Income taxes - Valuation Allowance (Details) - USD ($)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Income taxes
(Decrease) increase in valuation allowance	$ 11,118,000	$ (29,037,000)
Reduction of deferred tax assets		$ 20,547,000

Amount of increase (decrease) in the valuation allowance for a specified deferred tax asset.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 45
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=84176650&loc=d3e32123-109318

Name:	us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration