Fair Values Of Assets And Liabilities (Summary Of Carrying Amounts And Estimated Fair Values Of Financial Instruments) (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest bearing deposits with banks	$ 701,106		$ 998,437	$ 620,370
Available-for-sale securities, at fair value	2,214,281		1,792,078	1,782,648
Trading account securities	3,312		1,206	6,015
Federal Home Loan Bank and Federal Reserve Bank stock, at cost	90,308		91,582	80,951
Brokerage customer receivables	28,293		24,221	26,624
Mortgage loans held-for-sale, at fair value	347,005		351,290	363,303
Derivative assets	56,324		35,070	27,858
FDIC indemnification asset	0	$ 3,429	11,846	27,359	$ 46,115	$ 85,672
Accrued interest receivable and other	667,036		501,882	494,213
Federal Home Loan Bank advances	451,330		733,050	347,500
Other borrowings	259,978		196,465	51,483
Subordinated notes	140,000		140,000	140,000
Junior subordinated debentures	268,566		249,493	249,493
Derivative liabilities	57,429		36,921	31,451
Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	250,655		230,707	287,416
Interest bearing deposits with banks	701,106		998,437	620,370
Available-for-sale securities, at fair value	2,214,281		1,792,078	1,782,648
Trading account securities	3,312		1,206	6,015
Federal Home Loan Bank and Federal Reserve Bank stock, at cost	90,308		91,582	80,951
Brokerage customer receivables	28,293		24,221	26,624
Mortgage loans held-for-sale, at fair value	347,005		351,290	363,303
Total loans	16,484,820		14,636,107	14,306,664
Mortgage servicing rights	7,875		8,435	8,137
Nonqualified deferred compensation assets	8,342		7,951	7,927
Derivative assets	69,034		47,964	43,343
FDIC indemnification asset	0		11,846	27,359
Accrued interest receivable and other	192,572		169,156	170,517
Total financial assets	20,397,603		18,370,980	17,731,274
Non-maturity deposits	14,092,697		12,142,034	11,761,825
Deposits with stated maturities	4,135,772		4,139,810	4,303,421
Federal Home Loan Bank advances	451,330		733,050	347,500
Other borrowings	259,978		196,465	51,483
Subordinated notes	140,000		140,000	140,000
Junior subordinated debentures	268,566		249,493	249,493
Derivative liabilities	65,198		41,180	35,505
Accrued interest payable	11,364		8,001	8,995
Total financial liabilities	19,424,905		17,650,033	16,898,222
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	250,655		230,707	287,416
Interest bearing deposits with banks	701,106		998,437	620,370
Available-for-sale securities, at fair value	2,214,281		1,792,078	1,782,648
Trading account securities	3,312		1,206	6,015
Federal Home Loan Bank and Federal Reserve Bank stock, at cost	90,308		91,582	80,951
Brokerage customer receivables	28,293		24,221	26,624
Mortgage loans held-for-sale, at fair value	347,005		351,290	363,303
Total loans	17,284,375		15,346,266	15,001,394
Mortgage servicing rights	7,875		8,435	8,137
Nonqualified deferred compensation assets	8,342		7,951	7,927
Derivative assets	69,034		47,964	43,343
FDIC indemnification asset	0		11,846	27,359
Accrued interest receivable and other	192,572		169,156	170,517
Total financial assets	21,197,158		19,081,139	18,426,004
Non-maturity deposits	14,092,697		12,142,034	11,761,825
Deposits with stated maturities	4,137,856		4,143,161	4,303,717
Federal Home Loan Bank advances	459,154		738,113	352,516
Other borrowings	259,978		197,883	51,483
Subordinated notes	142,953		143,639	142,720
Junior subordinated debentures	268,058		250,305	250,452
Derivative liabilities	65,198		41,180	35,505
Accrued interest payable	11,364		8,001	8,995
Total financial liabilities	$ 19,437,258		$ 17,664,316	$ 16,907,213