Fair Value Measurements - Restricted Investments (Details) - TSG - USD ($) $ in Millions		1 Months Ended	3 Months Ended		9 Months Ended	12 Months Ended
	May 05, 2020	Nov. 30, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Agreement period		20 years
Number of common shares received under agreement (in shares)	0.2253	1,000,000
Common shares, sales restriction period, upon exercise of first option		1 year
Cumulative unrealized gains (losses)			$ 5	$ (2)	$ 8
Marketable Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, total			19		19	$ 14
Cumulative unrealized gains (losses)					9	$ 4
Restricted Cash and Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, total			$ 8		8
Cumulative unrealized gains (losses)					$ 3