Summary of significant accounting policies - Stock based compensation (Details) shares in Thousands |
12 Months Ended | ||
|---|---|---|---|
|
Dec. 31, 2016
shares
|
Dec. 31, 2015
shares
|
Dec. 31, 2014
item
$ / shares
|
|
| Assumptions that were used to calculate the grant date fair value of employee stock option grants | |||
| Dividends declared on common stock (in dollars per share) | $ 0 | ||
| Dividends paid on common stock (in dollars per share) | $ 0.00 | ||
| Number of dividend payouts anticipated in the foreseeable future | item | 0 | ||
| Stock options | |||
| Assumptions that were used to calculate the grant date fair value of employee stock option grants | |||
| Number of stock options granted | shares | 0 | 0 | |
| Expected term | 6 years 3 months | ||
| Expected volatility (as a percent) | 48.60% | ||
| Risk-free interest rate (as a percent) | 1.80% | ||
| Dividend yield (as a percent) | 0.00% | ||