Fair Value Measurements - Fair Value of Direct and Indirect Investments, Related Unfunded Commitments and Financial Support Provided (Details) - Principal Investments [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 97
Unfunded Commitments	26
Funded Commitments	1	$ 1
Funded Other	0	0
Direct Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1
Unfunded Commitments	0
Funded Commitments	0	0
Funded Other	0	0
Indirect Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	96
Unfunded Commitments	26
Funded Commitments	1	1
Funded Other	$ 0	$ 0