Common Shares (Activity Under At-The-Market Share Offering Program) (Details) (USD $)	9 Months Ended		3 Months Ended	9 Months Ended	1 Months Ended	3 Months Ended	9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011 2010 ATM Program [Member]	Sep. 30, 2011 2010 ATM Program [Member]	Nov. 04, 2011 2011 ATM Program [Member]	Sep. 30, 2011 2011 ATM Program [Member]	Sep. 30, 2011 2011 ATM Program [Member]
Total net consideration	$ 69,863,000	$ 134,617,000		$ 13,847,000		$ 32,736,800	$ 56,015,900
Common shares sold			0	252,500	0	506,200	875,400
Average price per common share sold				$ 55.81		$ 65.76	$ 65.24