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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Oct. 30, 2017	Oct. 27, 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	$ 0.30	$ 0.30
Percent of General Partner Interest Owned by Holding Company (in hundredths)	2.00%	2.00%	2.00%	2.00%
Net income	$ 69.8	$ 47.3	$ 212.8	$ 214.0
Declared distribution	25.5	25.5	76.6	76.6
Assumed allocation of undistributed net income	44.3	21.8	136.2	137.4
Assumed allocation of net income attributable to limited partner unitholders and general partner	$ 69.8	$ 47.3	$ 212.8	$ 214.0
Weighted-average units outstanding (in units)	250.3	250.3	250.3	250.3
Net income per unit (in dollars per unit)	$ 0.27	$ 0.19	$ 0.83	$ 0.84
Common Units
Reconciliation And Allocation of Net Income By Class of Stock [Line Items]
Declared distribution	$ 25.0	$ 25.0	$ 75.1	$ 75.1
Assumed allocation of undistributed net income	43.4	21.4	133.5	134.7
Assumed allocation of net income attributable to limited partner unitholders and general partner	$ 68.4	$ 46.4	$ 208.6	$ 209.8
Weighted-average units outstanding (in units)	250.3	250.3	250.3	250.3
Net income per unit (in dollars per unit)	$ 0.27	$ 0.19	$ 0.83	$ 0.84
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	0.9	0.4	2.7	2.7
Assumed allocation of net income attributable to limited partner unitholders and general partner	$ 1.4	$ 0.9	$ 4.2	$ 4.2
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