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Debt and liquidity (Revolving Facility) (Details) - USD ($) $ in Millions | Feb. 27, 2015 | Dec. 31, 2017 | Dec. 31, 2016 | Apr. 27, 2016 | Aug. 11, 2015 | Jul. 28, 2015 | Feb. 23, 2014
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Debt Instrument [Line Items] |  |  |  |  |  |  |
Minimum Liquidity |  |  |  | $ 25.0 |  |  |
12 Month Trailing EBITDA |  | $ 75.0 |  |  |  |  |
12 Month Trailing EBITDA Minimum (Low End) |  |  |  | 40.0 |  |  |
12 Month Trailing EBITDA Minimum (High End) |  |  |  | 35.0 |  |  |
Borrowings |  | 39.5 | $ 61.2 |  |  |  |
Outstanding letters of credit |  | 8.7 | 12.3 |  |  |  |
Revolving Credit Facility balance |  |  | $ 73.5 |  |  |  |
Term loan balance |  | $ 18.7 |  |  | $ 40.0 |  |
Amended and Restated Credit Agreement July 2015 [Member] | Minimum |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage | 0.35% |  |  |  |  |  |
Amended and Restated Credit Agreement July 2015 [Member] | Maximum |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage | 0.70% |  |  |  |  |  |
Amended and Restated Credit Agreement July 2015 [Member] | LIBOR | Minimum |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate | 2.25% |  |  |  |  |  |
Amended and Restated Credit Agreement July 2015 [Member] | LIBOR | Maximum |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate | 4.75% |  |  |  |  |  |
Revolving Credit Facility | Amended and Restated Credit Agreement |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Borrowing capacity |  |  |  |  |  |  | $ 400.0
Revolving Credit Facility | Amended and Restated Credit Agreement July 2015 [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Borrowing capacity |  |  |  | $ 225.0 |  | $ 375.0 |
Term Loan Facility | Amended and Restated Credit Agreement |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Borrowing capacity | $ 40.0 |  |  |  |  |  |
Term Loan Facility | Amended and Restated Credit Agreement July 2015 [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Borrowing capacity |  |  |  |  |  | $ 40.0 |

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- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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