Condensed consolidating financial information - Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities	$ (33.0)	$ (11.3)
Net cash provided by (used in) investing activities
Purchase of property, plant and equipment	(3.5)	(9.0)
Proceeds from sale of property, plant and equipment		13.5
Intercompany transactions	0.0	0.0
Net cash provided by (used in) investing activities	(3.5)	4.5
Net cash provided by (used in) financing activities
Borrowings under revolving credit facilities	0.3	0.2
Repayments under revolving credit facilities	(0.3)	(0.2)
Issuance of common stock	0.1	0.0
Intercompany transactions	0.0	0.0
Net cash provided by financing activities	0.1	0.0
CHANGE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	(36.4)	(6.8)
Cash, cash equivalents and restricted cash, beginning of period	168.0	133.5
Cash, cash equivalents and restricted cash, end of period	131.6	126.7
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	23.8	(30.4)
Net cash provided by (used in) investing activities	23.8	(30.4)
Net cash provided by (used in) financing activities
Borrowings under revolving credit facilities	0.0	0.0
Repayments under revolving credit facilities	0.0	0.0
Issuance of common stock	0.0
Intercompany transactions	(23.8)	30.4
Net cash provided by financing activities	(23.8)	30.4
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	(26.8)	(1.1)
Net cash provided by (used in) investing activities
Purchase of property, plant and equipment	(1.5)	(4.4)
Proceeds from sale of property, plant and equipment		0.0
Intercompany transactions	(23.8)	19.2
Net cash provided by (used in) investing activities	(25.3)	14.8
Net cash provided by (used in) financing activities
Borrowings under revolving credit facilities	0.3	0.2
Repayments under revolving credit facilities	(0.3)	(0.2)
Issuance of common stock	0.1
Intercompany transactions	(3.8)	(11.2)
Net cash provided by financing activities	(3.7)	(11.2)
CHANGE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	(55.8)	2.5
Cash, cash equivalents and restricted cash, beginning of period	64.3	36.7
Cash, cash equivalents and restricted cash, end of period	8.5	39.2
Combined Guarantor Subsidiaries [Member] \| Reportable Legal Entities [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities	(26.8)	(21.8)
Net cash provided by (used in) investing activities
Purchase of property, plant and equipment	(0.4)	(2.3)
Proceeds from sale of property, plant and equipment		0.0
Intercompany transactions	0.0	23.6
Net cash provided by (used in) investing activities	(0.4)	21.3
Net cash provided by (used in) financing activities
Borrowings under revolving credit facilities	0.0	0.0
Repayments under revolving credit facilities	0.0	0.0
Issuance of common stock	0.0
Intercompany transactions	27.2	0.8
Net cash provided by financing activities	27.2	0.8
CHANGE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	0.0	0.3
Cash, cash equivalents and restricted cash, beginning of period	0.7	0.5
Cash, cash equivalents and restricted cash, end of period	0.7	0.8
Combined Non-Guarantor Subsidiaries [Member] \| Reportable Legal Entities [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities	20.6	11.6
Net cash provided by (used in) investing activities
Purchase of property, plant and equipment	(1.6)	(2.3)
Proceeds from sale of property, plant and equipment		13.5
Intercompany transactions	0.0	(12.4)
Net cash provided by (used in) investing activities	(1.6)	(1.2)
Net cash provided by (used in) financing activities
Borrowings under revolving credit facilities	0.0	0.0
Repayments under revolving credit facilities	0.0	0.0
Issuance of common stock	0.0
Intercompany transactions	0.4	(20.0)
Net cash provided by financing activities	0.4	(20.0)
CHANGE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	19.4	(9.6)
Cash, cash equivalents and restricted cash, beginning of period	103.0	96.3
Cash, cash equivalents and restricted cash, end of period	$ 122.4	$ 86.7

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