Summary of Significant Accounting Policies - Disaggregated Revenue by Payor Type and Major Service (Details) - USD ($) $ in Thousands	3 Months Ended						6 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Revenue, net	$ 54,673	$ 52,441	$ 48,334	$ 38,441	$ 35,292	$ 31,762	$ 100,775	$ 67,054	$ 155,448	$ 105,495	$ 147,277	$ 99,129
Commercial Payors
Disaggregation of Revenue [Line Items]
Revenue, net	25,961			14,887					72,040	38,270
Non-contracted third-party payors
Disaggregation of Revenue [Line Items]
Revenue, net	2,686			3,126					9,016	10,302
Centers for Medicare & Medicaid
Disaggregation of Revenue [Line Items]
Revenue, net	15,379			10,952					42,573	29,317
Healthcare Institutions
Disaggregation of Revenue [Line Items]
Revenue, net	$ 10,647			$ 9,476					$ 31,819	$ 27,606