Financial Instruments Measured at Fair Value and Level of Fair Value Hierarchy of Inputs (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	$ 17,872.5		$ 17,789.5
Investments (excluding equity method investments)	18,015.4		17,831.8
Senior Notes	1,984.2		1,946.7
Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	2,004.9		1,424.0
Equity Securities | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	2,004.9	[1]	1,424.0	[1]
Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	15,110.5		15,999.5
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	522.1		522.9
Debt Securities | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	6,321.8		6,304.1
Debt Securities | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	821.1		816.0
Debt Securities | U.S. corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	2,842.5		3,515.7
Debt Securities | Foreign corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	1,950.1		2,198.5
Debt Securities | Mortgage and asset backed securities | RMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	1,611.2	[2]	1,662.5	[2]
Debt Securities | Mortgage and asset backed securities | CMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	627.6		510.1
Debt Securities | Mortgage and asset backed securities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	414.1		469.7
Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	757.1		366.0
Other invested assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	142.9	[3]	42.3	[3]
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments (excluding equity method investments)	2,004.9		1,424.0
Level 1 | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	2,004.9		1,424.0
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	2,004.9	[1]	1,424.0	[1]
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments (excluding equity method investments)	15,721.6		16,193.5
Senior Notes	1,984.2		1,946.7
Level 2 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	14,964.5		15,827.5
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	522.1		522.9
Level 2 | Debt Securities | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	6,321.8		6,304.1
Level 2 | Debt Securities | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	821.1		816.0
Level 2 | Debt Securities | U.S. corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	2,836.4		3,485.3
Level 2 | Debt Securities | Foreign corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	1,950.1		2,198.5
Level 2 | Debt Securities | Mortgage and asset backed securities | RMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	1,549.3	[2]	1,602.9	[2]
Level 2 | Debt Securities | Mortgage and asset backed securities | CMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	553.7		434.0
Level 2 | Debt Securities | Mortgage and asset backed securities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	410.0		463.8
Level 2 | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	757.1		366.0
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments (excluding equity method investments)	288.9		214.3
Level 3 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	146.0		172.0
Level 3 | Debt Securities | U.S. corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	6.1		30.4
Level 3 | Debt Securities | Mortgage and asset backed securities | RMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	61.9	[2]	59.6	[2]
Level 3 | Debt Securities | Mortgage and asset backed securities | CMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	73.9		76.1
Level 3 | Debt Securities | Mortgage and asset backed securities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	4.1		5.9
Level 3 | Other invested assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	$ 142.9	[3]	$ 42.3	[3]

[1] Of the $2,004.9 million and $1,424.0 million of fair value as of March 31, 2013 and December 31, 2012, respectively, $775.0 million and $542.2 million, respectively, related to certain energy sector businesses.

[2] Primarily includes government agency pass-through securities guaranteed by a government agency or government sponsored enterprise, among other types of RMBS.

[3] Includes partnership and non-marketable equity investments accounted for on an AFS basis.