Summary of Estimated Fair Values of Financial Instruments not Carried at Fair Value (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	$ 508,170		$ 0
Other long-term debt	0	[1]	1,439	[1]
Federal Home Loan Bank Advances
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	215,000		0
Federal Funds Purchased
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	287,000		0
Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	6,170	[2]	0	[2]
5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	347,944		347,793
5.70% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	0		143,969	[3]
6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	55,130	[4]	55,075	[4]
7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior Subordinated Debentures	55,240		55,372
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	906,680		1,114,948
Non-marketable securities (cost and equity method accounting)	386,784		267,508
Net commercial loans	7,345,239		6,192,578
Net consumer loans	745,606		687,557
FHLB and FRB stock	39,301		39,189
Accrued interest receivable	62,441		58,108
Deposits	17,575,816		16,553,787
Time Deposits	149,250		155,749
Accrued interest payable	1,938		6,689
Commitments to extend credit	0		0
Other long-term debt			1,439
Carrying Amount | Federal Home Loan Bank Advances
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	215,000
Carrying Amount | Federal Funds Purchased
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	287,000
Carrying Amount | Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	6,170
Carrying Amount | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	347,944		347,793
Carrying Amount | 5.70% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes			143,969
Carrying Amount | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	55,130		55,075
Carrying Amount | 7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior Subordinated Debentures	55,240		55,372
Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	906,680		1,114,948
Non-marketable securities (cost and equity method accounting)	0		0
Net commercial loans	0		0
Net consumer loans	0		0
FHLB and FRB stock	0		0
Accrued interest receivable	0		0
Deposits	17,575,816		16,553,787
Time Deposits	0		0
Accrued interest payable	0		0
Commitments to extend credit	0		0
Other long-term debt			0
Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1) | Federal Home Loan Bank Advances
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	215,000
Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1) | Federal Funds Purchased
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	287,000
Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	6,170
Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1) | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	0		0
Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1) | 5.70% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes			0
Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1) | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	0		0
Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1) | 7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior Subordinated Debentures	0		0
Estimated Fair Value | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0		0
Non-marketable securities (cost and equity method accounting)	0		0
Net commercial loans	0		0
Net consumer loans	0		0
FHLB and FRB stock	0		0
Accrued interest receivable	62,441		58,108
Deposits	0		0
Time Deposits	149,031		155,346
Accrued interest payable	1,938		6,689
Commitments to extend credit	0		0
Other long-term debt			0
Estimated Fair Value | Significant Other Observable Inputs (Level 2) | Federal Home Loan Bank Advances
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	0
Estimated Fair Value | Significant Other Observable Inputs (Level 2) | Federal Funds Purchased
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	0
Estimated Fair Value | Significant Other Observable Inputs (Level 2) | Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	0
Estimated Fair Value | Significant Other Observable Inputs (Level 2) | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	398,220		362,786
Estimated Fair Value | Significant Other Observable Inputs (Level 2) | 5.70% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes			145,184
Estimated Fair Value | Significant Other Observable Inputs (Level 2) | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	62,486		57,746
Estimated Fair Value | Significant Other Observable Inputs (Level 2) | 7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior Subordinated Debentures	52,268		51,526
Estimated Fair Value | Significant Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0		0
Non-marketable securities (cost and equity method accounting)	421,123		290,393
Net commercial loans	7,542,349		6,336,705
Net consumer loans	766,903		627,733
FHLB and FRB stock	39,301		39,189
Accrued interest receivable	0		0
Deposits	0		0
Time Deposits	0		0
Accrued interest payable	0		0
Commitments to extend credit	21,467		21,232
Other long-term debt			1,439
Estimated Fair Value | Significant Unobservable Inputs (Level 3) | Federal Home Loan Bank Advances
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	0
Estimated Fair Value | Significant Unobservable Inputs (Level 3) | Federal Funds Purchased
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	0
Estimated Fair Value | Significant Unobservable Inputs (Level 3) | Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	0
Estimated Fair Value | Significant Unobservable Inputs (Level 3) | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	0		0
Estimated Fair Value | Significant Unobservable Inputs (Level 3) | 5.70% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes			0
Estimated Fair Value | Significant Unobservable Inputs (Level 3) | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	0		0
Estimated Fair Value | Significant Unobservable Inputs (Level 3) | 7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior Subordinated Debentures	$ 0		$ 0

[1] Represents long-term notes payable related to one of our debt fund investments. The last payment related to the notes was made in April 2012.

[2] Represents cash collateral received from our counterparty for our interest rate swap agreement related to our 6.05% Subordinated Notes.

[3] At December 31, 2011, included in the carrying value of our 5.70% Senior Notes was $2.6 million related to the fair value of the interest rate swap associated with the notes.

[4] At September 30, 2012 and December 31, 2011, included in the carrying value of our 6.05% Subordinated Notes were $9.5 million and $8.8 million, respectively, related to the fair value of the interest rate swap associated with the notes.