Fair Value Measurements - Narrative (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Fair Value of Financial Instruments [Line Items]
Investments not required to be disclosed	$ 349	$ 322
NUG contracts
Fair Value of Financial Instruments [Line Items]
Period of future observable data to determine contract price	2 years