Reserve for Property-Liability Insurance Claims and Claims Expense (Details 2) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Total auto
Increase (decrease) in claims and claims expense
Net increase (decrease) to reserves	$ (365)	$ (381)	$ (179)
Catastrophes
Increase (decrease) in claims and claims expense
Losses from catastrophes, net of recoveries	2,350	3,820	2,210
Reserve reestimates | Other personal lines
Increase (decrease) in claims and claims expense
Net increase (decrease) to reserves	(30)	94	15
Catastrophe loss reestimates
Increase (decrease) in claims and claims expense
Net increase (decrease) to reserves	410	130	163
Catastrophe loss reestimates | Homeowners
Increase (decrease) in claims and claims expense
Net increase (decrease) to reserves	$ (321)	$ (69)	$ (23)