Annual report pursuant to Section 13 and 15(d)

Investment in Joint Ventures - Additional Information (Detail)

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Investment in Joint Ventures - Additional Information (Detail) - Sovran HHF, Sovran HHF II, 191 III, SERS, HIERS, Urban Box, McDonald, Merrick, RAP and N32 [Member] - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]      
Management and call center fee earned $ 7.8 $ 6.6 $ 4.9
Minimum [Member]      
Schedule of Equity Method Investments [Line Items]      
Management and call center fee as a percentage of revenues 6.00%    
Maximum [Member]      
Schedule of Equity Method Investments [Line Items]      
Management and call center fee as a percentage of revenues 7.00%