Common Shares (Activity Under At-The-Market Share Offering Program) (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Document Fiscal Year Focus	2012
Total net consideration	$ 693,355,000	$ 106,571,000	$ 231,651,000
Common shares sold	11,192	1,751	4,868
2010 and 2011 ATM Programs [Member]
Total net consideration	128,128,000	106,570,600	231,650,500
Common shares sold	1,971,400	1,751,000	4,867,700
Average price per common share sold	$ 66.01	$ 61.95	$ 48.37
2012 ATM Program [Member]
Total net consideration	$ 173,607,500
Common shares sold	2,607,900
Average price per common share sold	$ 67.63