Fair Value (Details 3) (Fair value measurements on a recurring basis, USD $) In Millions, except Per Share data, unless otherwise specified	6 Months Ended
	Jun. 30, 2012 item
Fair value inputs assets quantitative information
Fair value (in dollars)	$ 83
No. of holdings	27
Maximum
Unobservable input
Securities priced via unobservable inputs as a percentage of total portfolio	0.70%
Indemnity
Fair value inputs assets quantitative information
Fair value (in dollars)	5
No. of holdings	3
Indemnity | Corporate debt securities | Market approach
Fair value inputs assets quantitative information
Fair value (in dollars)	1
No. of holdings	1
Unobservable input
Non-binding broker quote (in dollars per share)	$ 115.77
Indemnity | Collateralized debt obligations (CDO) | Income approach
Fair value inputs assets quantitative information
Fair value (in dollars)	4
No. of holdings	2
Unobservable input
Projected LIBOR rate (as a percent)	0.47%
Indemnity | Collateralized debt obligations (CDO) | Income approach | Minimum
Unobservable input
Repayment at maturity (as a percent)	42.00%
Discount rate (as a percent)	7.50%
Indemnity | Collateralized debt obligations (CDO) | Income approach | Maximum
Unobservable input
Repayment at maturity (as a percent)	100.00%
Discount rate (as a percent)	15.00%
Indemnity | Collateralized debt obligations (CDO) | Income approach | Weighted average
Unobservable input
Repayment at maturity (as a percent)	90.10%
Discount rate (as a percent)	10.80%
Exchange
Fair value inputs assets quantitative information
Fair value (in dollars)	78
No. of holdings	24
Exchange | Corporate debt securities | Market approach
Fair value inputs assets quantitative information
Fair value (in dollars)	23
No. of holdings	7
Unobservable input
Comparable security yield (as a percent)	6.00%
Exchange | Corporate debt securities | Market approach | Minimum
Unobservable input
Non-binding broker quote (in dollars per share)	$ 103.50
Comparable transaction EBITA multiples (as a multiple)	7.3
Exchange | Corporate debt securities | Market approach | Maximum
Unobservable input
Non-binding broker quote (in dollars per share)	$ 118.00
Comparable transaction EBITA multiples (as a multiple)	17.1
Exchange | Corporate debt securities | Market approach | Weighted average
Unobservable input
Non-binding broker quote (in dollars per share)	$ 111.80
Comparable transaction EBITA multiples (as a multiple)	7.5
Exchange | Collateralized debt obligations (CDO) | Market approach
Fair value inputs assets quantitative information
Fair value (in dollars)	3
No. of holdings	4
Exchange | Collateralized debt obligations (CDO) | Market approach | Minimum
Unobservable input
Non-binding broker quote (in dollars per share)	$ 3.00
Exchange | Collateralized debt obligations (CDO) | Market approach | Maximum
Unobservable input
Non-binding broker quote (in dollars per share)	$ 84.00
Exchange | Collateralized debt obligations (CDO) | Market approach | Weighted average
Unobservable input
Non-binding broker quote (in dollars per share)	$ 64.08
Exchange | Collateralized debt obligations (CDO) | Income approach
Fair value inputs assets quantitative information
Fair value (in dollars)	24
No. of holdings	6
Unobservable input
Projected LIBOR rate (as a percent)	0.47%
Exchange | Collateralized debt obligations (CDO) | Income approach | Minimum
Unobservable input
Repayment at maturity (as a percent)	42.00%
Discount rate (as a percent)	7.00%
Exchange | Collateralized debt obligations (CDO) | Income approach | Maximum
Unobservable input
Repayment at maturity (as a percent)	100.00%
Discount rate (as a percent)	15.00%
Exchange | Collateralized debt obligations (CDO) | Income approach | Weighted average
Unobservable input
Repayment at maturity (as a percent)	94.20%
Discount rate (as a percent)	9.30%
Exchange | States & political subdivisions | Market approach
Fair value inputs assets quantitative information
Fair value (in dollars)	4
No. of holdings	1
Unobservable input
Comparable security yield (as a percent)	0.49%
Added yield due to lack of marketability (as a percent)	1.00%
Exchange | Other debt securities | Income approach
Fair value inputs assets quantitative information
Fair value (in dollars)	5
No. of holdings	1
Unobservable input
Repayment at maturity (as a percent)	100.00%
Discount rate (as a percent)	1.85%
Exchange | Nonredeemable preferred stock | Market approach
Fair value inputs assets quantitative information
Fair value (in dollars)	5
No. of holdings	1
Unobservable input
Comparable security yield (as a percent)	7.51%
Exchange | Common stock | Market approach
Fair value inputs assets quantitative information
Fair value (in dollars)	$ 14
No. of holdings	4
Exchange | Common stock | Market approach | Minimum
Unobservable input
Comparable transaction EBITA multiples (as a multiple)	7.3
Discount for lack of marketability (as a percent)	5.00%
Exchange | Common stock | Market approach | Maximum
Unobservable input
Comparable transaction EBITA multiples (as a multiple)	17.1
Discount for lack of marketability (as a percent)	30.00%
Exchange | Common stock | Market approach | Weighted average
Unobservable input
Comparable transaction EBITA multiples (as a multiple)	7.5
Discount for lack of marketability (as a percent)	30.00%