Fair Value Measurements - Narrative (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Accounting Standards Update [Extensible List]			us-gaap:AccountingStandardsUpdate201613Member		us-gaap:AccountingStandardsUpdate201613Member
Carrying value
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total debt	$ 21,200,000,000		$ 21,200,000,000		$ 21,500,000,000
Fair Value, Inputs, Level 2 | Fair value
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total debt	23,200,000,000		23,200,000,000		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	6,000,000	$ 3,000,000
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	395,000,000		395,000,000
Outstanding borrowings	25,000,000		25,000,000
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	22,000,000		22,000,000		$ 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	$ 11,000,000		$ 11,000,000