Fair value measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Level 3
Fair value measurements
Assets measured at fair value	$ 0	$ 0
Recurring
Fair value measurements
Assets measured at fair value	98,343	150,642
Recurring | Money market funds, included in cash and cash equivalents
Fair value measurements
Money market funds	97,998	150,151
Recurring | Money market funds, included in restricted cash
Fair value measurements
Money market funds	179	252
Recurring | Money market funds, included in other assets
Fair value measurements
Money market funds	166	239
Recurring | Level 1
Fair value measurements
Assets measured at fair value	98,343	150,642
Recurring | Level 1 | Money market funds, included in cash and cash equivalents
Fair value measurements
Money market funds	97,998	150,151
Recurring | Level 1 | Money market funds, included in restricted cash
Fair value measurements
Money market funds	179	252
Recurring | Level 1 | Money market funds, included in other assets
Fair value measurements
Money market funds	$ 166	$ 239