Investments - Other Long-Term Investments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Other Long Term Investments [Line Items]
Percentage of the committed amounts expected to be disbursed in the next fiscal year	26.00%
Other long-term investments	$ 1,901	$ 1,518
Unfunded Commitments	1,472
Real Estate Entities [Member]
Other Long Term Investments [Line Items]
Other long-term investments	679	591
Unfunded Commitments	376
Security Partnerships [Member]
Other Long Term Investments [Line Items]
Other long-term investments	1,045	863
Unfunded Commitments	1,063
Other Long Term Investments [Member]
Other Long Term Investments [Line Items]
Other long-term investments	177	$ 64
Unfunded Commitments	$ 33