Fair Value Measurements - Fair Value Measurement (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Assets [Abstract]
Derivative contracts, at fair value (asset)	$ 1,927	$ 182
Liabilities [Abstract]
Securities sold, not yet purchased, at fair value	1,819	1,190
Derivative contracts at fair value (liability)	142	1,224
Recurring measurement
Assets [Abstract]
Investments	9,293	9,732
Derivative contracts, at fair value (asset)	1,927	182
Assets, Fair Value Disclosure	11,220	9,914
Liabilities [Abstract]
Securities sold, not yet purchased, at fair value	1,819	1,190
Other liabilities	15	13
Derivative contracts at fair value (liability)	142	1,224
Financial and Nonfinancial Liabilities, Fair Value Disclosure	1,976	2,427
Recurring measurement | Level 1
Assets [Abstract]
Investments	9,057	9,448
Derivative contracts, at fair value (asset)	0	0
Assets, Fair Value Disclosure	9,057	9,448
Liabilities [Abstract]
Securities sold, not yet purchased, at fair value	1,819	1,190
Other liabilities	0	0
Derivative contracts at fair value (liability)	0	0
Financial and Nonfinancial Liabilities, Fair Value Disclosure	1,819	1,190
Recurring measurement | Level 2
Assets [Abstract]
Investments	233	281
Derivative contracts, at fair value (asset)	1,927	182
Assets, Fair Value Disclosure	2,160	463
Liabilities [Abstract]
Securities sold, not yet purchased, at fair value	0	0
Other liabilities	12	7
Derivative contracts at fair value (liability)	142	1,224
Financial and Nonfinancial Liabilities, Fair Value Disclosure	154	1,231
Recurring measurement | Level 3
Assets [Abstract]
Investments	3	3
Derivative contracts, at fair value (asset)	0	0
Assets, Fair Value Disclosure	3	3
Liabilities [Abstract]
Securities sold, not yet purchased, at fair value	0	0
Other liabilities	3	6
Derivative contracts at fair value (liability)	0	0
Financial and Nonfinancial Liabilities, Fair Value Disclosure	$ 3	$ 6