Investments (Details 2) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Amortized cost
Total fixed maturities	$ 7,440
Estimated fair value
Total fixed maturities	8,170	7,840
Indemnity
Amortized cost
Due in one year or less	168
Due after one year through five years	255
Due after five years through ten years	45
Due after ten years	63
Total fixed maturities	531
Estimated fair value
Due in one year or less	169
Due after one year through five years	259
Due after five years through ten years	49
Due after ten years	70
Total fixed maturities	547	548
Exchange
Amortized cost
Due in one year or less	527
Due after one year through five years	2,241
Due after five years through ten years	2,799
Due after ten years	1,342
Total fixed maturities	6,909
Estimated fair value
Due in one year or less	537
Due after one year through five years	2,427
Due after five years through ten years	3,165
Due after ten years	1,494
Total fixed maturities	$ 7,623	$ 7,292