Debt and Financial Instruments - Additional Information (Detail)						1 Months Ended	3 Months Ended				9 Months Ended			27 Months Ended		30 Months Ended
		May 02, 2018 USD ($)	Apr. 05, 2018 USD ($)	Mar. 27, 2018 USD ($) Option	Feb. 13, 2018 USD ($)	Aug. 31, 2018 USD ($)	Sep. 30, 2018 USD ($) MortgageLoan		Sep. 30, 2016 USD ($) Derivative	Jun. 30, 2016 USD ($) Derivative	Sep. 30, 2018 USD ($) MortgageLoan		Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) MortgageLoan		Sep. 30, 2018 USD ($) MortgageLoan		Dec. 31, 2017 USD ($) MortgageLoan
Debt Instrument [Line Items]
Outstanding borrowings under revolving credit agreement							$ 195,000,000				$ 195,000,000			$ 195,000,000		$ 195,000,000		$ 181,050,000
Available borrowing capacity							$ 105,000,000				105,000,000			$ 105,000,000		$ 105,000,000
Repayments of mortgage loan											$ 22,964,000		$ 43,656,000
Number of non-recourse mortgages repaid | MortgageLoan							3				3			3		3
Refinanced the property with fixed rate mortgage loan											$ 13,000,000		$ 13,200,000
Number of non-recourse mortgages | MortgageLoan							9				9			9		9		11
Mortgage loan fair value							$ 65,400,000				$ 65,400,000			$ 65,400,000		$ 65,400,000		$ 76,300,000
Balance of Non Recourse Mortgages							65,778,000	[1]			65,778,000	[1]		65,778,000	[1]	65,778,000	[1]	$ 75,700,000
Interest Rate Cap
Debt Instrument [Line Items]
Number of interest rate cap agreements | Derivative									1	1
Notional amount									$ 30,000,000	$ 30,000,000
Premium paid									$ 55,000	$ 115,000
Expiration date of interest rate cap									Mar. 31, 2019	Mar. 31, 2019
Derivative interest rate cap, payment received or accrued from counterparties														83,000		83,000
Three Mortgages Utilizing Borrowings under Credit Agreement
Debt Instrument [Line Items]
Repayments of mortgage loan											$ 21,700,000
Refinanced the property with fixed rate mortgage loan							13,000,000
Sparks Medical Building/Vista Medical Terrace Floating Rate Mortgage Loan
Debt Instrument [Line Items]
Repayments of mortgage loan					$ 4,100,000
Centennial Hills Medical Office Building floating rate mortgage loan
Debt Instrument [Line Items]
Repayments of mortgage loan			$ 9,700,000
Rosenberg Children's Medical Plaza Fixed Rate Mortgage Loan
Debt Instrument [Line Items]
Repayments of mortgage loan		$ 7,900,000
Refinanced the property with fixed rate mortgage loan						$ 13,000,000
Maturity Date											2033-09
Balance of Non Recourse Mortgages	[1]						13,000,000				$ 13,000,000			13,000,000		13,000,000
LIBOR | Interest Rate Cap
Debt Instrument [Line Items]
Derivative instruments, LIBOR rate									1.50%	1.50%
Credit Agreement
Debt Instrument [Line Items]
Outstanding borrowing				$ 300,000,000			$ 300,000,000				$ 300,000,000			$ 300,000,000		$ 300,000,000
Increase in borrowing capacity				$ 50,000,000
Unsecured revolving amended credit agreement terminated date				2022-03
Number of additional six month extension options | Option				2
Proceeds from Lines of Credit				$ 50,000,000
Credit facility, Interest Rate Terms											one, two, three, or six month LIBOR plus an applicable margin ranging from 1.10% to 1.35% or at the Base Rate plus an applicable margin ranging from 0.10% to 0.35%.
Base rate description											the greater of: (a) the administrative agent’s prime rate; (b) the federal funds effective rate plus 1/2 of 1%, and; (c) one month LIBOR plus 1%.
Facility fee payable on commitment											0.20%
Credit Agreement | LIBOR
Debt Instrument [Line Items]
Margin points added to the reference rate											1.15%
Credit Agreement | Base Rate
Debt Instrument [Line Items]
Margin points added to the reference rate											0.15%
Credit Agreement | Swingline/Short-Term Loans
Debt Instrument [Line Items]
Outstanding borrowing				30,000,000
Credit Agreement | Letters of Credit
Debt Instrument [Line Items]
Outstanding borrowing				$ 40,000,000
Credit Agreement | Minimum
Debt Instrument [Line Items]
Facility fee payable on commitment				0.15%
Credit Agreement | Minimum | LIBOR
Debt Instrument [Line Items]
Margin points added to the reference rate				1.10%
Margin points added to the base rate				1.00%
Credit Agreement | Minimum | Base Rate
Debt Instrument [Line Items]
Margin points added to the reference rate				0.10%
Credit Agreement | Minimum | Federal Funds Effective Rate
Debt Instrument [Line Items]
Margin points added to the base rate				0.50%
Credit Agreement | Maximum
Debt Instrument [Line Items]
Facility fee payable on commitment				0.35%
Credit Agreement | Maximum | LIBOR
Debt Instrument [Line Items]
Margin points added to the reference rate				1.35%
Credit Agreement | Maximum | Base Rate
Debt Instrument [Line Items]
Margin points added to the reference rate				0.35%

[1] All mortgage loans require monthly principal payments through maturity and either fully amortize or include a balloon principal payment upon maturity.