Fair Value Measurements Fair Value Measurement (Details) - USD ($) $ in Millions		Mar. 31, 2019	Dec. 31, 2018
Liabilities [Abstract]
Securities sold, not yet purchased, at fair value		$ 447	$ 468
Derivative contracts at fair value (liability)		722	36
Recurring measurement
Assets [Abstract]
Investments		7,922	8,182
Derivative contracts, at fair value (asset)	[1]	27	524
Assets, Fair Value Disclosure		7,949	8,706
Liabilities [Abstract]
Securities sold, not yet purchased, at fair value		447	468
Other liabilities		16	2
Derivative contracts at fair value (liability)		722	36
Financial and Nonfinancial Liabilities, Fair Value Disclosure		1,185	506
Recurring measurement | Level 1
Assets [Abstract]
Investments		7,598	7,493
Derivative contracts, at fair value (asset)	[1]	0	7
Assets, Fair Value Disclosure		7,598	7,500
Liabilities [Abstract]
Securities sold, not yet purchased, at fair value		447	468
Other liabilities		0	0
Derivative contracts at fair value (liability)		0	0
Financial and Nonfinancial Liabilities, Fair Value Disclosure		447	468
Recurring measurement | Level 2
Assets [Abstract]
Investments		321	317
Derivative contracts, at fair value (asset)	[1]	27	517
Assets, Fair Value Disclosure		348	834
Liabilities [Abstract]
Securities sold, not yet purchased, at fair value		0	0
Other liabilities		16	2
Derivative contracts at fair value (liability)		722	36
Financial and Nonfinancial Liabilities, Fair Value Disclosure		738	38
Recurring measurement | Level 3
Assets [Abstract]
Investments		3	372
Derivative contracts, at fair value (asset)	[1]	0	0
Assets, Fair Value Disclosure		3	372
Liabilities [Abstract]
Securities sold, not yet purchased, at fair value		0	0
Other liabilities		0	0
Derivative contracts at fair value (liability)		0	0
Financial and Nonfinancial Liabilities, Fair Value Disclosure		$ 0	$ 0

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