Common Shares (Activity Under At-The-Market Share Offering Program) (Details) (USD $)	9 Months Ended		3 Months Ended		9 Months Ended		1 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 2012 ATM Program [Member]	Sep. 30, 2013 2012 ATM Program [Member]	Sep. 30, 2014 2012 ATM Program [Member]	Sep. 30, 2013 2012 ATM Program [Member]	Nov. 01, 2014 Subsequent Event [Member] 2012 ATM Program [Member]	Oct. 31, 2014 Subsequent Event [Member] 2012 ATM Program [Member]
Total net consideration	$ 50,535,000	$ 40,098,000	$ 50,535,300	$ 0	$ 50,535,300	$ 40,097,600
Common shares sold			688,300	0	688,300	555,100	0
Average price per share			$ 74.60	$ 0.00	$ 74.60	$ 73.73
Maximum aggregate offering price of remaining common shares available for sale								$ 31,300,000