Nature of operations and summary of significant accounting policies (Stock-based Compensation) (Details)	12 Months Ended
Dec. 31, 2017 USD ($) Installment Tranche
Time Shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of vesting annual installments \| Installment	3
Performance Shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of tranches \| Tranche	3
Award vesting period	3 years
Shares based awards expense recognized \| $	$ 0

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration