Amortized Cost or Cost and Fair Value of Available For Sale Securities (Detail) - USD ($) $ in Thousands		Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost		$ 17,768,100	$ 17,446,100
Gross Unrealized Gains		690,600	829,300
Gross Unrealized Losses		(148,700)	(145,700)
Fair Value		18,310,000	18,129,700
Equity securities, cost		2,701,247	2,366,035
Equity securities, gross unrealized gains		482,200	530,300
Equity securities, gross unrealized losses		(39,900)	(80,800)
Equity securities, fair value		3,143,572	2,815,484
Debt securities, amortized cost		14,465,377	14,364,430
Debt securities, gross unrealized gains		208,400	299,000
Debt securities, gross unrealized losses		(108,800)	(64,900)
Debt securities, fair value		14,564,927	14,598,641
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost		601,500	715,600
Fair Value		601,500	715,600
Mortgage and asset backed securities | Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost	[1]	1,549,600	1,531,200
Debt securities, gross unrealized gains	[1]	5,100	2,200
Debt securities, gross unrealized losses	[1]	(5,200)	(13,500)
Debt securities, fair value	[1]	1,549,500	1,519,900
Mortgage and asset backed securities | RMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,611,000	1,646,900
Debt securities, gross unrealized gains		13,500	20,700
Debt securities, gross unrealized losses		(19,300)	(11,700)
Debt securities, fair value	[2]	1,605,200	1,655,900
Mortgage and asset backed securities | CMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,159,100	1,104,200
Debt securities, gross unrealized gains		15,000	22,500
Debt securities, gross unrealized losses		(5,300)	(1,400)
Debt securities, fair value		1,168,800	1,125,300
U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		790,700	541,200
Debt securities, gross unrealized gains		2,400	3,400
Debt securities, gross unrealized losses		(11,600)	(3,500)
Debt securities, fair value		781,500	541,100
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		4,913,500	5,067,300
Debt securities, gross unrealized gains		96,100	139,300
Debt securities, gross unrealized losses		(28,100)	(9,100)
Debt securities, fair value		4,981,500	5,197,500
Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		885,100	876,700
Debt securities, gross unrealized gains		17,300	23,700
Debt securities, gross unrealized losses		(2,000)
Debt securities, fair value		900,400	900,400
U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		2,139,100	2,136,500
Debt securities, gross unrealized gains		28,600	39,500
Debt securities, gross unrealized losses		(26,500)	(21,200)
Debt securities, fair value		2,141,200	2,154,800
Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,417,200	1,460,500
Debt securities, gross unrealized gains		30,400	47,700
Debt securities, gross unrealized losses		(10,800)	(4,500)
Debt securities, fair value		1,436,800	1,503,700
Common Stock
Schedule of Available-for-sale Securities [Line Items]
Equity securities, cost		2,701,300	2,366,000
Equity securities, gross unrealized gains		482,200	530,300
Equity securities, gross unrealized losses		(39,900)	(80,800)
Equity securities, fair value		$ 3,143,600	$ 2,815,500

[1] Includes $935.6 million and $900.7 million of collateralized loan obligations as of June 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.