Subsequent Event (Details) (USD $) In Millions, except Share data, unless otherwise specified	9 Months Ended		0 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Nov. 08, 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)			$ 62.05
Net cash proceeds from issuance of common units	$ 1,134.7	$ 658.6	$ 553.4