Financial Instruments at Fair Value and Level of Fair Value Hierarchy of Inputs (Detail) - USD ($) $ in Thousands		Sep. 30, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of equities		$ 3,471,705	$ 2,718,902
Estimated fair value of available for sale debt securities		15,884,928	15,618,470
Estimated fair value of investments (excluding equity method investments and loans)	[1]	20,417,000	19,051,900
Senior notes and other debt	[2]	2,850,400	2,468,700
Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of equities		3,467,100	2,714,600
Preferred Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of equities		4,600	4,300
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		2,011,800	1,360,100
Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		2,608,500	2,656,800
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		828,400	879,500
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		3,338,000	3,545,700
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		1,223,800	1,323,100
RMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	[3]	1,955,800	2,611,400
CMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		909,900	890,300
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]	3,008,700	2,351,600
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)		1,060,400	714,200
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]	0	300
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of equities		3,465,000	2,711,100
Estimated fair value of available for sale debt securities		0	0
Estimated fair value of investments (excluding equity method investments and loans)		3,465,000	2,711,100
Senior notes and other debt		0	0
Level 1 | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of equities		3,465,000	2,711,100
Level 1 | Preferred Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of equities		0	0
Level 1 | 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		0	0
Level 1 | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		0	0
Level 1 | 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		0	0
Level 1 | 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		0	0
Level 1 | 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		0	0
Level 1 | RMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	[3]	0	0
Level 1 | CMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		0	0
Level 1 | 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]	0	0
Level 1 | 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)		0	0
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]	0	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of equities		5,400	6,500
Estimated fair value of available for sale debt securities		13,687,800	13,886,400
Estimated fair value of investments (excluding equity method investments and loans)		14,753,600	14,607,100
Senior notes and other debt		2,364,800	1,911,800
Level 2 | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of equities		2,100	3,500
Level 2 | Preferred Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of equities		3,300	3,000
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		2,011,800	1,360,100
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		2,608,500	2,656,800
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		826,700	879,500
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		2,696,800	2,914,100
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		1,060,700	1,133,600
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]	1,953,800	2,608,900
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		909,900	884,500
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]	1,619,600	1,448,900
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)		1,060,400	714,200
Level 2 | 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]	0	0
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of equities		1,300	1,300
Estimated fair value of available for sale debt securities		2,197,100	1,732,100
Estimated fair value of investments (excluding equity method investments and loans)		2,198,400	1,733,700
Senior notes and other debt		485,600	556,900
Level 3 | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of equities		0	0
Level 3 | Preferred Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of equities		1,300	1,300
Level 3 | 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		0	0
Level 3 | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		0	0
Level 3 | 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		1,700	0
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		641,200	631,600
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		163,100	189,500
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]	2,000	2,500
Level 3 | CMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		0	5,800
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]	1,389,100	902,700
Level 3 | 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)		0	0
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]	$ 0	$ 300

[1] This table includes debt and equity securities, as well as partnership and non-marketable equity investments 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.

[2] See Note 8 to Notes to Consolidated Financial Statements set forth in Part II, Item 8, “Financial Statements and Supplementary Data” of the 2020 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 $1,360.0 million and $878.4 million of collateralized loan obligations as of September 30, 2021 and December 31, 2020, respectively.

[5] Includes partnership and non-marketable equity investments accounted for at fair value and excludes investments accounted for using the equity method.