Summary of significant accounting policies - Recent adopted changes in accounting principle and Recent accounting pronouncements (Details) - Accounting Standards Update 2016-13 $ in Millions	Jan. 01, 2020 USD ($)
Summary of significant accounting policies
Allowance for credit loss upon adoption of new accounting guidance	$ (6.8)
Cumulative effect of an accounting change	$ 14.9

Amount after tax of increase (decrease) to equity or (increase) decrease to net assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption.

No definition available.

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