Estimated Carrying Values and Fair Values of Consolidated Financial Instruments (Detail) (USD $)	Jun. 30, 2011		Dec. 31, 2010
Assets
Investments (excluding equity method investments)	$ 4,726,900,000		$ 4,622,700,000
Liabilities
Senior Notes	298,979,000		298,923,000
Carrying Value
Assets
Investments (excluding equity method investments)	4,726,900,000	[1]	4,622,700,000	[1]
Liabilities
Senior Notes	299,000,000	[2]	298,900,000	[2]
Fair Value
Assets
Investments (excluding equity method investments)	4,726,900,000	[1]	4,622,700,000	[1]
Liabilities
Senior Notes	$ 308,900,000	[2]	$ 291,800,000	[2]

[1] This table includes available-for-sale investments (securities as well as partnership investments carried at fair value that are included in other invested assets). This table excludes investments accounted for using the equity method (Homesite, ORX and other investments) and certain loans receivable that are carried at cost, all of which are included in other invested assets. The fair value of short-term investments approximates amortized cost. The fair value of all other categories of investments is discussed below.

[2] See Note 7 to the Notes to the Consolidated Financial Statements set forth in Item 8 of the 2010 10-K.