Fair value measurements (Details) - Recurring - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair value measurements
Assets measured at fair value	$ 114,404	$ 150,642
Money market funds, included in cash and cash equivalents
Fair value measurements
Money market funds	114,059	150,151
Money market funds, included in restricted cash
Fair value measurements
Money market funds	179	252
Money market funds, included in other assets
Fair value measurements
Money market funds	166	239
Level 1
Fair value measurements
Assets measured at fair value	114,404	150,642
Level 1 | Money market funds, included in cash and cash equivalents
Fair value measurements
Money market funds	114,059	150,151
Level 1 | Money market funds, included in restricted cash
Fair value measurements
Money market funds	179	252
Level 1 | Money market funds, included in other assets
Fair value measurements
Money market funds	$ 166	$ 239