Debt (Details) $ in Thousands		1 Months Ended			12 Months Ended
	Dec. 21, 2018 USD ($) item	Jun. 30, 2019 USD ($)	May 31, 2018	May 31, 2017	Dec. 31, 2018 USD ($)	Dec. 31, 2019 USD ($)
Debt
Repayment					$ 5,500
Estimated future principal payments due
2022						$ 25,000
2023						25,000
Total debt						$ 50,000
2015 Credit Facility
Debt
Principal amount					5,500
Floor rate			5.50%	4.50%
Repayment					5,500
2015 Credit Facility | Maximum
Debt
Loss on extinguishment					$ (100)
2015 Credit Facility | Prime rate
Debt
Variable interest rate			0.75%	1.25%
2018 Credit Facility
Debt
Maximum amount outstanding	$ 75,000
Number of tranche | item	3
Borrowing	$ 25,000	$ 25,000
Period for commencing monthly principal payments	36 months
Amortization period	24 months
Prepayment fee in first year	1.00%
Prepayment fee in second year	0.50%
Prepayment fee in third year	0.25%
Issuance costs	$ 800
Debt default interest rate addition (as a percent)	4.00
2018 Credit Facility | One-month U.S. LIBOR
Debt
Variable interest rate	5.50%
2018 Credit Facility | Term loan
Debt
Principal amount of each tranche of term loans	$ 25,000