Amortized Cost or Cost and Fair Value of Available For Sale Securities (Detail) (USD $)	Sep. 30, 2013		Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	$ 17,710,400,000		$ 17,395,800,000
Gross Unrealized Gains	515,600,000		512,200,000
Gross Unrealized Losses	(200,400,000)		(118,500,000)
Fair value	18,025,600,000		17,789,500,000
Equity securities, cost	1,743,002,000		1,436,540,000
Equity securities, gross unrealized gains	311,700,000		90,300,000
Equity securities, gross unrealized losses	(19,500,000)		(102,800,000)
Equity securities, fair value	2,035,188,000		1,424,014,000
Debt securities, amortized cost	15,018,103,000		15,593,278,000
Debt securities, gross unrealized gains	203,900,000		421,900,000
Debt securities, gross unrealized losses	(180,900,000)		(15,700,000)
Debt securities, fair value	15,041,110,000		15,999,538,000
U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost	1,000,700,000		514,900,000
Debt securities, gross unrealized gains	5,000,000		8,100,000
Debt securities, gross unrealized losses	(5,200,000)		(100,000)
Debt securities, fair value	1,000,500,000		522,900,000
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost	6,003,000,000		6,122,600,000
Debt securities, gross unrealized gains	73,800,000		185,900,000
Debt securities, gross unrealized losses	(101,900,000)		(4,400,000)
Debt securities, fair value	5,974,900,000		6,304,100,000
Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost	1,061,300,000		800,900,000
Debt securities, gross unrealized gains	7,800,000		15,600,000
Debt securities, gross unrealized losses	(6,300,000)		(500,000)
Debt securities, fair value	1,062,800,000		816,000,000
U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost	2,163,200,000		3,448,100,000
Debt securities, gross unrealized gains	30,500,000		69,700,000
Debt securities, gross unrealized losses	(20,200,000)		(2,100,000)
Debt securities, fair value	2,173,500,000		3,515,700,000
Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost	1,749,200,000		2,137,700,000
Debt securities, gross unrealized gains	32,100,000		61,100,000
Debt securities, gross unrealized losses	(4,000,000)		(300,000)
Debt securities, fair value	1,777,300,000		2,198,500,000
Mortgage and asset backed securities | RMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost	1,585,900,000		1,617,400,000
Debt securities, gross unrealized gains	38,800,000		50,800,000
Debt securities, gross unrealized losses	(33,000,000)		(5,700,000)
Debt securities, fair value	1,591,700,000	[1]	1,662,500,000	[1]
Mortgage and asset backed securities | CMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost	854,400,000		486,600,000
Debt securities, gross unrealized gains	13,100,000		26,100,000
Debt securities, gross unrealized losses	(9,000,000)		(2,600,000)
Debt securities, fair value	858,500,000		510,100,000
Mortgage and asset backed securities | Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost	600,400,000		465,100,000
Debt securities, gross unrealized gains	2,800,000		4,600,000
Debt securities, gross unrealized losses	(1,300,000)
Debt securities, fair value	601,900,000		469,700,000
Common Stock
Schedule of Available-for-sale Securities [Line Items]
Equity securities, cost	1,743,000,000	[2]	1,436,500,000	[2]
Equity securities, gross unrealized gains	311,700,000	[2]	90,300,000	[2]
Equity securities, gross unrealized losses	(19,500,000)	[2]	(102,800,000)	[2]
Equity securities, fair value	2,035,200,000	[2]	1,424,000,000	[2]
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	949,300,000		366,000,000
Fair value	$ 949,300,000		$ 366,000,000

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

[2] Of the $2,035.2 million and $1,424.0 million of fair value as of September 30, 2013 and December 31, 2012, respectively, $347.3 million and $542.2 million, respectively, related to certain energy sector businesses.