Revenue - Contract balances (Details) $ in Millions	3 Months Ended
Mar. 31, 2019 USD ($)
Revenue from Contract with Customer [Abstract]
Increase (decrease) in unbilled receivables	$ 35.0
Billing for the reporting period	519.0
Revenue recognized but unbilled	484.0
Increase or decrease in deferred revenue	6.5
Revenue recognized	15.0
Deferrals of customer payments	$ 8.2

Amount of decrease in right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time from transfer to receivable due to right to consideration becoming unconditional.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130551-203045

Name:	us-gaap_ContractWithCustomerAssetReclassifiedToReceivable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration