Investments (Amortized cost by greater than Twenty Percent) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Available For Sale Securities In Unrealized Loss Position With Loss Severity Greater Than 20 Percent [Line Items]
Fair Value	$ 444	$ 1,055
Gross Unrealized Losses	198	625
Gross Unrealized OTTI	129	155
Number of securities	148	256
Less than six months
Available For Sale Securities In Unrealized Loss Position With Loss Severity Greater Than 20 Percent [Line Items]
Fair Value	34	385
Gross Unrealized Losses	9	125
Gross Unrealized OTTI	1	31
Number of securities	14	56
Six months or greater, but less than nine months
Available For Sale Securities In Unrealized Loss Position With Loss Severity Greater Than 20 Percent [Line Items]
Fair Value		53
Gross Unrealized Losses		30
Gross Unrealized OTTI		12
Number of securities		18
Nine months or greater, but less than twelve months
Available For Sale Securities In Unrealized Loss Position With Loss Severity Greater Than 20 Percent [Line Items]
Fair Value	15	2
Gross Unrealized Losses	10	0
Gross Unrealized OTTI	0	1
Number of securities	3	7
Twelve months or greater
Available For Sale Securities In Unrealized Loss Position With Loss Severity Greater Than 20 Percent [Line Items]
Fair Value	395	615
Gross Unrealized Losses	179	470
Gross Unrealized OTTI	$ 128	$ 111
Number of securities	131	175