(Narrative) (Details) (USD $)	1 Months Ended	3 Months Ended		9 Months Ended		1 Months Ended	9 Months Ended		1 Months Ended		9 Months Ended
	Dec. 31, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Jun. 30, 2013 4 Year Facility	Sep. 30, 2013 4 Year Facility	Jun. 19, 2013 4 Year Facility	Jun. 30, 2013 3 Year Facility	Jun. 18, 2013 3 Year Facility	Sep. 30, 2013 Minimum 4 Year Facility	Sep. 30, 2013 Maximum	Sep. 30, 2013 Maximum 4 Year Facility
Debt Instrument [Line Items]
Long-term debt fair value	$ 916,000,000	$ 816,000,000		$ 816,000,000
Maximum borrowing capacity							1,000,000,000	1,000,000,000		1,200,000,000
Dividend declared per common share (in usd per share)	$ 2.50	$ 0.28	$ 0.255	$ 0.84	$ 0.765
Line of credit amount outstanding	$ 457,000,000
Line of credit, interest rate	0.48%
Line of credit, term at period end	28 days
Term of facility						4 years			3 years
Commitment fee							0.25%				0.20%		0.45%
Indebtedness to cash flow												3.25