Variable Interest Entities - Assets for Which Fair Value Option Was Elected (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Variable Interest Entity [Line Items]
Total gains (losses)	$ (18)	$ 7	$ (42)	$ 33
Variable Interest Entities
Variable Interest Entity [Line Items]
Total gains (losses)	9	0	19	6
Variable Interest Entities | Trading securities
Variable Interest Entity [Line Items]
Total gains (losses)	0	0	1	1
Variable Interest Entities | Investment funds
Variable Interest Entity [Line Items]
Total gains (losses)	$ 9	$ 0	$ 18	$ 5