Fair Value Measurements Fair Value Measurement (Details) - USD ($) $ in Millions		Dec. 31, 2018	Dec. 31, 2017
Liabilities
Securities sold, not yet purchased, at fair value		$ 468	$ 1,023
Derivative contracts, at fair value (Note 6)		36	1,275
Recurring measurement
Assets
Investments		8,182	9,920
Derivative contracts, at fair value(1)	[1]	524	0
Assets, Fair Value Disclosure		8,706	9,920
Liabilities
Securities sold, not yet purchased, at fair value		468	1,023
Other liabilities		2	1
Derivative contracts, at fair value (Note 6)		36	1,275
Financial and Nonfinancial Liabilities, Fair Value Disclosure		506	2,299
Recurring measurement | Level 1
Assets
Investments		7,493	9,378
Derivative contracts, at fair value(1)	[1]	7	0
Assets, Fair Value Disclosure		7,500	9,378
Liabilities
Securities sold, not yet purchased, at fair value		468	988
Other liabilities		0	0
Derivative contracts, at fair value (Note 6)		0	36
Financial and Nonfinancial Liabilities, Fair Value Disclosure		468	1,024
Recurring measurement | Level 2
Assets
Investments		317	264
Derivative contracts, at fair value(1)	[1]	517	0
Assets, Fair Value Disclosure		834	264
Liabilities
Securities sold, not yet purchased, at fair value		0	35
Other liabilities		2	1
Derivative contracts, at fair value (Note 6)		36	1,239
Financial and Nonfinancial Liabilities, Fair Value Disclosure		38	1,275
Recurring measurement | Level 3
Assets
Investments		372	278
Derivative contracts, at fair value(1)	[1]	0	0
Assets, Fair Value Disclosure		372	278
Liabilities
Securities sold, not yet purchased, at fair value		0	0
Other liabilities		0	0
Derivative contracts, at fair value (Note 6)		0	0
Financial and Nonfinancial Liabilities, Fair Value Disclosure		$ 0	$ 0

[1] Amounts are classified within other assets in our consolidated balance sheets.