Equity - Additional Information (Details) - USD ($)	1 Months Ended			12 Months Ended
	Jul. 31, 2015	Jul. 31, 2014	Jul. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Equity [Line Items]
Common shares issued through public offering and direct share purchase plan during period				2,266,191	2,600,795	36,012,313
Proceeds from issuance of common shares				$ 19,382,000	$ 23,563,000	$ 434,927,000
Shares, net of forfeitures				860,730	37,678	1,893,409
OP unit equivalent in common shares				1.13
OP units, redemptions				$ 0	$ 1,962,000	$ 0
Shares issued for units redeemed				32,780	29,086	202,241
Partners' capital account, exchanges and conversions				$ 165,000	$ 148,000	$ 1,053,000
OP units outstanding				3,393,000
Payments to acquire interest in joint venture				$ 4,022,000	$ 2,100,000	8,918,000
Joint Venture [Member] | Office Building [Member] | Philadelphia, Pennsylvania [Member]
Equity [Line Items]
Payments to acquire interest in joint venture	$ 4,022,000	$ 2,100,000
Real state, ownership percentage	100.00%
Joint Venture [Member] | Industrial Property [Member] | Long Island City, New York [Member]
Equity [Line Items]
Payments to acquire interest in joint venture			$ 8,918,000
Former LCIF II Partners [Member]
Equity [Line Items]
OP units redeemed					170,193
OP units, redemptions					$ 1,962,000
Common Stock [Member]
Equity [Line Items]
Number of shares authorized to be repurchased (in shares)	10,000,000
Repurchase of common shares (in shares)				2,216,799
Average cost per share (in dollars per share)				$ 8.29
Series C [Member]
Equity [Line Items]
Preferred shares outstanding				1,935,400	1,935,400
Dividend rate, per-dollar-amount				$ 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				$ 20,797,000	$ 25,813,000	$ 399,566,000
At The Market [Member]
Equity [Line Items]
Common shares issued through public offering and direct share purchase plan during period				0	0	3,409,927
Value authorized						$ 100,000,000
Value, new issues						$ 36,884,000