Investments and Cash - Distribution of Fixed-Maturity Securities by Contractual Maturity (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Amortized Cost
Amortized cost	$ 9,972		$ 9,488
Estimated Fair Value
Estimated fair value	11,258		10,615
Fixed Maturities [Member]
Amortized Cost
Due within one year	111
Due after one year through five years	1,961
Due after five years through 10 years	2,286
Due after 10 years	3,720
Amortized cost	9,972
Estimated Fair Value
Due within one year	112
Due after one year through five years	2,028
Due after five years through 10 years	2,422
Due after 10 years	3,985
Estimated fair value	10,491		9,711
Fixed Maturities [Member] | RMBS [Member]
Amortized Cost
Amortized cost	1,255
Estimated Fair Value
Estimated fair value	1,285	[1]	1,122	[1]
Fixed Maturities [Member] | CMBS [Member]
Amortized Cost
Amortized cost	639
Estimated Fair Value
Estimated fair value	$ 659	[1]	$ 549	[1]

[1] Government-agency obligations were approximately 44% of mortgage backed securities as of December 31, 2014 and 50% as of December 31, 2013 based on fair value.