20,833,837	19,465,510
Deposits with stated maturities	5,526,715	5,283,063	4,911,324
FHLB advances	596,689	429,830	669,406
Other borrowings	418,057	393,855	255,701
Subordinated notes	451,874	138,345	137,782
Junior subordinated debentures	250,697	263,846	257,465
Derivative liabilities	136,312	68,088	69,364
Accrued interest payable	17,503	16,025	9,802
Total financial liabilities	$ 29,411,039	$ 27,426,889	$ 25,776,354