Investments (Details 2) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Amortized cost
Total fixed maturities	$ 7,648
Estimated fair value
Total fixed maturities	8,312	8,159
Indemnity
Amortized cost
Due in one year or less	144
Due after one year through five years	170
Due after five years through ten years	34
Due after ten years	58
Total fixed maturities	406
Estimated fair value
Due in one year or less	145
Due after one year through five years	175
Due after five years through ten years	36
Due after ten years	65
Total fixed maturities	421	452
Exchange
Amortized cost
Due in one year or less	538
Due after one year through five years	2,570
Due after five years through ten years	2,753
Due after ten years	1,381
Total fixed maturities	7,242
Estimated fair value
Due in one year or less	548
Due after one year through five years	2,762
Due after five years through ten years	3,068
Due after ten years	1,513
Total fixed maturities	$ 7,891	$ 7,707