Equity - Additional Information (Details) - USD ($)	1 Months Ended	12 Months Ended
	Jul. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity [Line Items]
Common shares issued through public offering and direct share purchase plan during period (in shares)		0	577,823	2,266,191
Proceeds from issuance of common shares		$ 16,804,000	$ 12,186,000	$ 19,382,000
Preferred shares outstanding (in shares)		1,935,400	1,935,400
Shares issued, net of forfeitures (in shares)		835,234	1,084,835	860,730
OP unit equivalent in common shares (in unit per share)		1.13
Shares issued for units redeemed (in shares)		140,746	48,549	32,780
Partners' capital account, exchanges and conversions		$ 584,000	$ 210,000	$ 165,000
OP units outstanding (in units)		3,223,000
Payments to acquire interest in joint venture		$ 7,951,000	$ 0	4,022,000
Joint Venture [Member]
Equity [Line Items]
Payments to acquire interest in joint venture		$ 5,831,000
Philadelphia, Pennsylvania [Member] | Office Building [Member] | Joint Venture [Member]
Equity [Line Items]
Payments to acquire interest in joint venture	$ 4,022,000
Real state, ownership percentage	100.00%
Common Shares [Member]
Equity [Line Items]
Number of shares authorized to be repurchased (in shares)	10,000,000
Repurchase of common shares (in shares)		0	1,184,113
Average cost per share (usd per share)			$ 7.56
Series C [Member]
Equity [Line Items]
Preferred shares outstanding (in shares)		1,935,400
Dividend rate (usd per share)		$ 3.25
Preferred shares, liquidation preference		$ 96,770,000
Preferred shares to common shares conversion ratio		2.4339
Series C [Member] | Minimum [Member]
Equity [Line Items]
Common share closing price percent of conversion price		125.00%
Public Offering And Direct Share Purchase Plan Proceeds [Member]
Equity [Line Items]
Proceeds from issuance of common shares			$ 4,115,000	$ 20,797,000
At The Market [Member]
Equity [Line Items]
Common shares issued through public offering and direct share purchase plan during period (in shares)		1,593,603	976,109	0
Value authorized		$ 125,000,000
Value, new issues		$ 17,362,000	$ 10,498,000