Financial Instruments Measured at Fair Value and Level of Fair Value Hierarchy of Inputs (Detail) (USD $)	Sep. 30, 2014		Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale equities	$ 2,895,272,000		$ 2,229,453,000
Estimated fair value of available for sale debt securities	14,766,235,000		14,802,890,000
Estimated fair values of financial instruments	18,453,500,000		18,350,200,000
Investments (excluding equity method investments)	18,477,800,000		18,632,200,000
Senior Notes	2,271,400,000		1,887,700,000
Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale equities	2,895,300,000		2,229,400,000
U.S. Government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	377,700,000		955,000,000
Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	5,429,500,000		5,590,100,000
Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	910,500,000		975,400,000
U.S. corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	2,320,500,000		2,312,900,000
Foreign corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	1,613,700,000		1,831,700,000
Mortgage and asset backed securities | RMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	1,655,600,000	[1]	1,547,800,000	[1]
Mortgage and asset backed securities | CMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	1,135,900,000		885,600,000
Mortgage and asset backed securities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	1,322,800,000	[2]	704,400,000	[2]
Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	792,000,000		1,317,900,000
Other invested assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	24,300,000	[3]	282,000,000	[3]
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale equities	2,890,800,000		2,229,400,000
Investments (excluding equity method investments)	2,890,800,000		2,229,400,000
Level 1 | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale equities	2,890,800,000		2,229,400,000
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	13,695,400,000		14,376,400,000
Investments (excluding equity method investments)	14,487,400,000		15,694,300,000
Senior Notes	2,271,400,000		1,887,700,000
Level 2 | U.S. Government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	377,700,000		955,000,000
Level 2 | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	5,429,500,000		5,590,100,000
Level 2 | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	910,500,000		975,400,000
Level 2 | U.S. corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	2,274,200,000		2,285,400,000
Level 2 | Foreign corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	1,612,600,000		1,830,700,000
Level 2 | Mortgage and asset backed securities | RMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	1,590,700,000	[1]	1,469,000,000	[1]
Level 2 | Mortgage and asset backed securities | CMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	1,099,100,000		824,800,000
Level 2 | Mortgage and asset backed securities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	401,100,000	[2]	446,000,000	[2]
Level 2 | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	792,000,000		1,317,900,000
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale equities	4,500,000
Estimated fair value of available for sale debt securities	1,070,800,000		426,500,000
Investments (excluding equity method investments)	1,099,600,000		708,500,000
Level 3 | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale equities	4,500,000
Level 3 | U.S. corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	46,300,000		27,500,000
Level 3 | Foreign corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	1,100,000		1,000,000
Level 3 | Mortgage and asset backed securities | RMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	64,900,000	[1]	78,800,000	[1]
Level 3 | Mortgage and asset backed securities | CMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	36,800,000		60,800,000
Level 3 | Mortgage and asset backed securities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	921,700,000	[2]	258,400,000	[2]
Level 3 | Other invested assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	$ 24,300,000	[3]	$ 282,000,000	[3]

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

[2] Includes $887.5 million and $237.9 million of collateralized loan obligations as of September 30, 2014 and December 31, 2013, respectively.

[3] Includes partnership and non-marketable equity investments accounted for on an AFS basis, and excludes investments accounted for using the equity method and certain loans receivable that are carried at cost.