Condensed Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Accounts receivable, allowance for doubtful accounts	$ 2,370	$ 2,292
Preferred shares, par (in shares)	$ 0.01	$ 0.01
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	120,000
Common shares of beneficial interest, shares issued (in shares)	78,596	77,573
Common shares of beneficial interest, shares outstanding (in shares)	78,596	77,573
Series D Preferred Stock
Preferred shares, shares authorized (in shares)	2,000	2,000
Cumulative Perpetual Convertible Preferred Shares, shares issued (in shares)	2,000	2,000
Cumulative Perpetual Convertible Preferred Shares, shares outstanding (in shares)	2,000	2,000
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%