Common Stock of ROIC (Details Textual)		3 Months Ended	9 Months Ended
	May 01, 2018 USD ($) agreement $ / shares	Sep. 30, 2018 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) $ / shares shares	Sep. 30, 2017 USD ($)	Dec. 31, 2017 $ / shares	Jul. 31, 2013 USD ($)
Class of Stock [Line Items]
Common stock, par value (in dollars per share) | $ / shares		$ 0.0001	$ 0.0001		$ 0.0001
Proceeds from the sale of common stock			$ 25,703,000	$ 0
Registration expenditures			$ 376,000	$ 365,000
Stock repurchase program, authorized amount						$ 50,000,000.0
Sales Agreement
Class of Stock [Line Items]
Number of sales agreements entered into | agreement	5
Common stock, par value (in dollars per share) | $ / shares	$ 0.0001
Common shares that may be sold under a sales agreement, aggregate offering price, maximum	$ 250,000,000.00
Shares sold during the year (shares) | shares		1,251,376	1,251,376
Proceeds from the sale of common stock		$ 24,200,000	$ 24,200,000
Registration expenditures		$ 242,000	$ 242,000
Prior Sales Agreements
Class of Stock [Line Items]
Shares sold during the year (shares) | shares			75,314
Proceeds from the sale of common stock			$ 1,500,000
Registration expenditures			$ 19,000