Fair Values of Assets and Liabilities (Summary Of Carrying Amounts And Estimated Fair Values Of Financial Instruments) (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 231,464	$ 277,591	$ 217,148	$ 270,045
Interest bearing deposits with banks	980,380	1,063,242	1,007,468
Available-for-sale securities	1,895,688	1,803,666	1,803,733
Held-to-maturity securities	862,527	812,516	647,895
Trading account securities	1,682	995	714
Equity securities with readily determinable fair value	37,832	0	0
FHLB and FRB stock, at cost	104,956	89,989	78,904
Brokerage customer receivables	24,531	26,431	23,171
Mortgage loans held-for-sale, at fair value	411,505	313,592	288,964
MSRs	54,572	33,676	21,596	19,103
Accrued interest receivable and other	601,794	567,374	560,741
FHLB advances	915,000	559,663	227,585
Other borrowings	247,092	266,123	238,787
Subordinated notes	139,111	139,088	138,993
Junior subordinated debentures	253,566	253,566	253,566
Derivative liabilities	55,699	35,671	41,531
FDIC indemnification liability			18,263	$ 16,701
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	231,464	277,591	217,148
Interest bearing deposits with banks	980,380	1,063,242	1,007,468
Available-for-sale securities	1,895,688	1,803,666	1,803,733
Held-to-maturity securities	892,937	826,449	667,764
Trading account securities	1,682	995	714
Equity securities with readily determinable fair value	37,832	0	0
FHLB and FRB stock, at cost	104,956	89,989	78,904
Brokerage customer receivables	24,531	26,431	23,171
Mortgage loans held-for-sale, at fair value	411,505	313,592	288,964
Loans held-for-investment, at fair value	67,962	33,717	28,548
Loans held-for-investment, at amortized cost	21,994,172	21,607,080	19,954,869
MSRs	54,572	33,676	21,596
Nonqualified deferred compensation assets	11,724	11,065	10,347
Derivative assets	78,960	52,069	54,273
Accrued interest receivable and other	236,131	227,649	209,623
Total financial assets	27,024,496	26,367,211	24,367,122
Non-maturity deposits	18,941,201	18,775,977	17,509,429
Deposits with stated maturities	4,338,126	4,407,370	4,221,012
FHLB advances	915,000	559,663	227,585
Other borrowings	247,092	266,123	238,787
Subordinated notes	139,111	139,088	138,993
Junior subordinated debentures	253,566	253,566	253,566
Derivative liabilities	55,699	35,671	41,531
FDIC indemnification liability	0	0	18,264
Accrued interest payable	11,442	8,030	7,944
Total financial liabilities	24,901,237	24,445,488	22,657,111
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	231,464	277,591	217,148
Interest bearing deposits with banks	980,380	1,063,242	1,007,468
Available-for-sale securities	1,895,688	1,803,666	1,803,733
Held-to-maturity securities	862,527	812,516	647,895
Trading account securities	1,682	995	714
Equity securities with readily determinable fair value	37,832	0	0
FHLB and FRB stock, at cost	104,956	89,989	78,904
Brokerage customer receivables	24,531	26,431	23,171
Mortgage loans held-for-sale, at fair value	411,505	313,592	288,964
Loans held-for-investment, at fair value	67,962	33,717	28,548
Loans held-for-investment, at amortized cost	22,234,795	21,768,978	21,048,751
MSRs	54,572	33,676	21,596
Nonqualified deferred compensation assets	11,724	11,065	10,347
Derivative assets	78,960	52,069	54,273
Accrued interest receivable and other	236,131	227,649	209,623
Total financial assets	27,234,709	26,515,176	25,441,135
Non-maturity deposits	18,941,201	18,775,977	17,509,429
Deposits with stated maturities	4,344,584	4,350,004	4,210,294
FHLB advances	916,513	544,750	229,338
Other borrowings	247,092	266,123	238,787
Subordinated notes	140,889	144,266	141,180
Junior subordinated debentures	268,873	264,696	254,417
Derivative liabilities	55,699	35,671	41,531
FDIC indemnification liability	0	0	18,264
Accrued interest payable	11,442	8,030	7,944
Total financial liabilities	$ 24,926,293	$ 24,389,517	$ 22,651,184