Derivative instruments and hedging activities - Cash Flow Hedging on AOCI (Details) - Interest rate swaps - USD ($) $ in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Cumulative Amount of Gain/(Loss) Recognized in OCI on Derivative	$ (17.5)		$ 2.0
Cash Flow Hedging \| Interest expense
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Gain/(Loss) Reclassified from Accumulated OCI into Income	$ (1.1)	$ 0.0

Amount, before tax, of reclassification of gain (loss) from accumulated other comprehensive income (AOCI) for derivative instrument designated and qualifying as cash flow hedge included in assessment of hedge effectiveness.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=121641772&loc=d3e689-108580

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=121641772&loc=d3e637-108580

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5624171-113959

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration