Proposed Merger With PECO (Details) - Subsequent Event $ / shares in Units, ft² in Millions, $ in Millions	Jul. 17, 2018 USD ($) ft² $ / shares
Subsequent Event [Line Items]
Business combination, date of combination agreement	Jul. 17, 2018
Business acquisition, name of acquiring entity	PECO
Number of real estate properties, post merger transaction	321
Business combination, post merger transaction, net rentable area | ft²	36.6
Number of states in which surviving entity operates	33
Business combination, number of share convertible to acquirer stock	2.04
Business acquisition, value per share, acquiree stock | $ / shares	$ 22.54
Business acquisition, potential termination fee, payable by acquiree for third party proposal	$ 15.9
Business combination, contingent termination fee due to acquirer	31.7
Business combination, contingent termination due from acquirer	$ 75.6
Business Acquisition percentage of voting interests retained by acquirer	71.00%
Business combination, post-transaction acquirer ownership percentage	29.00%
Common Stock
Subsequent Event [Line Items]
Common stock of acquirer, per share, fair value | $ / shares	$ 11.05