Common Shares (Activity Under At-The-Market Share Offering Program) (Details) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Total net consideration	$ 66,225,000	$ 40,044,000	$ 693,355,000
Common shares sold	898	555	11,192
Two thousand fourteen ATM program [Domain]
Total net consideration	15,690,200
Common shares sold	209,700
Average price per common share sold	$ 76.28
2012 ATM Program [Member]
Total net consideration	50,535,300	40,044,100
Common shares sold	688,300	555,100
Average price per common share sold	$ 74.60	$ 73.73
2012 and 2011 Atm Programs [Domain]
Total net consideration			$ 301,735,500
Common shares sold			4,579,300
Average price per common share sold			$ 66.93