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.

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

Income taxes - Additional Information (Details) - USD ($) $ in Thousands	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustments to accumulated deficit for new accounting pronouncements		$ (1,824)
Accounting Standards Update 2016-16
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustments to accumulated deficit for new accounting pronouncements	$ 1,800