Significant Accounting Policies (Details 3) (USD $) In Millions, unless otherwise specified	12 Months Ended	1 Months Ended
	Dec. 31, 2011 M Y statement	Apr. 30, 2009 Adoption of other-than-temporary impairment guidance Retained earnings	Apr. 30, 2009 Adoption of other-than-temporary impairment guidance Accumulated other comprehensive income
Accounting pronouncements
Net impact of cumulative effect adjustment		$ (6)	$ 6
Number of separate but consecutive statements under new guidance for comprehensive income (in number of statements)	2
Cash and cash equivalents
Cash and cash equivalents, maximum maturity at the date of purchase (in months)	3
Property and casualty unpaid losses and loss expenses
Interest rate used to discount workers compensation loss reserves (as a percent)	2.50%
Agent bonus estimates
Performance period for agent bonuses (in months)	36
Period of prior year actual underwriting data used to estimate agent bonuses (in years)	2