Fair Values of Assets and Liabilities (Summary Of Carrying Amounts And Estimated Fair Values Of Financial Instruments) (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 279,993		$ 277,591	$ 251,952		$ 270,045
Interest bearing deposits with banks	1,137,044		1,063,242	1,218,728
Available-for-sale securities	2,164,985		1,803,666	1,665,903
Held-to-maturity securities	911,597		812,516	807,036
Trading account securities	688		995	643
Equity securities with readily determinable fair value	36,414		0	0
FHLB and FRB stock, at cost	99,998		89,989	87,192
Brokerage customer receivables	15,649		26,431	23,631
Mortgage loans held-for-sale, at fair value	338,111		313,592	370,282
MSRs	74,530	$ 63,194	33,676	29,414	$ 27,307	19,103
Accrued interest receivable and other	700,568		567,374	580,612
FHLB advances	615,000		559,663	468,962
Other borrowings	373,571		266,123	251,680
Subordinated notes	139,172		139,088	139,052
Junior subordinated debentures	253,566		253,566	253,566
Derivative liabilities	71,381		35,671	35,773
FDIC indemnification liability				15,472	$ 15,375	$ 16,701
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	279,993		277,591	251,952
Interest bearing deposits with banks	1,137,044		1,063,242	1,218,728
Available-for-sale securities	2,164,985		1,803,666	1,665,903
Held-to-maturity securities	966,438		826,449	819,340
Trading account securities	688		995	643
Equity securities with readily determinable fair value	36,414		0	0
FHLB and FRB stock, at cost	99,998		89,989	87,192
Brokerage customer receivables	15,649		26,431	23,631
Mortgage loans held-for-sale, at fair value	338,111		313,592	370,282
Loans held-for-investment, at fair value	77,883		33,717	29,704
Loans held-for-investment, at amortized cost	23,046,068		21,607,080	20,929,678
MSRs	74,530		33,676	29,414
Nonqualified deferred compensation assets	12,503		11,065	10,824
Derivative assets	96,153		52,069	48,198
Accrued interest receivable and other	254,879		227,649	225,435
Total financial assets	28,601,336		26,367,211	25,710,924
Non-maturity deposits	19,456,677		18,775,977	18,228,388
Deposits with stated maturities	5,460,038		4,407,370	4,666,675
FHLB advances	615,000		559,663	468,962
Other borrowings	373,571		266,123	251,680
Subordinated notes	139,172		139,088	139,052
Junior subordinated debentures	253,566		253,566	253,566
Derivative liabilities	71,381		35,671	35,773
FDIC indemnification liability	0		0	15,472
Accrued interest payable	15,374		8,030	9,177
Total financial liabilities	26,384,779		24,445,488	24,068,745
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	279,993		277,591	251,952
Interest bearing deposits with banks	1,137,044		1,063,242	1,218,728
Available-for-sale securities	2,164,985		1,803,666	1,665,903
Held-to-maturity securities	911,597		812,516	807,036
Trading account securities	688		995	643
Equity securities with readily determinable fair value	36,414		0	0
FHLB and FRB stock, at cost	99,998		89,989	87,192
Brokerage customer receivables	15,649		26,431	23,631
Mortgage loans held-for-sale, at fair value	338,111		313,592	370,282
Loans held-for-investment, at fair value	77,883		33,717	29,704
Loans held-for-investment, at amortized cost	23,261,545		21,768,978	21,064,801
MSRs	74,530		33,676	29,414
Nonqualified deferred compensation assets	12,503		11,065	10,824
Derivative assets	96,153		52,069	48,198
Accrued interest receivable and other	254,879		227,649	225,435
Total financial assets	28,761,972		26,515,176	25,833,743
Non-maturity deposits	19,456,677		18,775,977	18,228,388
Deposits with stated maturities	5,475,048		4,350,004	4,608,760
FHLB advances	615,342		544,750	454,753
Other borrowings	373,571		266,123	251,680
Subordinated notes	146,838		144,266	145,376
Junior subordinated debentures	258,488		264,696	240,305
Derivative liabilities	71,381		35,671	35,773
FDIC indemnification liability	0		0	15,472
Accrued interest payable	15,374		8,030	9,177
Total financial liabilities	$ 26,412,719		$ 24,389,517	$ 23,989,684