Revenue from contracts with customers Revenue from contracts with customers - contract balances (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Receivables	$ 599.5	$ 642.3
Customer Asset	1.8
Contract Liability	2.5
Revenue - revenue from performance obligation in prior periods	0.6
Capitalized costs to obtain contracts	1.8
Pro Forma under Old Revenue Recognition Standard
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Receivables		642.3
Customer Asset		0.4
Contract Liability		$ 5.6
Revenue recognized included in beginning balance	5.6
Accounting Standards Update 2014-09 \| Impact of New Revenue Recognition Standard
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Receivable - increase (decrease)	(42.8)
Contract Asset - increase (decrease)	1.4
Contract Liability - increase (decrease)	$ (3.1)

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.

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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.

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