Equity (Details Textual) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) shares $ / shares	Dec. 31, 2017 $ / shares shares	Jul. 27, 2017 USD ($)	Mar. 01, 2017 USD ($)	Feb. 22, 2016 USD ($)	Mar. 05, 2014 USD ($)
Common Stock Disclosures [Abstract]
Common shares of beneficial interest, shares authorized	500,000,000	500,000,000
Common shares of beneficial interest, par value (usd per share) | $ / shares	$ 0.01	$ 0.01
Preferred Stock Disclosures [Abstract]
Preferred shares of beneficial interest, shares authorized	100,000,000	100,000,000
Preferred shares of beneficial interest, par value (usd per share) | $ / shares	$ 0.01	$ 0.01
Preferred shares of beneficial interest, redemption price per share (usd per share) | $ / shares	$ 25.00
Preferred shares of beneficial interest, shares outstanding	10,000,000	10,000,000
Preferred stock, redemption after change in control	120 days
LTIP Units Disclosures [Abstract]
Operating Partnership outstanding (shares)	236,351	236,351
LTIP units, vested (shares)	145,598
6.50% Series C [Member]
Preferred Stock Disclosures [Abstract]
Preferred stock, dividend rate, percentage	6.50%	6.50%
Preferred shares of beneficial interest, shares outstanding	5,000,000	5,000,000
Share cap on each preferred shares	2.0325
6.375% Series D [Member]
Preferred Stock Disclosures [Abstract]
Preferred stock, dividend rate, percentage	6.375%	6.375%
Preferred shares of beneficial interest, shares outstanding	5,000,000	5,000,000
Share cap on each preferred shares	1.9794
ATMProgram [Member]
Common Stock Disclosures [Abstract]
March 2014 shelf registration statement maximum amount | $						$ 175,000
September 2012 shelf registration statement maximum amount | $						$ 170,000
Amount available under ATM program | $				$ 159,800
Share Repurchase Program [Member]
Common Stock Disclosures [Abstract]
Share repurchase program, authorized amount | $					$ 150,000
Share repurchased, shares	0
Remaining authorized repurchase amount | $	$ 56,600
Share Repurchase Program 2 [Member]
Common Stock Disclosures [Abstract]
Share repurchase program, authorized amount | $			$ 100,000