Unsecured Line of Credit and Term Notes - Borrowings Outstanding on Unsecured Line of Credit and Term Notes (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Unsecured Line Of Credit And Term Notes [Line Items]
Revolving line of credit borrowings	$ 331,000	$ 253,000
Total term notes payable - gross	1,400,000	1,400,000
Less: unamortized debt issuance costs	(8,634)	(9,323)
Less: unamortized senior term note discount	(2,986)	(3,152)
Term notes, net of debt issuance costs	1,388,380	1,387,525
Term Note Due June 4, 2020 [Member]
Unsecured Line Of Credit And Term Notes [Line Items]
Total term notes payable - gross	325,000	325,000
Term Note Due August 5, 2021 [Member]
Unsecured Line Of Credit And Term Notes [Line Items]
Total term notes payable - gross	100,000	100,000
Term Note Due April 8, 2024 [Member]
Unsecured Line Of Credit And Term Notes [Line Items]
Total term notes payable - gross	175,000	175,000
Senior Term Note Due July 1, 2026 [Member]
Unsecured Line Of Credit And Term Notes [Line Items]
Total term notes payable - gross	600,000	600,000
Term Note Due July 21, 2028 [Member]
Unsecured Line Of Credit And Term Notes [Line Items]
Total term notes payable - gross	$ 200,000	$ 200,000