Summary of Significant Accounting Policies - Liabilities Measured at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Debt	$ 9,132	$ 9,011
Level 1
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Debt	0	0
Level 2
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Debt	9,132	9,011
Level 3
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Debt	$ 0	$ 0