Investment in Joint Ventures - Additional Information (Detail) - Sovran HHF, Sovran HHF II, 191 III, SERS, RAP and Merrick [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Schedule of Equity Method Investments [Line Items]
Management and call center fee earned	$ 1.9	$ 1.3
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%