Equity (Details) - USD ($)				5 Months Ended	9 Months Ended
	Oct. 30, 2015	May. 01, 2015	May. 22, 2014	Sep. 30, 2015	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Equity, Class of Treasury Stock [Line Items]
Share repurchase program, authorized amount		$ 200,000,000.0
Share repurchase program, expiration date		Apr. 30, 2016
Common shares repurchased (in shares)				6,992,708
Common shares repurchased				$ 199,900,000	$ 199,944,000
Common shares of beneficial interest sold (in shares)			9,200,000
Common shares of beneficial interest, par value (in dollars per share)			$ 0.01	$ 0.01	$ 0.01		$ 0.01
Common shares of beneficial interest sold, price per share (in dollars per share)			$ 26.45
Common shares of beneficial interest sold, gross proceeds			$ 243,300,000
Common shares of beneficial interest sold, net proceeds			$ 232,800,000			$ 232,756,000
Subsequent Event
Equity, Class of Treasury Stock [Line Items]
Share repurchase program, authorized amount	$ 400,000,000.0
Share repurchase program, expiration date	Dec. 31, 2016
Share repurchase program, additional authorized amount	$ 200,000,000.0