Annual report pursuant to Section 13 and 15(d)

Unsecured Line of Credit and Term Notes - Borrowings Outstanding on Unsecured Line of Credit and Term Notes (Detail)

v3.8.0.1
Unsecured Line of Credit and Term Notes - Borrowings Outstanding on Unsecured Line of Credit and Term Notes (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Unsecured Line Of Credit And Term Notes [Line Items]    
Revolving line of credit borrowings $ 105,000 $ 253,000
Total term notes payable - gross 1,625,000 1,400,000
Less: unamortized debt issuance costs (10,962) (9,323)
Less: unamortized senior term note discount (4,949) (3,152)
Term notes payable 1,609,089 1,387,525
Term Note Due June 4, 2020 [Member]    
Unsecured Line Of Credit And Term Notes [Line Items]    
Total term notes payable - gross 100,000 325,000
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
Senior Term Note Due December 15, 2027 [Member]    
Unsecured Line Of Credit And Term Notes [Line Items]    
Total term notes payable - gross 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