Unsecured Line of Credit and Term Notes - Borrowings Outstanding on Unsecured Line of Credit and Term Notes (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Unsecured Line Of Credit And Term Notes [Line Items]
Revolving line of credit borrowings	$ 113,000	$ 105,000
Total term notes payable - gross	1,625,000	1,625,000
Less: unamortized debt issuance costs	(10,262)	(10,962)
Less: unamortized senior term note discount	(4,676)	(4,949)
Term notes payable	1,610,062	1,609,089
Term Note Due June 4, 2020 [Member]
Unsecured Line Of Credit And Term Notes [Line Items]
Total term notes payable - gross	100,000	100,000
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
Senior Term Note Due December 15, 2027 [Member]
Unsecured Line Of Credit And Term Notes [Line Items]
Total term notes payable - gross	450,000	450,000
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