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Subsequent Events (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended			1 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Feb. 28, 2013 Subsequent Event [Member]
Subsequent Event [Line Items]
Number of common units issued (in units)				9,200,000
Over-allotment of common units included in offering (in units)				1,200,000
Offering price of common unit (in dollars per share)				$ 54.56
Net cash proceeds from issuance of common units	$ 816.8	$ 542.9	$ 528.5	$ 486.6

Total units issued during the year due to the sale of units. All partners include general, limited and preferred partners.

The cash inflow from the issuance of common limited partners units during the period.