Investments - Real Estate and Other Long-Term Investments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Other Long Term Investments [Line Items]
Other long-term investments	$ 1,462	$ 1,404
Commitments to contribute to limited liability entities that hold real estate or loans to real estate	263
Commitments to contribute to entities that hold securities diversified by issuer and maturity date	$ 810
Percentage of the committed amounts to be disbursed by the end of next year	26.00%
Real Estate Entities [Member]
Other Long Term Investments [Line Items]
Other long-term investments	$ 738	814
Security Partnerships [Member]
Other Long Term Investments [Line Items]
Other long-term investments	650	501
Other Long Term Investments [Member]
Other Long Term Investments [Line Items]
Other long-term investments	$ 74	$ 89