Common Shares (Narrative) (Details) (USD $) In Millions, except Share data, unless otherwise specified			1 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	May 31, 2012 2012 ATM Program [Member]	May 31, 2011 2010 and 2011 ATM Programs [Member]	Sep. 30, 2013 Line of Credit [Member]
Maximum aggregate offering price of common shares			$ 300	$ 300
Amount of current borrowing capacity under unsecured credit facility					$ 500
Number of common and preferred stock authorized to issue	185,000,000
Common shares, authorized	175,000,000	175,000,000
Preferred shares, authorized	10,000,000