Investments (Details 2) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Amortized cost
Total fixed maturities	$ 7,418
Estimated fair value
Total fixed maturities	7,977	7,840
Indemnity
Amortized cost
Due in one year or less	150
Due after one year through five years	204
Due after five years through ten years	48
Due after ten years	77
Total fixed maturities	479
Estimated fair value
Due in one year or less	151
Due after one year through five years	209
Due after five years through ten years	51
Due after ten years	83
Total fixed maturities	494	548
Exchange
Amortized cost
Due in one year or less	509
Due after one year through five years	2,404
Due after five years through ten years	2,743
Due after ten years	1,283
Total fixed maturities	6,939
Estimated fair value
Due in one year or less	516
Due after one year through five years	2,559
Due after five years through ten years	3,016
Due after ten years	1,392
Total fixed maturities	$ 7,483	$ 7,292