Revenue from contracts with customers Revenue from contracts with customers - contract balances (Details) - USD ($) $ in Millions |
9 Months Ended | |
|---|---|---|
Sep. 30, 2018 |
Dec. 31, 2017 |
|
| New Accounting Pronouncements or Change in Accounting Principle [Line Items] | ||
| Receivables | $ 630.7 | $ 642.3 |
| Contract Asset | 2.5 | |
| Contract Liability | 3.2 | |
| Revenue - revenue from performance obligation in prior periods | 0.6 | |
| Capitalized costs to obtain contracts | 1.7 | |
| Pro Forma under Old Revenue Recognition Standard | ||
| New Accounting Pronouncements or Change in Accounting Principle [Line Items] | ||
| Receivables | 642.3 | |
| Contract Asset | 0.4 | |
| Contract Liability | $ 5.6 | |
| Revenue recognized included in beginning balance | 5.1 | |
| Accounting Standards Update 2014-09 | Impact of New Revenue Recognition Standard | ||
| New Accounting Pronouncements or Change in Accounting Principle [Line Items] | ||
| Receivable - increase (decrease) | (11.6) | |
| Contract asset increase (decrease) | 2.1 | |
| Contract liability - increase (decrease) | $ (2.4) |