Summary of Estimated Fair Values of Financial Instruments that are not Carried at Fair Value (Detail) (USD $) In Thousands	Sep. 30, 2011		Dec. 31, 2010
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net loans	$ 6,243,342		$ 5,439,110
Other short-term borrowings			37,245
Deposits	16,139,222		14,336,941
3.875% Convertible Notes | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Convertible notes			249,304
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Non-marketable securities (cost and equity method accounting)	252,704		217,449
Net loans	6,243,342		5,439,110
Other short-term borrowings			37,245
Deposits	16,139,222		14,336,941
Carrying Amount | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	347,744		347,601
Carrying Amount | 5.70% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	145,632	[1]	265,613	[1]
Carrying Amount | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	55,302	[2]	285,937	[2]
Carrying Amount | 7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior subordinated debentures	55,416		55,548
Carrying Amount | 4.99% long-term notes payable
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other long-term debt	5,463		5,257
3.875% Convertible Notes | Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Convertible notes			276,825
Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Non-marketable securities (cost and equity method accounting)	273,991		230,158
Net loans	6,302,330		5,466,252
Other short-term borrowings			37,245
Deposits	16,139,059		14,334,013
Commitments to extend credit	20,252		19,264
Estimated Fair Value | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	363,763		344,498
Estimated Fair Value | 5.70% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	149,695	[1]	277,301	[1]
Estimated Fair Value | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	58,916	[2]	298,101	[2]
Estimated Fair Value | 7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior subordinated debentures	50,722		49,485
Estimated Fair Value | 4.99% long-term notes payable
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other long-term debt	5,463		5,257
5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	347,744		347,601
5.70% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	145,632	[3]	265,613	[3]
6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	55,302	[4]	285,937	[4]
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	55,416		55,548
4.99% long-term notes payable
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other long-term debt	$ 5,463	[5]	$ 5,257	[5]

[1] At September 30, 2011 and December 31, 2010, included in the carrying value and estimated fair value of our 5.70% Senior Notes are $4.2 million and $15.7 million, respectively, related to the fair value of the interest rate swaps associated with the notes.

[2] At September 30, 2011 and December 31, 2010, included in the carrying value and estimated fair value of our 6.05% Subordinated Notes are $9.1 million and $36.3 million, respectively, related to the fair value of the interest rate swaps associated with the notes.

[3] At September 30, 2011 and December 31, 2010, included in the carrying value of our 5.70% Senior Notes are $4.2 million and $15.7 million, respectively, related to the fair value of the interest rate swap associated with the notes.

[4] At September 30, 2011 and December 31, 2010, included in the carrying value of our 6.05% Subordinated Notes are $9.1 million and $36.3 million, respectively, related to the fair value of the interest rate swap associated with the notes.

[5] 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.