Investments - Investment Segment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Schedule of Investments [Line Items]
Investments	$ 6,910		$ 6,910		$ 9,945
Securities sold, not yet purchased, at fair value	970		970		1,190
Investment Segment
Schedule of Investments [Line Items]
Investments	6,177		6,177		9,207
Securities sold, not yet purchased, at fair value	970		970		1,190
Unrealized gain (loss) on debt and equity securities still held	(535)	$ (513)	(1,560)	$ (99)
Investment Segment | Equity securities:
Schedule of Investments [Line Items]
Investments	6,077		6,077		9,062
Investment Segment | Basic materials
Schedule of Investments [Line Items]
Investments					281
Securities sold, not yet purchased, at fair value					209
Investment Segment | Consumer, non-cyclical
Schedule of Investments [Line Items]
Investments	1,136		1,136		2,085
Securities sold, not yet purchased, at fair value	254		254		29
Investment Segment | Consumer, cyclical
Schedule of Investments [Line Items]
Investments	1,776		1,776		2,427
Securities sold, not yet purchased, at fair value	450		450		379
Investment Segment | Energy
Schedule of Investments [Line Items]
Investments	1,884		1,884		1,717
Securities sold, not yet purchased, at fair value	266		266		124
Investment Segment | Financial
Schedule of Investments [Line Items]
Securities sold, not yet purchased, at fair value					152
Investment Segment | Technology
Schedule of Investments [Line Items]
Investments	1,207		1,207		2,425
Securities sold, not yet purchased, at fair value					217
Investment Segment | Industrial
Schedule of Investments [Line Items]
Investments	74		74		127
Investment Segment | Communication
Schedule of Investments [Line Items]
Securities sold, not yet purchased, at fair value					80
Investment Segment | Corporate debt securities
Schedule of Investments [Line Items]
Investments	$ 100		$ 100		$ 145