Summary of Estimated Fair Values of Financial Instruments that are not Carried at Fair Value (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net loans	$ 6,880,135		$ 5,439,110
Other short-term borrowings			37,245
Deposits	16,709,536		14,336,941
Other long-term debt	1,439	[1]	5,257	[1]
5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	347,793		347,601
5.70% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	143,969	[2]	265,613	[2]
6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	55,075	[3]	285,937	[3]
3.875% Convertible Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Convertible Notes			249,304
7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior subordinated debentures	55,372		55,548
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Non-marketable securities (cost and equity method accounting)	267,508		217,449
Net loans	6,880,135		5,439,110
Other short-term borrowings			37,245
Deposits	16,709,536		14,336,941
Other long-term debt	1,439		5,257
Carrying Amount | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	347,793		347,601
Carrying Amount | 5.70% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	143,969	[4]	265,613	[4]
Carrying Amount | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	55,075	[5]	285,937	[5]
Carrying Amount | 3.875% Convertible Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Convertible Notes			249,304
Carrying Amount | 7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior subordinated debentures	55,372		55,548
Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Non-marketable securities (cost and equity method accounting)	290,393		230,158
Net loans	6,964,438		5,466,252
Other short-term borrowings			37,245
Deposits	16,709,133		14,334,013
Other long-term debt	1,439		5,257
Commitments to extend credit	21,232		19,264
Estimated Fair Value | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	362,786		344,498
Estimated Fair Value | 5.70% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	145,184	[4]	277,301	[4]
Estimated Fair Value | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	57,746	[5]	298,101	[5]
Estimated Fair Value | 3.875% Convertible Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Convertible Notes			276,825
Estimated Fair Value | 7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior subordinated debentures	$ 51,526		$ 49,485

[1] Represents long-term notes payable related to one of our debt fund investments, and was payable beginning April 30, 2009 with the last payment due in April 2012.

[2] At December 31, 2011 and 2010, included in the carrying value of our 5.70% Senior Notes are $2.6 million and $15.7 million, respectively, related to the fair value of the interest rate swap associated with the notes.

[3] At December 31, 2011 and 2010, included in the carrying value of our 6.05% Subordinated Notes are $8.8 million and $36.3 million, respectively, related to the fair value of the interest rate swap associated with the notes.

[4] At December 31, 2011 and December 31, 2010, included in the carrying value and estimated fair value of our 5.70% Senior Notes are $2.6 million and $15.7 million, respectively, related to the fair value of the interest rate swaps associated with the notes.

[5] At December 31, 2011, and December 31, 2010, included in the carrying value and estimated fair value of our 6.05% Subordinated Notes, are $8.8 million and $36.3 million, respectively, related to the fair value of the interest rate swaps associated with the notes.