Lines of Credit - Composition of Partnership's Aggregate Bank Lines of Credit (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Line of Credit Facility [Line Items]
Line of credit facility	$ 690,000,000	$ 690,000,000
2013 Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of credit facility	400,000,000	400,000,000
Uncommitted Secured Credit Facilities [Member]
Line of Credit Facility [Line Items]
Line of credit facility	$ 290,000,000	$ 290,000,000