Condensed consolidating financial information - Cash Flows (Details) - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities	$ (30.4)	$ 3.1
Net cash provided by (used in) investing activities
Purchase of property, plant and equipment	(13.0)	(18.9)
Proceeds from sale of property, plant and equipment		13.6
Intercompany transactions	0.0	0.0
Net cash used in investing activities	(13.0)	(5.3)
Net cash provided by (used in) financing activities
Borrowings under revolving credit facilities	0.0	0.7
Repayments under revolving credit facilities	0.0	(0.7)
Issuance of common stock	0.2	0.2
Intercompany transactions	0.0	0.0
Net cash provided by financing activities	0.2	0.2
CHANGE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	(43.2)	(2.0)
Cash, cash equivalents and restricted cash, beginning of period	168.0	133.5
Cash, cash equivalents and restricted cash, end of period	124.8	131.5
Consolidating Adjustments [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities	0.0	0.0
Net cash provided by (used in) investing activities
Purchase of property, plant and equipment	0.0	0.0
Proceeds from sale of property, plant and equipment		0.0
Intercompany transactions	(27.0)	28.2
Net cash used in investing activities	(27.0)	28.2
Net cash provided by (used in) financing activities
Borrowings under revolving credit facilities		0.0
Repayments under revolving credit facilities		0.0
Issuance of common stock	0.0	0.0
Intercompany transactions	27.0	(28.2)
Net cash provided by financing activities	27.0	(28.2)
CHANGE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	0.0	0.0
Cash, cash equivalents and restricted cash, beginning of period	0.0	0.0
Cash, cash equivalents and restricted cash, end of period	0.0	0.0
The Company [Member] \| Reportable Legal Entities [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities	(42.9)	(8.0)
Net cash provided by (used in) investing activities
Purchase of property, plant and equipment	(4.2)	(7.8)
Proceeds from sale of property, plant and equipment		0.0
Intercompany transactions	17.2	11.8
Net cash used in investing activities	13.0	4.0
Net cash provided by (used in) financing activities
Borrowings under revolving credit facilities		0.7
Repayments under revolving credit facilities		(0.7)
Issuance of common stock	0.2	0.2
Intercompany transactions	(7.3)	(3.6)
Net cash provided by financing activities	(7.1)	(3.4)
CHANGE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	(37.0)	(7.4)
Cash, cash equivalents and restricted cash, beginning of period	64.3	36.8
Cash, cash equivalents and restricted cash, end of period	27.3	29.4
Combined Guarantor Subsidiaries [Member] \| Reportable Legal Entities [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities	(2.9)	(25.9)
Net cash provided by (used in) investing activities
Purchase of property, plant and equipment	(5.5)	(5.0)
Proceeds from sale of property, plant and equipment		0.0
Intercompany transactions	10.2	(27.9)
Net cash used in investing activities	4.7	(32.9)
Net cash provided by (used in) financing activities
Borrowings under revolving credit facilities		0.0
Repayments under revolving credit facilities		0.0
Issuance of common stock	0.0	0.0
Intercompany transactions	(1.8)	58.8
Net cash provided by financing activities	(1.8)	58.8
CHANGE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	0.0	0.0
Cash, cash equivalents and restricted cash, beginning of period	0.7	0.5
Cash, cash equivalents and restricted cash, end of period	0.7	0.5
Combined Non-Guarantor Subsidiaries [Member] \| Reportable Legal Entities [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities	15.4	37.0
Net cash provided by (used in) investing activities
Purchase of property, plant and equipment	(3.3)	(6.1)
Proceeds from sale of property, plant and equipment		13.6
Intercompany transactions	(0.4)	(12.1)
Net cash used in investing activities	(3.7)	(4.6)
Net cash provided by (used in) financing activities
Borrowings under revolving credit facilities		0.0
Repayments under revolving credit facilities		0.0
Issuance of common stock	0.0	0.0
Intercompany transactions	(17.9)	(27.0)
Net cash provided by financing activities	(17.9)	(27.0)
CHANGE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	(6.2)	5.4
Cash, cash equivalents and restricted cash, beginning of period	103.0	96.2
Cash, cash equivalents and restricted cash, end of period	$ 96.8	$ 101.6

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