Guarantee of Securities of Subsidiaries Statements of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	$ 637.0	$ 600.8	$ 576.4
INVESTING ACTIVITIES:
Capital expenditures	(708.4)	(590.4)	(374.5)
Proceeds from sale of operating assets	63.8	0.2	0.8
Proceeds from other recoveries	0.0	0.0	6.2
Advances to affiliates, net	0.0	0.0	0.0
Net cash provided by (used in) investing activities	(644.6)	(590.2)	(367.5)
FINANCING ACTIVITIES:
Proceeds from long-term debt, net of issuance cost	494.0	539.1	247.1
Repayment of borrowings from long-term debt and term loan	(575.0)	(250.0)	(725.0)
Proceeds from borrowings on revolving credit agreement	765.0	490.0	1,125.0
Repayment of borrowings on revolving credit agreement, including financing fees	(560.8)	(685.8)	(873.6)
Principal payment of capital lease obligation	(0.5)	(0.5)	(0.4)
Advances from affiliates, net	0.1	0.3	0.6
Distributions paid	(102.2)	(102.2)	(101.5)
Proceeds from sale of common units	0.0	0.0	113.1
Capital contributions from general partner	0.0	0.0	2.3
Net cash (used in) provided by financing activities	20.6	(9.1)	(212.4)
Increase (decrease) in cash and cash equivalents	13.0	1.5	(3.5)
Cash and cash equivalents at beginning of period	4.6	3.1	6.6
Cash and cash equivalents at end of period	17.6	4.6	3.1
Reportable Legal Entities | Parent Guarantor
Condensed Financial Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	46.9	37.3	27.9
INVESTING ACTIVITIES:
Capital expenditures	0.0	0.0	(1.0)
Proceeds from sale of operating assets	0.0	0.0	0.0
Proceeds from other recoveries			0.0
Advances to affiliates, net	54.9	65.2	(41.9)
Net cash provided by (used in) investing activities	54.9	65.2	(42.9)
FINANCING ACTIVITIES:
Proceeds from long-term debt, net of issuance cost	0.0	0.0	0.0
Repayment of borrowings from long-term debt and term loan	0.0	0.0	0.0
Proceeds from borrowings on revolving credit agreement	0.0	0.0	0.0
Repayment of borrowings on revolving credit agreement, including financing fees	0.0	0.0	0.0
Principal payment of capital lease obligation	0.0	0.0	0.0
Advances from affiliates, net	0.1	0.3	0.6
Distributions paid	(102.2)	(102.2)	(101.5)
Proceeds from sale of common units			113.1
Capital contributions from general partner			2.3
Net cash (used in) provided by financing activities	(102.1)	(101.9)	14.5
Increase (decrease) in cash and cash equivalents	(0.3)	0.6	(0.5)
Cash and cash equivalents at beginning of period	0.6	0.0	0.5
Cash and cash equivalents at end of period	0.3	0.6	0.0
Reportable Legal Entities | Subsidiary Issuer
Condensed Financial Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	(161.5)	(161.9)	(136.3)
INVESTING ACTIVITIES:
Capital expenditures	0.0	0.0	0.0
Proceeds from sale of operating assets	0.0	0.0	0.0
Proceeds from other recoveries			0.0
Advances to affiliates, net	(434.4)	(20.6)	(269.0)
Net cash provided by (used in) investing activities	(434.4)	(20.6)	(269.0)
FINANCING ACTIVITIES:
Proceeds from long-term debt, net of issuance cost	494.0	539.1	247.1
Repayment of borrowings from long-term debt and term loan	(300.0)	(250.0)	0.0
Proceeds from borrowings on revolving credit agreement	0.0	0.0	0.0
Repayment of borrowings on revolving credit agreement, including financing fees	(0.8)	(0.8)	(3.6)
Principal payment of capital lease obligation	0.0	0.0	0.0
Advances from affiliates, net	405.5	(104.3)	160.3
Distributions paid	0.0	0.0	0.0
Proceeds from sale of common units			0.0
Capital contributions from general partner			0.0
Net cash (used in) provided by financing activities	598.7	184.0	403.8
Increase (decrease) in cash and cash equivalents	2.8	1.5	(1.5)
Cash and cash equivalents at beginning of period	1.8	0.3	1.8
Cash and cash equivalents at end of period	4.6	1.8	0.3
Reportable Legal Entities | Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	751.6	725.4	684.8
INVESTING ACTIVITIES:
Capital expenditures	(708.4)	(590.4)	(373.5)
Proceeds from sale of operating assets	63.8	0.2	0.8
Proceeds from other recoveries			6.2
Advances to affiliates, net	(460.4)	39.1	(118.4)
Net cash provided by (used in) investing activities	(1,105.0)	(551.1)	(484.9)
FINANCING ACTIVITIES:
Proceeds from long-term debt, net of issuance cost	0.0	0.0	0.0
Repayment of borrowings from long-term debt and term loan	(275.0)	0.0	(725.0)
Proceeds from borrowings on revolving credit agreement	765.0	490.0	1,125.0
Repayment of borrowings on revolving credit agreement, including financing fees	(560.0)	(685.0)	(870.0)
Principal payment of capital lease obligation	(0.5)	(0.5)	(0.4)
Advances from affiliates, net	434.4	20.6	269.0
Distributions paid	0.0	0.0	0.0
Proceeds from sale of common units			0.0
Capital contributions from general partner			0.0
Net cash (used in) provided by financing activities	363.9	(174.9)	(201.4)
Increase (decrease) in cash and cash equivalents	10.5	(0.6)	(1.5)
Cash and cash equivalents at beginning of period	2.2	2.8	4.3
Cash and cash equivalents at end of period	12.7	2.2	2.8
Eliminations
Condensed Financial Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	0.0	0.0	0.0
INVESTING ACTIVITIES:
Capital expenditures	0.0	0.0	0.0
Proceeds from sale of operating assets	0.0	0.0	0.0
Proceeds from other recoveries			0.0
Advances to affiliates, net	839.9	(83.7)	429.3
Net cash provided by (used in) investing activities	839.9	(83.7)	429.3
FINANCING ACTIVITIES:
Proceeds from long-term debt, net of issuance cost	0.0	0.0	0.0
Repayment of borrowings from long-term debt and term loan	0.0	0.0	0.0
Proceeds from borrowings on revolving credit agreement	0.0	0.0	0.0
Repayment of borrowings on revolving credit agreement, including financing fees	0.0	0.0	0.0
Principal payment of capital lease obligation	0.0	0.0	0.0
Advances from affiliates, net	(839.9)	83.7	(429.3)
Distributions paid	0.0	0.0	0.0
Proceeds from sale of common units			0.0
Capital contributions from general partner			0.0
Net cash (used in) provided by financing activities	(839.9)	83.7	(429.3)
Increase (decrease) in cash and cash equivalents	0.0	0.0	0.0
Cash and cash equivalents at beginning of period	0.0	0.0	0.0
Cash and cash equivalents at end of period	$ 0.0	$ 0.0	$ 0.0