Financial Instruments Measured at Fair Value and Level of Fair Value Hierarchy of Inputs (Detail) - USD ($) $ in Thousands		Sep. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of equities		$ 2,092,267	$ 3,572,790
Estimated fair value of available for sale debt securities		14,488,289	11,823,968
Estimated fair value of investments (excluding equity method investments and loans)	[1]	17,582,000	16,291,300
Senior Notes and other debt	[2]	1,908,900	1,795,500
Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of equities		2,088,200	3,567,400
Preferred Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of equities		4,100	5,400
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		1,288,200	1,022,400
Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		2,348,700	2,214,700
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		756,900	947,900
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		3,326,000	2,385,300
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		1,454,100	1,353,300
RMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	[3]	1,935,700	1,387,900
CMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		821,500	533,300
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]	2,557,200	1,979,200
Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of investments (excluding equity method investments and loans)		968,500	893,800
Other invested assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of investments (excluding equity method investments and loans)	[5]	32,900	700
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of equities		2,084,900	3,563,900
Estimated fair value of investments (excluding equity method investments and loans)		2,117,500	3,563,900
Level 1 | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of equities		2,084,900	3,563,900
Level 1 | Other invested assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of investments (excluding equity method investments and loans)	[5]	32,600
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of equities		3,300	3,500
Estimated fair value of available for sale debt securities		12,893,300	10,004,500
Estimated fair value of investments (excluding equity method investments and loans)		13,865,100	10,901,800
Senior Notes and other debt		1,597,600	1,510,500
Level 2 | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of equities		3,300	3,500
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		1,288,200	1,022,400
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		2,348,700	2,214,700
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		756,900	947,900
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		2,754,700	1,959,600
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		1,298,000	1,226,400
Level 2 | RMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	[3]	1,934,000	1,387,900
Level 2 | CMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		821,500	533,300
Level 2 | 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]	1,691,300	712,300
Level 2 | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of investments (excluding equity method investments and loans)		968,500	893,800
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of equities		4,100	5,400
Estimated fair value of available for sale debt securities		1,595,000	1,819,500
Estimated fair value of investments (excluding equity method investments and loans)		1,599,400	1,825,600
Senior Notes and other debt		311,300	285,000
Level 3 | Preferred Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of equities		4,100	5,400
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		571,300	425,700
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		156,100	126,900
Level 3 | RMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	[3]	1,700
Level 3 | 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]	865,900	1,266,900
Level 3 | Other invested assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of investments (excluding equity method investments and loans)	[5]	$ 300	$ 700

[1] This table includes debt and equity securities, as well as partnership and non-marketable equity investments and derivatives accounted for 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 accounted for at unpaid principal balance. The fair value of short-term investments approximates amortized cost. The fair value of all other categories of investments is disclosed below.

[2] See Note 8 to Notes to Consolidated Financial Statements set forth in Part II, Item 8, “Financial Statements and Supplementary Data” of the 2018 Form 10-K 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 $842.1 million and $1,266.9 million of collateralized loan obligations as of September 30, 2019 and December 31, 2018, respectively.

[5] Includes partnership and non-marketable equity investments accounted for at fair value, and excludes investments accounted for using the equity method. Also, as further described in Note 3(d), other invested assets as of September 30, 2019 includes the fair value of an exchange-traded equity derivative index put option (the “Put Option”), which is classified as Level 1.