Guarantee of Securities of Subsidiaries Guarantee of Securities of Subsidiaries Statements of Cash Flows (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Statement of Cash Flows
Net cash provided by (used in) operating activities	$ 377.3	$ 305.5
Investing Activities:
Capital expenditures	(179.7)	(228.9)
Proceeds from sale of operating assets	2.2	0.6
Advances to affiliates, net	(3.1)	0.0
Net cash provided by (used in) investing activities	(180.6)	(228.3)
Financing Activities:
Proceeds from long-term debt, net of issuance cost	495.2	0.0
Repayment of borrowings from long-term debt	0.0	(185.0)
Proceeds from borrowings on revolving credit agreement	165.0	365.0
Repayment of borrowings on revolving credit agreement	(745.0)	(215.0)
Principal payment of finance lease obligation	(0.3)	(0.3)
Advances from affiliates, net	3.1	0.1
Distributions paid	(51.1)	(51.1)
Net cash (used in) provided by financing activities	(133.1)	(86.3)
(Decrease) increase in cash and cash equivalents	63.6	(9.1)
Cash and cash equivalents at beginning of period	3.6	17.6
Cash and cash equivalents at end of period	67.2	8.5
Reportable Legal Entities | Parent Guarantor
Statement of Cash Flows
Net cash provided by (used in) operating activities	37.0	31.4
Investing Activities:
Capital expenditures	0.0	0.0
Proceeds from sale of operating assets	0.0	0.0
Advances to affiliates, net	10.9	19.3
Net cash provided by (used in) investing activities	10.9	19.3
Financing Activities:
Proceeds from long-term debt, net of issuance cost	0.0
Repayment of borrowings from long-term debt		0.0
Proceeds from borrowings on revolving credit agreement	0.0	0.0
Repayment of borrowings on revolving credit agreement	0.0	0.0
Principal payment of finance lease obligation	0.0	0.0
Advances from affiliates, net	3.1	0.1
Distributions paid	(51.1)	(51.1)
Net cash (used in) provided by financing activities	(48.0)	(51.0)
(Decrease) increase in cash and cash equivalents	(0.1)	(0.3)
Cash and cash equivalents at beginning of period	0.3	0.3
Cash and cash equivalents at end of period	0.2	0.0
Reportable Legal Entities | Subsidiary Issuer
Statement of Cash Flows
Net cash provided by (used in) operating activities	(91.0)	(85.1)
Investing Activities:
Capital expenditures	0.0	0.0
Proceeds from sale of operating assets	0.0	0.0
Advances to affiliates, net	(274.7)	62.1
Net cash provided by (used in) investing activities	(274.7)	62.1
Financing Activities:
Proceeds from long-term debt, net of issuance cost	495.2
Repayment of borrowings from long-term debt		(185.0)
Proceeds from borrowings on revolving credit agreement	0.0	0.0
Repayment of borrowings on revolving credit agreement	0.0	0.0
Principal payment of finance lease obligation	0.0	0.0
Advances from affiliates, net	(124.9)	204.5
Distributions paid	0.0	0.0
Net cash (used in) provided by financing activities	370.3	19.5
(Decrease) increase in cash and cash equivalents	4.6	(3.5)
Cash and cash equivalents at beginning of period	1.6	4.6
Cash and cash equivalents at end of period	6.2	1.1
Reportable Legal Entities | Non-Guarantor Subsidiaries
Statement of Cash Flows
Net cash provided by (used in) operating activities	431.3	359.2
Investing Activities:
Capital expenditures	(179.7)	(228.9)
Proceeds from sale of operating assets	2.2	0.6
Advances to affiliates, net	110.9	(223.8)
Net cash provided by (used in) investing activities	(66.6)	(452.1)
Financing Activities:
Proceeds from long-term debt, net of issuance cost	0.0
Repayment of borrowings from long-term debt		0.0
Proceeds from borrowings on revolving credit agreement	165.0	365.0
Repayment of borrowings on revolving credit agreement	(745.0)	(215.0)
Principal payment of finance lease obligation	(0.3)	(0.3)
Advances from affiliates, net	274.7	(62.1)
Distributions paid	0.0	0.0
Net cash (used in) provided by financing activities	(305.6)	87.6
(Decrease) increase in cash and cash equivalents	59.1	(5.3)
Cash and cash equivalents at beginning of period	1.7	12.7
Cash and cash equivalents at end of period	60.8	7.4
Eliminations
Statement of Cash Flows
Net cash provided by (used in) operating activities	0.0	0.0
Investing Activities:
Capital expenditures	0.0	0.0
Proceeds from sale of operating assets	0.0	0.0
Advances to affiliates, net	149.8	142.4
Net cash provided by (used in) investing activities	149.8	142.4
Financing Activities:
Proceeds from long-term debt, net of issuance cost	0.0
Repayment of borrowings from long-term debt		0.0
Proceeds from borrowings on revolving credit agreement	0.0	0.0
Repayment of borrowings on revolving credit agreement	0.0	0.0
Principal payment of finance lease obligation	0.0	0.0
Advances from affiliates, net	(149.8)	(142.4)
Distributions paid	0.0	0.0
Net cash (used in) provided by financing activities	(149.8)	(142.4)
(Decrease) increase in cash and cash equivalents	0.0	0.0
Cash and cash equivalents at beginning of period	0.0	0.0
Cash and cash equivalents at end of period	$ 0.0	$ 0.0