Financial Instruments Measured at Fair Value and Level of Fair Value Hierarchy of Inputs (Detail) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale equities		$ 4,099,467	$ 3,109,523
Estimated fair value of available for sale debt securities		12,721,399	12,983,213
Estimated fair values of financial instruments		17,399,000	16,871,100
Total investments (excluding equity method investments and loans)	[1]	17,406,500	16,899,200
Senior Notes and other debt	[2]	1,614,600	1,584,300
Equity Securities | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale equities		4,094,500	3,109,500
Equity Securities | Preferred Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale equities		5,000
Debt Securities | 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		948,000	1,243,300
Debt Securities | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		3,682,100	4,185,800
Debt Securities | 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		1,006,600	1,047,100
Debt Securities | 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		2,433,000	2,193,100
Debt Securities | 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		1,499,800	1,088,800
Debt Securities | 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	[3]	995,600	1,000,400
Debt Securities | 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		551,700	734,800
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 value of available for sale debt securities	[4]	1,604,600	1,489,900
Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments		578,100	778,400
Other invested assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	[5]	7,500	28,100
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale equities		4,090,700	3,105,200
Total investments (excluding equity method investments and loans)		4,090,700	3,105,200
Level 1 | Equity Securities | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale equities		4,090,700	3,105,200
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale equities		6,900
Estimated fair value of available for sale debt securities		11,121,500	11,995,900
Total investments (excluding equity method investments and loans)		11,706,500	12,774,300
Senior Notes and other debt		1,513,600	1,491,500
Level 2 | Equity Securities | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale equities		3,800
Level 2 | Equity Securities | Preferred Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale equities		3,100
Level 2 | Debt Securities | 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		948,000	1,243,300
Level 2 | Debt Securities | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		3,682,100	4,185,800
Level 2 | Debt Securities | 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		1,006,600	1,047,100
Level 2 | Debt Securities | 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		2,173,000	2,120,200
Level 2 | Debt Securities | 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		1,424,600	1,088,400
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 value of available for sale debt securities	[3]	833,800	994,500
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 value of available for sale debt securities		550,100	730,500
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 value of available for sale debt securities	[4]	503,300	586,100
Level 2 | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments		578,100	778,400
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale equities		1,900	4,300
Estimated fair value of available for sale debt securities		1,599,900	987,300
Total investments (excluding equity method investments and loans)		1,609,300	1,019,700
Senior Notes and other debt		101,000	92,800
Level 3 | Equity Securities | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale equities			4,300
Level 3 | Equity Securities | Preferred Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale equities		1,900
Level 3 | Debt Securities | 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		260,000	72,900
Level 3 | Debt Securities | 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		75,200	400
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 value of available for sale debt securities	[3]	161,800	5,900
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 value of available for sale debt securities		1,600	4,300
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 value of available for sale debt securities	[4]	1,101,300	903,800
Level 3 | Other invested assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	[5]	$ 7,500	$ 28,100

[1] This table includes AFS investments (debt and equity securities, as well as partnership and non-marketable equity investments carried at fair value that are included in other invested assets). This table excludes investments accounted for using the equity method and commercial mortgage loans that are carried at unpaid principal balance. 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 8 for additional information on the Senior Notes and other debt.

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

[4] Includes $1,101.3 million and $903.8 million of collateralized loan obligations as of December 31, 2017 and 2016, respectively.

[5] Includes partnership and non-marketable equity investments accounted for on an AFS basis, and excludes investments accounted for using the equity method.