Derivative Financial Instruments - Additional Information (Details)			3 Months Ended									12 Months Ended
	Oct. 12, 2018 USD ($)	Feb. 20, 2015 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Mar. 31, 2016 USD ($) swap	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jul. 27, 2018	Apr. 23, 2018 USD ($)	Jul. 13, 2015 USD ($)
Derivatives
Interest rate agreement, notional amount			$ 513,208,000				$ 401,079,000					$ 513,208,000	$ 401,079,000
Unrealized cash flow hedge losses												1,894,000	0	$ 0
Realized cash flow hedge losses reclassified to net income												(169,000)	0	0
Interest rate cash flow hedge gain (loss) to be reclassified during next 12 months			(300,000)									(300,000)
Net income			4,895,000	$ 5,669,000	$ 5,945,000	$ 6,983,000	5,768,000	$ 10,461,000	$ 4,943,000	$ 8,753,000		23,492,000	29,925,000	$ 42,755,000
Senior Unsecured Term Loan Facility
Derivatives
Aggregate capacity under the credit facility		$ 50,000,000.0
Senior Unsecured Term Loan Facility | Operating Partnership
Derivatives
Aggregate capacity under the credit facility		$ 125,000,000.0
Interest rate swaps
Derivatives
Number of interest rate swaps hedge relationship terminated (swap) | swap											2
One Month Libor | Senior Unsecured Term Loan Facility | Minimum
Derivatives
Stated interest rate, basis spread on variable rate		1.35%
One Month Libor | Senior Unsecured Term Loan Facility | Maximum
Derivatives
Stated interest rate, basis spread on variable rate		1.95%
One Month Libor | Interest rate swaps
Derivatives
Interest rate agreement, notional amount		$ 50,000,000.0															$ 6,500,000.0
Fixed rate interest rate swap		2.00%															3.05%
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent | Interest rate swaps
Derivatives
Net income											$ 700,000
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Gain (Loss), Net, Cash Flow Hedge, Noncontrolling Interest | Interest rate swaps
Derivatives
Net income											$ 400,000
Not Designated as Hedging Instrument
Derivatives
Interest rate agreement, notional amount			450,000,000				401,079,000					450,000,000	401,079,000
Not Designated as Hedging Instrument | Interest rate swaps
Derivatives
Interest rate agreement, notional amount			$ 100,000,000				$ 56,079,000					$ 100,000,000	$ 56,079,000
Not Designated as Hedging Instrument | Interest rate swaps | Operating Partnership
Derivatives
Interest rate agreement, notional amount																$ 50,000,000
Fixed rate interest rate swap																2.783%
Johns Hopkins Village
Derivatives
Effective interest rate															4.19%
Secured Debt | Lightfoot Marketplace | Line of Credit
Derivatives
Aggregate capacity under the credit facility	$ 17,900,000.0
Proceeds from lines of credit	$ 10,500,000
Initial Tranche | Secured Debt | Lightfoot Marketplace | Line of Credit
Derivatives
Effective interest rate	4.77%
Stabilization Of Property | Initial Tranche | Secured Debt | Lightfoot Marketplace | Line of Credit
Derivatives
Effective interest rate	4.62%