Income taxes - Summary of Change in Valuation Allowance (Details) - USD ($) |
12 Months Ended | |
|---|---|---|
Dec. 31, 2016 |
Dec. 31, 2015 |
|
| Valuation Allowance [Abstract] | ||
| Valuation allowance, at beginning of year | $ 887,000 | $ 0 |
| Increase in valuation allowance | 7,602,000 | 887,000 |
| Valuation allowance, at end of year | $ 8,489,000 | $ 887,000 |