Derivatives and Hedging Activities (Details) - USD ($)				3 Months Ended				6 Months Ended		12 Months Ended
	Apr. 21, 2017	Apr. 09, 2015	Feb. 03, 2014	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2018	Dec. 31, 2016
Derivative instruments and hedging activities disclosures
Amount of gain (loss) recorded in accumulated other comprehensive income				$ (323,000)		$ (1,223,000)		$ (10,000)	$ (4,421,000)
Cash Flow Hedging
Derivative instruments and hedging activities disclosures
Earnings related to derivatives due to hedge ineffectiveness					$ 0		$ 0
Interest Rate Swap | Designated as Hedging Instrument | Cash Flow Hedging
Derivative instruments and hedging activities disclosures
Amount borrowed under term loan or revolving credit facility	$ 50,000,000	$ 75,000,000	$ 100,000,000
Term of loan facility		5 years	5 years
Derivative, Fixed Interest Rate	2.98%	2.93%	3.23%
Interest rate swap terminated	$ 50,000,000
Amount of gain (loss) recorded in accumulated other comprehensive income				(300,000)		1,200,000		100,000	4,400,000
Amount reclassified to interest expense				200,000		$ 500,000		500,000	$ 900,000
Amount recognized in unearned revenue, prepaid rent and other liabilities				100,000				100,000			$ 400,000
Derivative asset recognized in other assets				$ 400,000				$ 400,000			$ 300,000
Interest Rate Swap | Designated as Hedging Instrument | Cash Flow Hedging | Forecast
Derivative instruments and hedging activities disclosures
Amount estimated to be reclassified as an increase to interest expense										$ 200,000
LIBOR | Interest Rate Swap | Designated as Hedging Instrument | Cash Flow Hedging
Derivative instruments and hedging activities disclosures
Reference rate		one-month LIBOR