Financial Instruments Measured at Fair Value and Level of Fair Value Hierarchy of Inputs (Detail) - USD ($) $ in Thousands		Sep. 30, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale equities		$ 3,012,070	$ 2,815,484
Estimated fair value of available for sale debt securities		14,530,255	14,598,641
Estimated fair values of financial instruments		17,904,400	18,129,700
Investments (excluding equity method investments)		17,935,900	18,153,800
Senior Notes		1,888,500	1,948,600
Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments		362,000	715,600
Other invested assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	[1]	31,500	24,100
Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale equities		3,012,100	2,815,500
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		1,071,500	541,100
Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		4,766,300	5,197,500
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		919,300	900,400
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		2,175,400	2,154,800
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		1,316,000	1,503,700
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	[2]	1,491,200	1,655,900
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		1,137,900	1,125,300
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	[3]	1,652,700	1,519,900
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale equities		3,006,000	2,805,300
Investments (excluding equity method investments)		3,006,000	2,805,300
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		3,006,000	2,805,300
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale equities		6,100	10,200
Estimated fair value of available for sale debt securities		13,467,900	13,581,300
Investments (excluding equity method investments)		13,836,000	14,307,100
Senior Notes		1,888,500	1,948,600
Level 2 | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments		362,000	715,600
Level 2 | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale equities		6,100	10,200
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		1,071,500	541,100
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		4,766,300	5,197,500
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		919,300	900,400
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		2,123,800	2,118,100
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		1,316,000	1,497,700
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	[2]	1,475,300	1,637,700
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		1,117,000	1,102,000
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	[3]	678,700	586,800
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		1,062,400	1,017,300
Investments (excluding equity method investments)		1,093,900	1,041,400
Level 3 | Other invested assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	[1]	31,500	24,100
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		51,600	36,700
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			6,000
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	[2]	15,900	18,200
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		20,900	23,300
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	[3]	$ 974,000	$ 933,100

[1] 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.

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

[3] Includes $952.4 million and $900.7 million of collateralized loan obligations as of September 30, 2015 and December 31, 2014, respectively.