Assets and Liabilities Measured at Fair Value Realized Unrealized Gain Loss (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Realized and unrealized gains (losses) on financial instruments, net (note 7)	$ (27)	$ 4	$ 92	$ 280
Fair Value Option Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Realized and unrealized gains (losses) on financial instruments, net (note 7)	10	130	154	552
Exchangeable Senior Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Realized and unrealized gains (losses) on financial instruments, net (note 7)	(52)	(91)	(9)	(230)
Indemnification Assets
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Realized and unrealized gains (losses) on financial instruments, net (note 7)	$ 15		(49)
Other Financial Instruments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Realized and unrealized gains (losses) on financial instruments, net (note 7)		$ (35)	$ (4)	$ (42)