Fair Value Measurements - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Carrying value
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total debt	$ 20,500,000,000		$ 20,500,000,000		$ 21,500,000,000
Accounting Standards Update [Extensible List]			us-gaap:AccountingStandardsUpdate201613Member		us-gaap:AccountingStandardsUpdate201613Member
Fair Value, Inputs, Level 2 | Fair value
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total debt	22,800,000,000		$ 22,800,000,000		$ 22,600,000,000
Line of Credit | Revolving credit facility
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Maximum borrowing capacity	3,500,000,000		3,500,000,000
Lending Joint Ventures
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other (expense) income	4,000,000	$ 2,000,000	10,000,000	$ 5,000,000
Variable-Rate Term Loan Facilities Due March 2023 | Line of Credit | Term loan facility
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Maximum borrowing capacity	390,000,000		390,000,000
Outstanding borrowings	5,000,000		5,000,000
Variable-Rate Revolving Credit Facilities Due March 2023 | Line of Credit | Revolving credit facility
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Maximum borrowing capacity	45,000,000		45,000,000
Financial Guarantee | Lending Joint Ventures
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Carrying value of non-contingent liability	20,000,000		20,000,000		$ 26,000,000
Cumulative Effect, Period of Adoption, Adjustment | Fair Value, Inputs, Level 3
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Contingent liability	$ 10,000,000		$ 10,000,000