Investments (Details 2) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Amortized cost
Total fixed maturities	$ 7,453
Estimated fair value
Total fixed maturities	8,159	7,840
Indemnity
Amortized cost
Due in one year or less	116
Due after one year through five years	222
Due after five years through ten years	38
Due after ten years	61
Total fixed maturities	437
Estimated fair value
Due in one year or less	116
Due after one year through five years	227
Due after five years through ten years	41
Due after ten years	68
Total fixed maturities	452	548
Exchange
Amortized cost
Due in one year or less	512
Due after one year through five years	2,302
Due after five years through ten years	2,816
Due after ten years	1,386
Total fixed maturities	7,016
Estimated fair value
Due in one year or less	524
Due after one year through five years	2,486
Due after five years through ten years	3,161
Due after ten years	1,536
Total fixed maturities	$ 7,707	$ 7,292