Fair Value Measurements - Fair Value Measurement (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Assets [Abstract]
Derivative contracts, at fair value (asset)	$ 2,866	$ 182
Liabilities [Abstract]
Securities sold, not yet purchased, at fair value	459	1,190
Derivative contracts at fair value (liability)	81	1,224
Recurring measurement
Assets [Abstract]
Investments	7,986	9,732
Derivative contracts, at fair value (asset)	2,866	182
Assets, Fair Value Disclosure	10,852	9,914
Liabilities [Abstract]
Securities sold, not yet purchased, at fair value	459	1,190
Other liabilities	12	13
Derivative contracts at fair value (liability)	81	1,224
Financial and Nonfinancial Liabilities, Fair Value Disclosure	552	2,427
Recurring measurement | Level 1
Assets [Abstract]
Investments	7,748	9,448
Derivative contracts, at fair value (asset)	0	0
Assets, Fair Value Disclosure	7,748	9,448
Liabilities [Abstract]
Securities sold, not yet purchased, at fair value	459	1,190
Other liabilities	0	0
Derivative contracts at fair value (liability)	0	0
Financial and Nonfinancial Liabilities, Fair Value Disclosure	459	1,190
Recurring measurement | Level 2
Assets [Abstract]
Investments	235	281
Derivative contracts, at fair value (asset)	2,866	182
Assets, Fair Value Disclosure	3,101	463
Liabilities [Abstract]
Securities sold, not yet purchased, at fair value	0	0
Other liabilities	6	7
Derivative contracts at fair value (liability)	81	1,224
Financial and Nonfinancial Liabilities, Fair Value Disclosure	87	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	6	6
Derivative contracts at fair value (liability)	0	0
Financial and Nonfinancial Liabilities, Fair Value Disclosure	$ 6	$ 6