Cash Distributions and Net Income per Unit (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jul. 30, 2013
Quarterly distributions [Abstract]
Cash distribution to class B units	$ 0.30	$ 0.30	$ 0.60	$ 0.60
Percent of general partner interest owned by holding company	2.00%	2.00%	2.00%	2.00%
Cash distribution declared and paid to common units	$ 0.5325	$ 0.5325	$ 1.065	$ 1.0625
Dividends Payable, Amount Per Share					$ 0.5325
Reconciliation And Allocation of Net Income By Class of Stock [Line Items]
Net Income	$ 70.4	$ 65.1	$ 171.8	$ 157.7
Less: Net loss attributable to noncontrolling interests	(0.1)	0	(0.1)	0
Less: Net income attributable to predecessor equity				0.2
Net income attributable to controlling interest	70.5		171.9	157.5
Declared distribution	135.6	114.9	263.8	229.8
Assumed allocation of undistributed net loss	(65.1)	(49.8)	(91.9)	(72.3)
Assumed allocation of net income attributable to limited partner unitholders and general partner	70.5	65.1	171.9	157.5
Common Units [Member]
Reconciliation And Allocation of Net Income By Class of Stock [Line Items]
Declared distribution	117.3	98.5	227.9	196.9
Assumed allocation of undistributed net loss	(57.6)	(43.5)	(81.2)	(63.1)
Assumed allocation of net income attributable to limited partner unitholders and general partner	59.7	55.0	146.7	133.8
Weighted-average units outstanding (in units)	212.3	184.9	210.0	183.8
Net income per unit	0.28	0.30	0.70	0.73
Class B Units [Member]
Reconciliation And Allocation of Net Income By Class of Stock [Line Items]
Declared distribution	6.9	6.8	13.7	13.7
Assumed allocation of undistributed net loss	(6.2)	(5.3)	(8.9)	(7.8)
Assumed allocation of net income attributable to limited partner unitholders and general partner	0.7	1.5	4.8	5.9
Weighted-average units outstanding (in units)	22.9	22.9	22.9	22.9
Net income per unit	0.03	0.07	0.21	0.26
General Partner and IDRs [Member]
Reconciliation And Allocation of Net Income By Class of Stock [Line Items]
Declared distribution	11.4	9.6	22.2	19.2
Assumed allocation of undistributed net loss	(1.3)	(1.0)	(1.8)	(1.4)
Assumed allocation of net income attributable to limited partner unitholders and general partner	$ 10.1	$ 8.6	$ 20.4	$ 17.8