Common Shares (Narrative) (Details) (USD $)	1 Months Ended	3 Months Ended			1 Months Ended	3 Months Ended	1 Months Ended		3 Months Ended	1 Months Ended
	Jan. 31, 2012	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011	Mar. 31, 2010 2010 ATM Program [Member]	Mar. 31, 2011 2010 ATM Program [Member]	Apr. 27, 2012 2011 ATM Program [Member]	May 31, 2011 2011 ATM Program [Member]	Mar. 31, 2012 2011 ATM Program [Member]	May 31, 2012 2012 ATM Program [Member]	Jan. 31, 2012 January 2012 Public Equity Offering [Member]
Maximum aggregate offering price of common shares					$ 250,000,000			$ 300,000,000		$ 300,000,000
Maximum aggregate offering price of remaining common shares available for sale							132,600,000		160,600,000
Common shares sold						71,300	400,000		704,200		6,612,500
Total net consideration		$ 435,911,000	$ 3,795,000			$ 3,794,600	$ 27,600,000		$ 44,291,700		$ 391,600,000
Number of common and preferred stock authorized to issue		110,000,000
Common shares, authorized		100,000,000		100,000,000
Preferred shares, authorized		10,000,000
Percentage of voting interests acquired	80.00%
Number of unconsolidated equity method investments acquired	12	12