Summary of Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Accounting Policies [Abstract]
General partnership interest percentage	98.89%		98.89%		99.93%
Depreciation expense	$ 18.4	$ 18.5	$ 36.6	$ 35.2