Income taxes - Changes in valuation allowance (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Changes in the valuation allowance
Balance at the beginning of the year	$ 34,990	$ 36,331	$ 19,548
Additions charged to operations		16,527	16,783
Effect of U.S. tax reform law changes		(17,868)
Decrease credited to operations	(1,180)
Balance at the end of the year	$ 33,810	$ 34,990	$ 36,331
Period of cumulative results for determination of releasing valuation allowance	3 years

Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32537-109319

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- Definition

Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations, attributable to increase (decrease) in the income tax rates.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32698-109319

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 45
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=84176650&loc=d3e32059-109318

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32687-109319

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- Definition

Represents the period of cumulative results for determination of releasing valuation allowance.

+ References

No definition available.

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- Definition

The amount of the decrease in the period in the valuation allowance for a specified deferred tax asset.

+ References

No definition available.

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- Definition

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

+ References

No definition available.

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- References

No definition available.

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