Income tax Income tax (Details) - USD ($)	3 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Reclassification of stranded tax effects due to Tax Reform	$ 2,000,000
Deferred Tax Assets, Operating Loss Carryforwards, Domestic	59,000,000	$ 52,000,000
SEC Schedule, 12-09, Valuation Allowances and Reserves, Amount	0
Unrecognized Tax Benefits, Income Tax Penalties Accrued	$ 0	$ 0

Amount before allocation of valuation allowances of deferred tax asset attributable to deductible domestic operating loss carryforwards. Excludes state and local operating loss carryforwards.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32621-109319

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 25
-Paragraph 20
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=115928272&loc=d3e28680-109314

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32632-109319

Name:	us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsDomestic
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant