| Income taxes - Components of Income Tax Expense (Benefit) (Details) - USD ($) $ in Thousands | 12 Months Ended | ||
|---|---|---|---|
| Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | |
| United States: | |||
| Federal | $ 400 | $ 459 | $ 752 | 
| State and local | 97 | (8) | 140 | 
| Total current tax expense (benefit) | 497 | 451 | 892 | 
| United States: | |||
| Total income tax expense (benefit) | $ 497 | $ 451 | $ 892 | 
| X | ||||||||||
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- Definition Amount of current federal tax expense (benefit) pertaining to income (loss) from continuing operations. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef 
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| X | ||||||||||
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- Definition Amount of current income tax expense (benefit) pertaining to taxable income (loss) from continuing operations. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef 
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| X | ||||||||||
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- References No definition available. 
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| X | ||||||||||
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- Definition Amount of current state and local tax expense (benefit) pertaining to income (loss) from continuing operations. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef 
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| X | ||||||||||
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- References No definition available. 
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| X | ||||||||||
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- Definition Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations. Reference 1: http://www.xbrl.org/2003/role/disclosureRef 
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