Cash Distributions and Net Income per Unit (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Jul. 31, 2017	Jul. 28, 2017
Reconciliation And Allocation of Net Income By Class of Stock [Line Items]
Cash distribution declared and paid to common units per common unit (in dollars per unit)	$ 0.10	$ 0.10	$ 0.20	$ 0.20
Percent of General Partner Interest Owned by Holding Company (in hundredths)	2.00%	2.00%	2.00%	2.00%
Net income	$ 23.7	$ 65.7	$ 143.0	$ 166.7
Declared distribution	25.6	25.6	51.1	51.1
Assumed allocation of undistributed net income	(1.9)	40.1	91.9	115.6
Assumed allocation of net income attributable to limited partner unitholders and general partner	$ 23.7	$ 65.7	$ 143.0	$ 166.7
Weighted-average units outstanding (in units)	250.3	250.3	250.3	250.3
Net income per unit (in dollars per unit)	$ 0.09	$ 0.26	$ 0.56	$ 0.65
Common Units
Reconciliation And Allocation of Net Income By Class of Stock [Line Items]
Declared distribution	$ 25.1	$ 25.1	$ 50.1	$ 50.1
Assumed allocation of undistributed net income	(1.9)	39.3	90.1	113.3
Assumed allocation of net income attributable to limited partner unitholders and general partner	$ 23.2	$ 64.4	$ 140.2	$ 163.4
Weighted-average units outstanding (in units)	250.3	250.3	250.3	250.3
Net income per unit (in dollars per unit)	$ 0.09	$ 0.26	$ 0.56	$ 0.65
General Partner and IDRs
Reconciliation And Allocation of Net Income By Class of Stock [Line Items]
Declared distribution	$ 0.5	$ 0.5	$ 1.0	$ 1.0
Assumed allocation of undistributed net income	0.0	0.8	1.8	2.3
Assumed allocation of net income attributable to limited partner unitholders and general partner	$ 0.5	$ 1.3	$ 2.8	$ 3.3
Subsequent Event
Reconciliation And Allocation of Net Income By Class of Stock [Line Items]
Percent of General Partner Interest Owned by Holding Company (in hundredths)						2.00%
Dividends Payable, Amount Per Share					$ 0.10