Fair Values of Assets and Liabilities (Summary Of Carrying Amounts And Estimated Fair Values Of Financial Instruments) (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 304,642		$ 277,591	$ 296,161		$ 270,045
Interest bearing deposits with banks	1,221,407		1,063,242	1,011,635
Available-for-sale securities	1,940,787		1,803,666	1,649,636
Held-to-maturity securities	851,842		812,516	787,489
Trading account securities	862		995	1,987
Equity securities with readily determinable fair value	37,839		0	0
FHLB and FRB stock, at cost	96,699		89,989	80,812
Brokerage customer receivables	16,649		26,431	23,281
Mortgage loans held-for-sale, at fair value	455,712		313,592	382,837
MSRs	63,194	$ 54,572	33,676	27,307	$ 21,596	19,103
Accrued interest receivable and other	666,673		567,374	577,359
FHLB advances	667,000		559,663	318,270
Other borrowings	255,701		266,123	277,710
Subordinated notes	139,148		139,088	139,029
Junior subordinated debentures	253,566		253,566	253,566
Derivative liabilities	69,364		35,671	37,651
FDIC indemnification liability				15,375	$ 18,263	$ 16,701
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	304,642		277,591	296,161
Interest bearing deposits with banks	1,221,407		1,063,242	1,011,635
Available-for-sale securities	1,940,787		1,803,666	1,649,636
Held-to-maturity securities	890,834		826,449	793,376
Trading account securities	862		995	1,987
Equity securities with readily determinable fair value	37,839		0	0
FHLB and FRB stock, at cost	96,699		89,989	80,812
Brokerage customer receivables	16,649		26,431	23,281
Mortgage loans held-for-sale, at fair value	455,712		313,592	382,837
Loans held-for-investment, at fair value	67,181		33,717	30,173
Loans held-for-investment, at amortized cost	22,543,379		21,607,080	20,763,278
MSRs	63,194		33,676	27,307
Nonqualified deferred compensation assets	12,053		11,065	10,556
Derivative assets	92,156		52,069	49,703
Accrued interest receivable and other	240,790		227,649	215,291
Total financial assets	27,984,184		26,367,211	25,336,033
Non-maturity deposits	19,465,510		18,775,977	18,086,300
Deposits with stated maturities	4,899,969		4,407,370	4,519,392
FHLB advances	667,000		559,663	318,270
Other borrowings	255,701		266,123	277,710
Subordinated notes	139,148		139,088	139,029
Junior subordinated debentures	253,566		253,566	253,566
Derivative liabilities	69,364		35,671	37,651
FDIC indemnification liability	0		0	15,375
Accrued interest payable	9,802		8,030	6,460
Total financial liabilities	25,760,060		24,445,488	23,653,753
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	304,642		277,591	296,161
Interest bearing deposits with banks	1,221,407		1,063,242	1,011,635
Available-for-sale securities	1,940,787		1,803,666	1,649,636
Held-to-maturity securities	851,842		812,516	787,489
Trading account securities	862		995	1,987
Equity securities with readily determinable fair value	37,839		0	0
FHLB and FRB stock, at cost	96,699		89,989	80,812
Brokerage customer receivables	16,649		26,431	23,281
Mortgage loans held-for-sale, at fair value	455,712		313,592	382,837
Loans held-for-investment, at fair value	67,181		33,717	30,173
Loans held-for-investment, at amortized cost	22,767,838		21,768,978	21,921,002
MSRs	63,194		33,676	27,307
Nonqualified deferred compensation assets	12,053		11,065	10,556
Derivative assets	92,156		52,069	49,703
Accrued interest receivable and other	240,790		227,649	215,291
Total financial assets	28,169,651		26,515,176	26,487,870
Non-maturity deposits	19,465,510		18,775,977	18,086,300
Deposits with stated maturities	4,911,324		4,350,004	4,503,645
FHLB advances	669,406		544,750	316,799
Other borrowings	255,701		266,123	277,710
Subordinated notes	137,782		144,266	143,126
Junior subordinated debentures	257,465		264,696	253,330
Derivative liabilities	69,364		35,671	37,651
FDIC indemnification liability	0		0	15,375
Accrued interest payable	9,802		8,030	6,460
Total financial liabilities	$ 25,776,354		$ 24,389,517	$ 23,640,396