Cash Distributions and Net Income per Unit (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Oct. 29, 2012
Quarterly distributions [Abstract]
Cash distribution to class B units	$ 0.30	$ 0.30	$ 0.90	$ 0.90
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	$ 0.5250	$ 1.5950	$ 1.5675
Dividends Payable, Amount Per Share					$ 0.5325
Reconciliation And Allocation of Net Income By Class of Stock [Line Items]
Net Income	$ 59.0	$ 47.2	$ 216.7	$ 145.4
Less: Net income attributable to predecessor equity			(0.2)
Net income attributable to limited partner unitholders and general partner			216.5
Declared distribution	128.2	107.8	364.8	317.6
Assumed allocation of undistributed net loss	(69.2)	(60.6)	(148.3)	(172.2)
Assumed allocation of net income attributable to limited partner unitholders and general partner	59.0		216.5
Assumed allocation of net income		47.2		145.4
Common Units [Member]
Reconciliation And Allocation of Net Income By Class of Stock [Line Items]
Declared distribution	110.6	92.7	313.7	273.6
Assumed allocation of undistributed net loss	(60.6)	(52.5)	(129.5)	(149.0)
Assumed allocation of net income attributable to limited partner unitholders and general partner	50.0		184.2
Assumed allocation of net income		40.2		124.6
Weighted-average units outstanding (in units)	192.6	175.7	186.8	172.5
Net income per unit	0.26	0.23	0.99	0.72
Class B Units [Member]
Reconciliation And Allocation of Net Income By Class of Stock [Line Items]
Declared distribution	6.8	6.9	20.6	20.6
Assumed allocation of undistributed net loss	(7.2)	(6.9)	(15.8)	(19.7)
Assumed allocation of net income attributable to limited partner unitholders and general partner	(0.4)		4.8
Assumed allocation of net income		0		0.9
Weighted-average units outstanding (in units)	22.9	22.9	22.9	22.9
Net income per unit	(0.02)	0.00	0.21	0.04
General Partner and IDRs [Member]
Reconciliation And Allocation of Net Income By Class of Stock [Line Items]
Declared distribution	10.8	8.2	30.5	23.4
Assumed allocation of undistributed net loss	(1.4)	(1.2)	(3.0)	(3.5)
Assumed allocation of net income attributable to limited partner unitholders and general partner	9.4		27.5
Assumed allocation of net income		$ 7.0		$ 19.9