Fair Value Measurements - Fair Value of Direct and Indirect Investments, Related Unfunded Commitments and Financial Support Provided (Details) - Principal Investments [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 84
Unfunded Commitments	25
Funded Commitments	1	$ 0
Funded Other	1	0
Direct Investments [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	1
Unfunded Commitments	0
Funded Commitments	0	0
Funded Other	1	0
Indirect Investments [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	83
Unfunded Commitments	25
Funded Commitments	1	0
Funded Other	$ 0	$ 0