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	Dec. 31, 2012	Dec. 31, 2011
Investment [Line Items]
Investment securities, Unrealized loss position, 0-12 Months, Fair Value	$ 3,968	$ 5,927
Investment securities, Unrealized loss position, 0-12 Months, Gross Unrealized Loss	(48.6)	(288.9)
Investment securities, Unrealized loss position, Over 12 Months, Fair Value	577	1,481
Investment securities, Unrealized loss position, Over 12 Months, Gross Unrealized Loss	(49.9)	(245.9)
Investment securities, Unrealized loss position, Total Fair Value	4,545	7,408
Investment securities, Unrealized loss position, Total Gross Unrealized Loss	(98.5)	(534.8)
U.S. Treasury and agency
Investment [Line Items]
Investment securities, Unrealized loss position, 0-12 Months, Fair Value	440
Investment securities, Unrealized loss position, 0-12 Months, Gross Unrealized Loss	(1.4)
Investment securities, Unrealized loss position, Over 12 Months, Fair Value	0
Investment securities, Unrealized loss position, Over 12 Months, Gross Unrealized Loss	0
Investment securities, Unrealized loss position, Total Fair Value	440
Investment securities, Unrealized loss position, Total Gross Unrealized Loss	(1.4)
Foreign
Investment [Line Items]
Investment securities, Unrealized loss position, 0-12 Months, Fair Value	1,234	1,801
Investment securities, Unrealized loss position, 0-12 Months, Gross Unrealized Loss	(8.6)	(82.2)
Investment securities, Unrealized loss position, Over 12 Months, Fair Value	88	529
Investment securities, Unrealized loss position, Over 12 Months, Gross Unrealized Loss	(5.8)	(40.0)
Investment securities, Unrealized loss position, Total Fair Value	1,322	2,330
Investment securities, Unrealized loss position, Total Gross Unrealized Loss	(14.4)	(122.2)
Corporate securities
Investment [Line Items]
Investment securities, Unrealized loss position, 0-12 Months, Fair Value	1,026	3,084
Investment securities, Unrealized loss position, 0-12 Months, Gross Unrealized Loss	(22.7)	(148.2)
Investment securities, Unrealized loss position, Over 12 Months, Fair Value	85	268
Investment securities, Unrealized loss position, Over 12 Months, Gross Unrealized Loss	(7.9)	(32.2)
Investment securities, Unrealized loss position, Total Fair Value	1,111	3,352
Investment securities, Unrealized loss position, Total Gross Unrealized Loss	(30.6)	(180.4)
Mortgage backed-securities
Investment [Line Items]
Investment securities, Unrealized loss position, 0-12 Months, Fair Value	855	440
Investment securities, Unrealized loss position, 0-12 Months, Gross Unrealized Loss	(3.8)	(7.5)
Investment securities, Unrealized loss position, Over 12 Months, Fair Value	356	586
Investment securities, Unrealized loss position, Over 12 Months, Gross Unrealized Loss	(32.6)	(170.2)
Investment securities, Unrealized loss position, Total Fair Value	1,211	1,026
Investment securities, Unrealized loss position, Total Gross Unrealized Loss	(36.4)	(177.7)
States, municipalities, and political subdivisions
Investment [Line Items]
Investment securities, Unrealized loss position, 0-12 Months, Fair Value	316	30
Investment securities, Unrealized loss position, 0-12 Months, Gross Unrealized Loss	(3.0)	(0.4)
Investment securities, Unrealized loss position, Over 12 Months, Fair Value	48	98
Investment securities, Unrealized loss position, Over 12 Months, Gross Unrealized Loss	(3.6)	(3.5)
Investment securities, Unrealized loss position, Total Fair Value	364	128
Investment securities, Unrealized loss position, Total Gross Unrealized Loss	(6.6)	(3.9)
Total fixed maturities
Investment [Line Items]
Investment securities, Unrealized loss position, 0-12 Months, Fair Value	3,871	5,355
Investment securities, Unrealized loss position, 0-12 Months, Gross Unrealized Loss	(39.5)	(238.3)
Investment securities, Unrealized loss position, Over 12 Months, Fair Value	577	1,481
Investment securities, Unrealized loss position, Over 12 Months, Gross Unrealized Loss	(49.9)	(245.9)
Investment securities, Unrealized loss position, Total Fair Value	4,448	6,836
Investment securities, Unrealized loss position, Total Gross Unrealized Loss	(89.4)	(484.2)
Equity securities
Investment [Line Items]
Investment securities, Unrealized loss position, 0-12 Months, Fair Value	29	484
Investment securities, Unrealized loss position, 0-12 Months, Gross Unrealized Loss	(4.2)	(42.3)
Investment securities, Unrealized loss position, Over 12 Months, Fair Value	0
Investment securities, Unrealized loss position, Over 12 Months, Gross Unrealized Loss	0
Investment securities, Unrealized loss position, Total Fair Value	29	484
Investment securities, Unrealized loss position, Total Gross Unrealized Loss	(4.2)	(42.3)
Other investments
Investment [Line Items]
Investment securities, Unrealized loss position, 0-12 Months, Fair Value	68	88
Investment securities, Unrealized loss position, 0-12 Months, Gross Unrealized Loss	(4.9)	(8.3)
Investment securities, Unrealized loss position, Over 12 Months, Fair Value	0
Investment securities, Unrealized loss position, Over 12 Months, Gross Unrealized Loss	0
Investment securities, Unrealized loss position, Total Fair Value	68	88
Investment securities, Unrealized loss position, Total Gross Unrealized Loss	$ (4.9)	$ (8.3)