Revenue from contracts with customers - Significant Changes in Contract Liabilities (Details) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Change in Contract with Customer, Liability [Roll Forward]
Contract liabilities at start of period	$ 6.4	$ 9.4
Decrease due to amortization of revenue that was included in the beginning contract liability balance	(9.3)	(4.6)
Increase due to cash received, excluding amounts recognized as revenue	5.9	1.6
Contract liabilities at end of period	$ 3.0	$ 6.4

Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118944033&loc=SL49130531-203044

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130549-203045

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118944033&loc=SL49130532-203044

Name:	us-gaap_ContractWithCustomerLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant