Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Accounts receivable, allowance for doubtful accounts	$ 1,914	$ 1,861
Common shares of beneficial interest, par (in usd per share)	$ 0.01	$ 0.01
Common shares of beneficial interest, shares authorized (in shares)	120,000,000	120,000,000
Common shares of beneficial interest, shares issued (in shares)	79,343,000	79,272,000
Common shares of beneficial interest, shares outstanding (in shares)	79,343,000	79,272,000
Series D Preferred Stock
Preferred shares, par (in usd per share)	$ 0.01	$ 0.01
Preferred shares, shares authorized (in shares)	2,000,000	2,000,000
Cumulative Perpetual Convertible Preferred Shares, shares issued (in shares)	1,849,000	1,849,000
Cumulative Perpetual Convertible Preferred Shares, liquidation preference (in usd per share)	$ 50	$ 50
Cumulative Perpetual Convertible Preferred Shares, dividend rate percentage	7.25%	7.25%