Cash Distributions and Net Income per Unit (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Nov. 03, 2015	Oct. 31, 2015
Reconciliation And Allocation of Net Income By Class of Stock [Line Items]
Cash distribution declared and paid to common units (in units)	$ 0.10	$ 0.10	$ 0.30	$ 0.30
Percent of general partner interest owned by holding company	2.00%	2.00%	2.00%	2.00%
Net income	$ 38.3	$ 28.4	$ 156.4	$ 109.9
Less: Net loss attributable to noncontrolling interests	0.0	(0.8)	0.0	(86.9)
Net income attributable to controlling interests	38.3	29.2	156.4	196.8
Declared distribution	25.5	24.8	76.6	74.5
Assumed allocation of undistributed net income	12.8	4.4	79.8	122.3
Assumed allocation of net income attributable to limited partner unitholders and general partner	$ 38.3	$ 29.2	$ 156.4	$ 196.8
Weighted-average number of common units outstanding (in units)	250.3	243.3	248.3	243.3
Net income per common unit (in units)	$ 0.15	$ 0.12	$ 0.62	$ 0.79
Subsequent Event
Reconciliation And Allocation of Net Income By Class of Stock [Line Items]
Percent of general partner interest owned by holding company					2.00%
Dividends Payable, Amount Per Share						$ 0.10
Common Units
Reconciliation And Allocation of Net Income By Class of Stock [Line Items]
Declared distribution	$ 25.0	$ 24.3	$ 75.1	$ 73.0
Assumed allocation of undistributed net income	12.5	4.3	78.2	119.9
Assumed allocation of net income attributable to limited partner unitholders and general partner	$ 37.5	$ 28.6	$ 153.3	$ 192.9
Weighted-average number of common units outstanding (in units)	250.3	243.3	248.3	243.3
Net income per common unit (in units)	$ 0.15	$ 0.12	$ 0.62	$ 0.79
General Partner and IDRs
Reconciliation And Allocation of Net Income By Class of Stock [Line Items]
Declared distribution	$ 0.5	$ 0.5	$ 1.5	$ 1.5
Assumed allocation of undistributed net income	0.3	0.1	1.6	2.4
Assumed allocation of net income attributable to limited partner unitholders and general partner	$ 0.8	$ 0.6	$ 3.1	$ 3.9