Investments (Details 2) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Amortized cost
Total fixed maturities	$ 7,490
Estimated fair value
Total fixed maturities	8,079	7,840
Indemnity
Amortized cost
Due in one year or less	158
Due after one year through five years	222
Due after five years through ten years	45
Due after ten years	70
Total fixed maturities	495
Estimated fair value
Due in one year or less	159
Due after one year through five years	227
Due after five years through ten years	48
Due after ten years	76
Total fixed maturities	510	548
Exchange
Amortized cost
Due in one year or less	522
Due after one year through five years	2,366
Due after five years through ten years	2,779
Due after ten years	1,328
Total fixed maturities	6,995
Estimated fair value
Due in one year or less	530
Due after one year through five years	2,517
Due after five years through ten years	3,073
Due after ten years	1,449
Total fixed maturities	$ 7,569	$ 7,292