Revenue from contracts with customers Revenue from contracts with customers - contract balances (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Receivables	$ 633.9	$ 642.3
Contract Asset	1.5
Contract Liability	6.9
Revenue - revenue from performance obligation in prior periods	0.3
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	3.1
Accounting Standards Update 2014-09 \| Impact of New Revenue Recognition Standard
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Receivable - increase (decrease)	(8.4)
Contract asset increase (decrease)	1.1
Contract liability - increase (decrease)	$ 1.3

Amount due from customers or clients, within one year of the balance sheet date (or the normal operating cycle, whichever is longer), for goods or services (including trade receivables) that have been delivered or sold in the normal course of business, reduced to the estimated net realizable fair value by an allowance established by the entity of the amount it deems uncertain of collection.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section S99
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Amount, after accumulated amortization and accumulated impairment loss, of asset recognized from cost incurred to obtain or fulfill contract with customer.

+ References

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Amount of increase (decrease) in revenue recognized for cumulative catch-up adjustment from change in estimate of transaction price which increases (decreases) right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time. Includes, but is not limited to, change in assessment of whether estimate of variable consideration is constrained.

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Amount, after allowance for credit loss, of right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time.

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Amount of revenue recognized that was previously included in balance of obligation to transfer good or service to customer for which consideration from customer has been received or is due.

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The increase (decrease) during the reporting period in receivables arising from the contracting of goods and services, net for uncollectible accounts.

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