Fair value measurements (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Money Market Funds Included In Other Assets
Fair value measurements
Money market funds	$ 67
Level 1 | Money Market Funds Included In Other Assets
Fair value measurements
Money market funds	67
Recurring
Fair value measurements
Assets measured at fair value	164,024	$ 89,870
Recurring | Money Market Funds In Cash And Cash Equivalents
Fair value measurements
Money market funds	163,705	89,553
Recurring | Money Market Funds Included In Restricted Cash
Fair value measurements
Money market funds	252
Recurring | Money Market Funds Included In Other Assets
Fair value measurements
Money market funds		317
Recurring | Level 1
Fair value measurements
Assets measured at fair value	164,024	89,870
Recurring | Level 1 | Money Market Funds In Cash And Cash Equivalents
Fair value measurements
Money market funds	163,705	89,553
Recurring | Level 1 | Money Market Funds Included In Restricted Cash
Fair value measurements
Money market funds	$ 252
Recurring | Level 1 | Money Market Funds Included In Other Assets
Fair value measurements
Money market funds		$ 317