Lines of Credit - Composition of Partnership's Aggregate Bank Lines of Credit (Detail) - USD ($)	Dec. 31, 2020	Dec. 31, 2019	Sep. 28, 2018
Line Of Credit Facility [Line Items]
Line of credit facility	$ 890,000,000	$ 790,000,000
2018 Credit Facility [Member]
Line Of Credit Facility [Line Items]
Line of credit facility	500,000,000	500,000,000	$ 500,000,000
Uncommitted Secured Credit Facilities [Member]
Line Of Credit Facility [Line Items]
Line of credit facility	$ 390,000,000	$ 290,000,000