Estimated Fair Values of Financial Instruments (Detail) (USD $)	Dec. 31, 2011		Dec. 31, 2010
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	$ 4,647,000,000		$ 4,597,900,000
Investments (excluding equity method investments)	4,670,600,000		4,622,700,000
Senior Notes	299,035,000		298,923,000
Mortgage and asset backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	860,500,000
Equity Securities | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	871,000,000	[1]	1,500,700,000	[1]
Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	2,679,500,000		2,832,400,000
Debt Securities | U.S. Government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	267,800,000		337,800,000
Debt Securities | Mortgage and asset backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	860,500,000	[2],[3]	866,500,000	[2],[3]
Debt Securities | States, municipalities and political subdivision bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	1,113,600,000		1,068,500,000
Debt Securities | Foreign bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	83,500,000		114,200,000
Debt Securities | Corporate bonds and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	354,100,000		445,400,000
Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	1,096,500,000		264,800,000
Partnership Investment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets	23,600,000	[4]	24,800,000	[4]
Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	314,800,000	[5]	291,800,000	[5]
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments (excluding equity method investments)	1,193,100,000		1,894,400,000
Level 1 | Equity Securities | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	871,000,000	[1]	1,500,700,000	[1]
Level 1 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	267,800,000		307,300,000
Level 1 | Debt Securities | U.S. Government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	267,800,000		307,300,000
Level 1 | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	54,300,000		86,400,000
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments (excluding equity method investments)	3,453,900,000		2,703,500,000
Level 2 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	2,411,700,000		2,525,100,000
Level 2 | Debt Securities | U.S. Government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments			30,500,000
Level 2 | Debt Securities | Mortgage and asset backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	860,500,000	[3]	866,500,000	[3]
Level 2 | Debt Securities | States, municipalities and political subdivision bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	1,113,600,000		1,068,500,000
Level 2 | Debt Securities | Foreign bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	83,500,000		114,200,000
Level 2 | Debt Securities | Corporate bonds and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	354,100,000		445,400,000
Level 2 | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	1,042,200,000		178,400,000
Level 2 | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	314,800,000		291,800,000
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments (excluding equity method investments)	23,600,000		24,800,000
Level 3 | Partnership Investment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets	$ 23,600,000	[4]	$ 24,800,000	[4]

[1] Of the $871.0 million of fair value as of December 31, 2011, $573.3 million related to certain energy sector businesses. Of the $1,500.7 million of fair value as of December 31, 2010, $1,004.8 million related to certain energy sector businesses.

[2] Of the $860.5 million of fair value as of December 31, 2011, $497.3 million related to residential mortgage-backed securities, or "RMBS," $144.7 million related to commercial mortgage-backed securities, or "CMBS" and $218.5 million related to other asset-backed securities. Of the $866.5 million of fair value as of December 31, 2010, $499.9 million related to RMBS, $173.4 million related to CMBS and $193.2 million related to other asset-backed securities.

[3] Of the $860.5 million of fair value as of December 31, 2011, $497.3 million related to RMBS, $144.7 million related to CMBS and $218.5 million related to other asset-backed securities. Of the $866.5 million of fair value as of December 31, 2010, $499.9 million related to RMBS, $173.4 million related to CMBS and $193.2 million related to other asset-backed securities.

[4] Level 3 securities consist of partnership investments. The carrying value of partnership investments of $23.6 million as of December 31, 2011 decreased by $1.2 million from the December 31, 2010 carrying value of $24.8 million, due primarily to net return of capital during 2011.

[5] See Note 7.