Investments (Aggregate Fair Value And Gross Unrealized Loss By Length Of Time Security Has Continuously Been In Unrealized Loss Position) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Investment [Line Items]
Investment securities, Unrealized loss position, 0-12 Months, Fair Value	$ 2,412	$ 5,927
Investment securities, Unrealized loss position, 0-12 Months, Gross Unrealized Loss	(49.2)	(288.9)
Investment securities, Unrealized loss position, Over 12 Months, Fair Value	852	1,481
Investment securities, Unrealized loss position, Over 12 Months, Gross Unrealized Loss	(64.2)	(245.9)
Investment securities, Unrealized loss position, Total Fair Value	3,264	7,408
Investment securities, Unrealized loss position, Total Gross Unrealized Loss	(113.4)	(534.8)
Foreign Government Debt Securities [Member]
Investment [Line Items]
Investment securities, Unrealized loss position, 0-12 Months, Fair Value	839	1,801
Investment securities, Unrealized loss position, 0-12 Months, Gross Unrealized Loss	(14.8)	(82.2)
Investment securities, Unrealized loss position, Over 12 Months, Fair Value	186	529
Investment securities, Unrealized loss position, Over 12 Months, Gross Unrealized Loss	(9.7)	(40.0)
Investment securities, Unrealized loss position, Total Fair Value	1,025	2,330
Investment securities, Unrealized loss position, Total Gross Unrealized Loss	(24.5)	(122.2)
Corporate Securities [Member]
Investment [Line Items]
Investment securities, Unrealized loss position, 0-12 Months, Fair Value	663	3,084
Investment securities, Unrealized loss position, 0-12 Months, Gross Unrealized Loss	(20.1)	(148.2)
Investment securities, Unrealized loss position, Over 12 Months, Fair Value	195	268
Investment securities, Unrealized loss position, Over 12 Months, Gross Unrealized Loss	(15.5)	(32.2)
Investment securities, Unrealized loss position, Total Fair Value	858	3,352
Investment securities, Unrealized loss position, Total Gross Unrealized Loss	(35.6)	(180.4)
Collateralized Mortgage Backed Securities [Member]
Investment [Line Items]
Investment securities, Unrealized loss position, 0-12 Months, Fair Value	225	440
Investment securities, Unrealized loss position, 0-12 Months, Gross Unrealized Loss	(1.3)	(7.5)
Investment securities, Unrealized loss position, Over 12 Months, Fair Value	412	586
Investment securities, Unrealized loss position, Over 12 Months, Gross Unrealized Loss	(35.0)	(170.2)
Investment securities, Unrealized loss position, Total Fair Value	637	1,026
Investment securities, Unrealized loss position, Total Gross Unrealized Loss	(36.3)	(177.7)
US States and Political Subdivisions Debt Securities [Member]
Investment [Line Items]
Investment securities, Unrealized loss position, 0-12 Months, Fair Value	29	30
Investment securities, Unrealized loss position, 0-12 Months, Gross Unrealized Loss	(1.3)	(0.4)
Investment securities, Unrealized loss position, Over 12 Months, Fair Value	59	98
Investment securities, Unrealized loss position, Over 12 Months, Gross Unrealized Loss	(4.0)	(3.5)
Investment securities, Unrealized loss position, Total Fair Value	88	128
Investment securities, Unrealized loss position, Total Gross Unrealized Loss	(5.3)	(3.9)
Fixed Maturities [Member]
Investment [Line Items]
Investment securities, Unrealized loss position, 0-12 Months, Fair Value	1,756	5,355
Investment securities, Unrealized loss position, 0-12 Months, Gross Unrealized Loss	(37.5)	(238.3)
Investment securities, Unrealized loss position, Over 12 Months, Fair Value	852	1,481
Investment securities, Unrealized loss position, Over 12 Months, Gross Unrealized Loss	(64.2)	(245.9)
Investment securities, Unrealized loss position, Total Fair Value	2,608	6,836
Investment securities, Unrealized loss position, Total Gross Unrealized Loss	(101.7)	(484.2)
Equity Securities [Member]
Investment [Line Items]
Investment securities, Unrealized loss position, 0-12 Months, Fair Value	543	484
Investment securities, Unrealized loss position, 0-12 Months, Gross Unrealized Loss	(5.4)	(42.3)
Investment securities, Unrealized loss position, Total Fair Value	543	484
Investment securities, Unrealized loss position, Total Gross Unrealized Loss	(5.4)	(42.3)
Other Long-term Investments [Member]
Investment [Line Items]
Investment securities, Unrealized loss position, 0-12 Months, Fair Value	113	88
Investment securities, Unrealized loss position, 0-12 Months, Gross Unrealized Loss	(6.3)	(8.3)
Investment securities, Unrealized loss position, Total Fair Value	113	88
Investment securities, Unrealized loss position, Total Gross Unrealized Loss	$ (6.3)	$ (8.3)