Common shareholder's equity, mezzanine equity, and non-controlling interests - Additional paid-in surplus (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Additional paid-in surplus (Details)
Reimbursement for performance shares		$ 1.4	$ 2.5
Reimbursement for retention bonuses	$ 1.9		9.0
Total Additional paid-in surplus	$ 1.9	$ 1.4	$ 11.5

The cash inflow from parent as a source of financing that is recorded as additional paid in capital.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3255-108585

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