Investments - Other Long-Term Investments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Other Long Term Investments [Line Items]
Percentage of the committed amounts expected to be disbursed in the next fiscal year	31.00%
Other long-term investments	$ 1,518	$ 1,462
Unfunded Commitments	1,178
Real Estate Entities [Member]
Other Long Term Investments [Line Items]
Other long-term investments	591	738
Unfunded Commitments	270
Security Partnerships [Member]
Other Long Term Investments [Line Items]
Other long-term investments	863	650
Unfunded Commitments	876
Other Long Term Investments [Member]
Other Long Term Investments [Line Items]
Other long-term investments	64	$ 74
Unfunded Commitments	$ 32