Common Shares (Activity Under At-The-Market Share Offering Program) (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Total net consideration			$ 40,098,000	$ 693,375,000
2012 ATM Program [Member]
Total net consideration	0	88,915,900	40,097,600	173,627,600
Common shares sold	0	1,302,500	555,100	2,607,900
Average price per common share sold	$ 0.00	$ 69.34	$ 73.73	$ 67.63
Maximum aggregate offering price of remaining common shares available for sale	82,700,000		82,700,000
2010 and 2011 ATM Programs [Member]
Total net consideration		$ 0		$ 128,128,000
Common shares sold		0		1,971,400
Average price per common share sold		$ 0.00		$ 66.01