Equity (Details Textual) $ / shares in Units, $ in Millions				9 Months Ended
	Sep. 21, 2016 $ / shares shares	Jun. 09, 2016 USD ($) $ / shares shares	Mar. 11, 2016 $ / shares shares	Sep. 30, 2016 USD ($) shares $ / shares	Dec. 31, 2015 $ / shares shares	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
Stock Repurchase Program, Authorized Amount | $				$ 150.0
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	14,000,000
LTIP Units Disclosures [Abstract]
Operating Partnership outstanding				236,351	236,351
LTIP units, vested				54,845
6.375% Series D [Member]
Preferred Stock Disclosures [Abstract]
Preferred Stock, Dividend Rate, Percentage		6.375%
Stock Issued During Period, Shares, New Issues		5,000,000
Shares Issued, Price Per Share | $ / shares		$ 25.00
ProceedsFromIssuanceOfPreferredStockNet | $		$ 121.1
Preferred shares of beneficial interest, shares outstanding				5,000,000
Share cap on each preferred shares				1.9794
8.00% Series B [Member]
Preferred Stock Disclosures [Abstract]
Stock Redeemed or Called During Period, Shares	3,400,000
Preferred Stock, Dividend Rate, Percentage	8.00%
Dividends Payable, Amount Per Share | $ / shares	$ 0.38
Preferred shares of beneficial interest, shares outstanding				0	3,400,000
Series A Preferred Stock [Member]
Preferred Stock Disclosures [Abstract]
Stock Redeemed or Called During Period, Shares			5,600,000
Preferred Stock, Dividend Rate, Percentage			7.875%
Dividends Payable, Amount Per Share | $ / shares			$ 0.31
Preferred shares of beneficial interest, shares outstanding				0	5,600,000
6.50% Series C [Member]
Preferred Stock Disclosures [Abstract]
Preferred Stock, Dividend Rate, Percentage				6.50%
Preferred shares of beneficial interest, shares outstanding				5,000,000	5,000,000
Share cap on each preferred shares				2.0325
ATMProgram [Member]
Common Stock Disclosures [Abstract]
Mar 2014 Shelf Registration Stmt Maximum Amount | $						$ 175.0
Sept 2012 Shelf Registration Stmt Maximum Amount | $						$ 170.0
Amount Available under ATM Program | $				$ 159.8