Amortized Cost or Cost and Fair Value of Available For Sale Securities (Detail) - USD ($) $ in Thousands		Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost		$ 17,571,900	$ 17,446,100
Gross Unrealized Gains		576,600	829,300
Gross Unrealized Losses		(244,100)	(145,700)
Fair Value		17,904,400	18,129,700
Equity securities, cost		2,826,902	2,366,035
Equity securities, gross unrealized gains		329,300	530,300
Equity securities, gross unrealized losses		(144,100)	(80,800)
Equity securities, fair value		3,012,070	2,815,484
Debt securities, amortized cost		14,382,992	14,364,430
Debt securities, gross unrealized gains		247,300	299,000
Debt securities, gross unrealized losses		(100,000)	(64,900)
Debt securities, fair value		14,530,255	14,598,641
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost		362,000	715,600
Fair Value		362,000	715,600
Mortgage and asset backed securities | Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost	[1]	1,664,800	1,531,200
Debt securities, gross unrealized gains	[1]	5,300	2,200
Debt securities, gross unrealized losses	[1]	(17,400)	(13,500)
Debt securities, fair value	[1]	1,652,700	1,519,900
Mortgage and asset backed securities | RMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,483,400	1,646,900
Debt securities, gross unrealized gains		16,100	20,700
Debt securities, gross unrealized losses		(8,300)	(11,700)
Debt securities, fair value	[2]	1,491,200	1,655,900
Mortgage and asset backed securities | CMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,124,500	1,104,200
Debt securities, gross unrealized gains		17,100	22,500
Debt securities, gross unrealized losses		(3,700)	(1,400)
Debt securities, fair value		1,137,900	1,125,300
U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,067,700	541,200
Debt securities, gross unrealized gains		8,200	3,400
Debt securities, gross unrealized losses		(4,400)	(3,500)
Debt securities, fair value		1,071,500	541,100
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		4,660,000	5,067,300
Debt securities, gross unrealized gains		120,100	139,300
Debt securities, gross unrealized losses		(13,800)	(9,100)
Debt securities, fair value		4,766,300	5,197,500
Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		898,700	876,700
Debt securities, gross unrealized gains		21,100	23,700
Debt securities, gross unrealized losses		(500)
Debt securities, fair value		919,300	900,400
U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		2,183,200	2,136,500
Debt securities, gross unrealized gains		30,900	39,500
Debt securities, gross unrealized losses		(38,700)	(21,200)
Debt securities, fair value		2,175,400	2,154,800
Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,300,700	1,460,500
Debt securities, gross unrealized gains		28,500	47,700
Debt securities, gross unrealized losses		(13,200)	(4,500)
Debt securities, fair value		1,316,000	1,503,700
Common Stock
Schedule of Available-for-sale Securities [Line Items]
Equity securities, cost		2,826,900	2,366,000
Equity securities, gross unrealized gains		329,300	530,300
Equity securities, gross unrealized losses		(144,100)	(80,800)
Equity securities, fair value		$ 3,012,100	$ 2,815,500

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

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