Investments - Fixed Maturities by Maturity Periods (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013
Amortized Cost:
Due in one year or less	$ 1,149
Due after one year through five years	5,042
Due after five years through ten years	4,695
Due after ten years	2,666
Mortgage and other asset-backed securities	963
Total	14,515
Fair Value:
Due in one year or less	1,169
Due after one year through five years	5,472
Due after five years through ten years	5,319
Due after ten years	3,520
Mortgage and other asset-backed securities	1,104
Total	$ 16,584