Investments (Details 2) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Amortized cost
Total fixed maturities	$ 7,364
Estimated fair value
Total fixed maturities	7,840	7,543
Indemnity
Amortized cost
Due in one year or less	152
Due after one year through five years	253
Due after five years through ten years	47
Due after ten years	83
Total fixed maturities	535
Estimated fair value
Due in one year or less	152
Due after one year through five years	256
Due after five years through ten years	52
Due after ten years	88
Total fixed maturities	548	264
Exchange
Amortized cost
Due in one year or less	506
Due after one year through five years	2,397
Due after five years through ten years	2,669
Due after ten years	1,257
Total fixed maturities	6,829
Estimated fair value
Due in one year or less	513
Due after one year through five years	2,519
Due after five years through ten years	2,898
Due after ten years	1,362
Total fixed maturities	$ 7,292	$ 7,279