Fair Values Of Assets And Liabilities (Summary Of Carrying Amounts And Estimated Fair Values Of Financial Instruments) (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest bearing deposits with banks	$ 697,799,000	$ 998,437,000	$ 540,964,000
Available-for-sale securities, at fair value	1,721,030,000	1,792,078,000	1,949,697,000
Trading account securities	7,811,000	1,206,000	1,068,000
Federal Home Loan Bank and Federal Reserve Bank stock, at cost	92,948,000	91,582,000	78,524,000
Brokerage customer receivables	25,287,000	24,221,000	26,884,000
Derivative assets	45,987,000	35,070,000	29,880,000
FDIC indemnification asset	10,224,000	11,846,000	60,298,000	85,672,000
Accrued interest receivable and other	537,117,000	501,882,000	549,705,000
Federal Home Loan Bank advances	416,036,000	733,050,000	387,672,000
Other borrowings	187,006,000	196,465,000	231,086,000
Subordinated notes	140,000,000	140,000,000	0
Junior subordinated debentures	249,493,000	249,493,000	249,493,000
Derivative liabilities	47,698,000	36,921,000	34,990,000
Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	290,872,000	230,707,000	342,738,000
Interest bearing deposits with banks	697,799,000	998,437,000	540,964,000
Available-for-sale securities, at fair value	1,721,030,000	1,792,078,000	1,949,697,000
Trading account securities	7,811,000	1,206,000	1,068,000
Federal Home Loan Bank and Federal Reserve Bank stock, at cost	92,948,000	91,582,000	78,524,000
Brokerage customer receivables	25,287,000	24,221,000	26,884,000
Mortgage loans held-for-sale, at fair value	446,355,000	351,290,000	215,231,000
Total loans	15,162,753,000	14,636,107,000	13,445,638,000
Mortgage servicing rights	7,852,000	8,435,000	8,719,000
Nonqualified deferred compensation assets	8,718,000	7,951,000	7,783,000
Derivative assets	62,984,000	47,964,000	51,529,000
FDIC indemnification asset	10,224,000	11,846,000	60,298,000
Accrued interest receivable and other	181,998,000	169,156,000	169,580,000
Total financial assets	18,716,631,000	18,370,980,000	16,898,653,000
Non-maturity deposits	12,927,014,000	12,142,034,000	10,944,521,000
Deposits with stated maturities	4,011,755,000	4,139,810,000	4,184,524,000
Federal Home Loan Bank advances	416,036,000	733,050,000	387,672,000
Other borrowings	187,006,000	196,465,000	231,086,000
Subordinated notes	140,000,000	140,000,000	0
Junior subordinated debentures	249,493,000	249,493,000	249,493,000
Derivative liabilities	55,225,000	41,180,000	37,797,000
Accrued interest payable	8,583,000	8,001,000	7,218,000
Total financial liabilities	17,995,112,000	17,650,033,000	16,042,311,000
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	290,872,000	230,707,000	342,738,000
Interest bearing deposits with banks	697,799,000	998,437,000	540,964,000
Available-for-sale securities, at fair value	1,721,030,000	1,792,078,000	1,949,697,000
Trading account securities	7,811,000	1,206,000	1,068,000
Federal Home Loan Bank and Federal Reserve Bank stock, at cost	92,948,000	91,582,000	78,524,000
Brokerage customer receivables	25,287,000	24,221,000	26,884,000
Mortgage loans held-for-sale, at fair value	446,355,000	351,290,000	215,231,000
Total loans	15,868,532,000	15,346,266,000	14,078,788,000
Mortgage servicing rights	7,852,000	8,435,000	8,719,000
Nonqualified deferred compensation assets	8,718,000	7,951,000	7,783,000
Derivative assets	62,984,000	47,964,000	51,529,000
FDIC indemnification asset	10,224,000	11,846,000	60,298,000
Accrued interest receivable and other	181,998,000	169,156,000	169,580,000
Total financial assets	19,422,410,000	19,081,139,000	17,531,803,000
Non-maturity deposits	12,927,014,000	12,142,034,000	10,944,521,000
Deposits with stated maturities	4,017,565,000	4,143,161,000	4,197,918,000
Federal Home Loan Bank advances	422,305,000	738,113,000	393,145,000
Other borrowings	187,006,000	197,883,000	231,086,000
Subordinated notes	147,851,000	143,639,000	0
Junior subordinated debentures	250,196,000	250,305,000	250,578,000
Derivative liabilities	55,225,000	41,180,000	37,797,000
Accrued interest payable	8,583,000	8,001,000	7,218,000
Total financial liabilities	$ 18,015,745,000	$ 17,664,316,000	$ 16,062,263,000