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Business and summary of significant accounting policies (Narrative) (Details) $ in Millions | 5 Months Ended | 7 Months Ended | 12 Months Ended
--- | --- | --- | ---
Dec. 31, 2015 USD ($) | Aug. 14, 2015 USD ($) | Dec. 31, 2017 USD ($) category | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Business And Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |
Number of major product categories | category |  |  | 2 |  |
Depreciation expense | $ 18.8 | $ 26.7 | $ 50.4 | $ 63.4 |
Capital expenditures within accounts payable |  |  | 13.6 |  |
Bank fees capitalized |  |  | 0.4 | 0.7 |
Amortization of capitalized bank fees | 0.0 | 2.1 | 0.3 | 0.2 |
Environmental remediation, compliance and management expenses |  |  | 8.0 | 8.3 | $ 6.5
Accrued liability relating to environmental remediation |  |  | 2.1 | 5.2 |
Deferred costs |  |  | 0.0 | 0.0 |
Amortized costs | $ 2.1 | $ 4.3 | $ 3.3 | $ 7.0 |
Commodity derivative contracts | Minimum |  |  |  |  |
Business And Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |
Derivative maturities |  |  | 3 years |  |
Commodity derivative contracts | Maximum |  |  |  |  |
Business And Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |
Derivative maturities |  |  | 5 years |  |

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- Definition Amount of expense for newly incurred environmental loss contingencies. Includes, but is not limited to, remediation expense and interest accretion. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 5.Y.Q2) -URI http://asc.fasb.org/extlink&oid=27011672&loc=d3e149879-122751 + Details
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