Lines of Credit - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2016 USD ($) Bank	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
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]
Expiry of unsecured revolving line of credit, month and year	2018-11
Number of banks with which agreement made | Bank	12
Number of years for unsecured revolving line of credit	5 years
Line of credit facility	$ 400,000,000	400,000,000
Debt instrument basis spread on LIBOR minimum on a three day advance notice	1.25%
Debt instrument basis spread on LIBOR maximum on a three day advance notice	2.00%
Debt instrument basis spread on variable rate minimum on same day borrowing	0.25%
Debt instrument basis spread on variable rate maximum on same day borrowing	1.00%
Federal funds effective rate plus 1%	1.00%
LIBOR rate plus 1%	1.00%
Maximum leverage ratio required to be maintained by partnership	35.00%
Required minimum partnership capital, net of reserve for anticipated withdrawals	$ 1,382,000,000
Additional issuances of partnership capital	50.00%
Amount outstanding under credit facility	$ 0	0
Partnership draws against lines of credit	0	0	$ 0
Uncommitted Secured Credit Facilities [Member]
Line of Credit Facility [Line Items]
Line of credit facility	290,000,000	290,000,000
Amount outstanding under credit facility	0	0
Partnership draws against lines of credit	$ 0	$ 0	$ 0