Amortized Cost or Cost and Fair Value of Available For Sale Securities (Detail) (USD $)	Jun. 30, 2014		Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	$ 17,863,200,000		$ 17,998,400,000
Gross Unrealized Gains	836,800,000		592,900,000
Gross Unrealized Losses	(68,200,000)		(241,100,000)
Fair Value	18,631,800,000		18,350,200,000
Equity securities, cost	2,419,324,000		1,804,698,000
Equity securities, gross unrealized gains	540,900,000		426,600,000
Equity securities, gross unrealized losses	(9,400,000)		(1,900,000)
Equity securities, fair value	2,950,824,000		2,229,453,000
Debt securities, amortized cost	14,828,719,000		14,875,750,000
Debt securities, gross unrealized gains	295,900,000		166,300,000
Debt securities, gross unrealized losses	(58,800,000)		(239,200,000)
Debt securities, fair value	15,065,785,000		14,802,890,000
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	615,200,000		1,317,900,000
Fair Value	615,200,000		1,317,900,000
U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost	399,500,000		982,100,000
Debt securities, gross unrealized gains	2,800,000		3,000,000
Debt securities, gross unrealized losses	(1,400,000)		(30,100,000)
Debt securities, fair value	400,900,000		955,000,000
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost	5,537,100,000		5,650,500,000
Debt securities, gross unrealized gains	115,300,000		51,700,000
Debt securities, gross unrealized losses	(24,100,000)		(112,100,000)
Debt securities, fair value	5,628,300,000		5,590,100,000
Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost	941,500,000		979,300,000
Debt securities, gross unrealized gains	13,600,000		4,600,000
Debt securities, gross unrealized losses	(1,200,000)		(8,500,000)
Debt securities, fair value	953,900,000		975,400,000
U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost	2,319,700,000		2,307,200,000
Debt securities, gross unrealized gains	50,900,000		29,400,000
Debt securities, gross unrealized losses	(5,800,000)		(23,700,000)
Debt securities, fair value	2,364,800,000		2,312,900,000
Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost	1,752,400,000		1,810,600,000
Debt securities, gross unrealized gains	45,800,000		27,000,000
Debt securities, gross unrealized losses	(900,000)		(5,900,000)
Debt securities, fair value	1,797,300,000		1,831,700,000
Mortgage and asset backed securities | RMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost	1,707,000,000		1,559,500,000
Debt securities, gross unrealized gains	42,200,000		36,100,000
Debt securities, gross unrealized losses	(20,800,000)		(47,800,000)
Debt securities, fair value	1,728,400,000	[1]	1,547,800,000	[1]
Mortgage and asset backed securities | CMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost	1,021,700,000		881,600,000
Debt securities, gross unrealized gains	21,000,000		12,800,000
Debt securities, gross unrealized losses	(1,600,000)		(8,800,000)
Debt securities, fair value	1,041,100,000		885,600,000
Mortgage and asset backed securities | Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost	1,149,800,000	[2]	705,000,000	[2]
Debt securities, gross unrealized gains	4,300,000	[2]	1,700,000	[2]
Debt securities, gross unrealized losses	(3,000,000)	[2]	(2,300,000)	[2]
Debt securities, fair value	1,151,100,000	[2]	704,400,000	[2]
Common Stock
Schedule of Available-for-sale Securities [Line Items]
Equity securities, cost	2,419,300,000		1,804,700,000
Equity securities, gross unrealized gains	540,900,000		426,600,000
Equity securities, gross unrealized losses	(9,400,000)		(1,900,000)
Equity securities, fair value	$ 2,950,800,000		$ 2,229,400,000

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

[2] Includes $770.0 million and $237.9 million of collateralized loan obligations as of June 30, 2014 and December 31, 2013, respectively.