Common Shares (Activity Under At-The-Market Share Offering Program) (Details) (USD $)	3 Months Ended			1 Months Ended	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2011 2010 ATM Program [Member]	Apr. 27, 2012 2011 ATM Program [Member]	Mar. 31, 2012 2011 ATM Program [Member]
Total net consideration	$ 435,911,000	$ 3,795,000	$ 3,794,600	$ 27,600,000	$ 44,291,700
Common shares sold			71,300	400,000	704,200
Average price per common share sold			$ 54.06	$ 65.23	$ 63.89