Leases - Maturities of lease liabilities under non-cancellable leases (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Jan. 01, 2019
Rent Payments
Remainder of 2019	$ 4,290
2020	8,750
2021	9,004
2022	9,000
2023	9,173
Thereafter	43,711
Total lease payments	83,928
Less: imputed interest	20,521
Present value of lease liabilities	63,407	$ 65,800
Sublease Receipts
2019	559
2020	888
2021	775
2022	804
2023	822
Thereafter	1,382
Sublease Receipts	5,230
Total Cash Outflows
2019	3,731
2020	7,862
2021	8,229
2022	8,196
2023	8,351
Thereafter	42,329
Total Cash Outflows	$ 78,698