Common Shares of Beneficial Interest (Schedule of ATM Equity Program) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Class of Stock [Line Items]
Shares issued (in shares)	123,585,000us-gaap_CommonStockSharesIssued	122,489,000us-gaap_CommonStockSharesIssued
ATM Equity Program [Member]
Class of Stock [Line Items]
Shares issued (in shares)	809,100
Weighted average price per share (in dollars per share)	$ 36.29
Gross proceeds	$ 29,360

Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.29)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 30

-Article 5

Name:	us-gaap_CommonStockSharesIssued
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant