Condensed consolidating financial information - Cash Flows (Details) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities	$ (59.0)	$ 50.4
Net cash provided by (used in) investing activities
Purchase of property, plant and equipment	(49.3)	(23.6)
Proceeds from sale of property, plant and equipment	0.0	14.5
Intercompany transactions	0.0	0.0
Net cash (used in) investing activities	(49.3)	(9.1)
Net cash provided by (used in) financing activities
Borrowings under revolving credit facilities	14.3	1.0
Repayments under revolving credit facilities	0.0	(1.0)
Revolving credit facility	14.3
Issuance of common stock	0.2	0.3
Intercompany transactions	0.0	0.0
Net cash provided by financing activities	14.5	0.3
CHANGE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	(93.8)	41.6
Cash, cash equivalents and restricted cash, beginning of period	168.0	133.5
Cash, cash equivalents and restricted cash, end of period	74.2	175.1
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	(80.0)	(0.6)
Net cash (used in) investing activities	(80.0)	(0.6)
Net cash provided by (used in) financing activities
Borrowings under revolving credit facilities		0.0
Repayments under revolving credit facilities		0.0
Revolving credit facility	0.0
Issuance of common stock	0.0	0.0
Intercompany transactions	80.0	0.6
Net cash provided by financing activities	80.0	0.6
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	(51.2)	(25.5)
Net cash provided by (used in) investing activities
Purchase of property, plant and equipment	(4.0)	(8.8)
Proceeds from sale of property, plant and equipment		0.0
Intercompany transactions	39.5	34.2
Net cash (used in) investing activities	35.5	25.4
Net cash provided by (used in) financing activities
Borrowings under revolving credit facilities		1.0
Repayments under revolving credit facilities		(1.0)
Revolving credit facility	14.3
Issuance of common stock	0.2	0.3
Intercompany transactions	(37.1)	33.6
Net cash provided by financing activities	(22.6)	33.9
CHANGE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	(38.3)	33.8
Cash, cash equivalents and restricted cash, beginning of period	64.3	36.8
Cash, cash equivalents and restricted cash, end of period	26.0	70.6
Combined Guarantor Subsidiaries [Member] \| Reportable Legal Entities [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities	(0.5)	14.3
Net cash provided by (used in) investing activities
Purchase of property, plant and equipment	(38.9)	(5.9)
Proceeds from sale of property, plant and equipment		0.9
Intercompany transactions	40.9	(26.0)
Net cash (used in) investing activities	2.0	(31.0)
Net cash provided by (used in) financing activities
Borrowings under revolving credit facilities		0.0
Repayments under revolving credit facilities		0.0
Revolving credit facility	0.0
Issuance of common stock	0.0	0.0
Intercompany transactions	(1.7)	16.8
Net cash provided by financing activities	(1.7)	16.8
CHANGE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	(0.2)	0.1
Cash, cash equivalents and restricted cash, beginning of period	0.7	0.5
Cash, cash equivalents and restricted cash, end of period	0.5	0.6
Combined Non-Guarantor Subsidiaries [Member] \| Reportable Legal Entities [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities	(7.3)	61.6
Net cash provided by (used in) investing activities
Purchase of property, plant and equipment	(6.4)	(8.9)
Proceeds from sale of property, plant and equipment		13.6
Intercompany transactions	(0.4)	(7.6)
Net cash (used in) investing activities	(6.8)	(2.9)
Net cash provided by (used in) financing activities
Borrowings under revolving credit facilities		0.0
Repayments under revolving credit facilities		0.0
Revolving credit facility	0.0
Issuance of common stock	0.0	0.0
Intercompany transactions	(41.2)	(51.0)
Net cash provided by financing activities	(41.2)	(51.0)
CHANGE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	(55.3)	7.7
Cash, cash equivalents and restricted cash, beginning of period	103.0	96.2
Cash, cash equivalents and restricted cash, end of period	$ 47.7	$ 103.9

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