Revenue Recognition - Disaggregation (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Total revenues	$ 83,240	$ 77,909	$ 74,355	$ 66,824	$ 66,965	$ 65,280	$ 63,069	$ 62,340	$ 302,328	$ 257,654	$ 209,625
Cumulative catch-up adjustments									0	10,200
Decrease to loss from operations									(116,287)	(114,628)	(135,341)
Adjustment
Disaggregation of Revenue [Line Items]
Decrease to loss from operations										$ 10,200
Net loss per share, basic and diluted (in dollars per share)										$ 0.18
United States
Disaggregation of Revenue [Line Items]
Total revenues									260,846	$ 225,931	185,533
Americas, excluding U.S.
Disaggregation of Revenue [Line Items]
Total revenues									3,218	3,472	3,028
Europe, Middle East, India, and Africa
Disaggregation of Revenue [Line Items]
Total revenues									15,434	20,866	13,998
Asia Pacific and Other
Disaggregation of Revenue [Line Items]
Total revenues									22,830	7,385	7,066
Insurance carriers
Disaggregation of Revenue [Line Items]
Total revenues									210,919	193,895	163,861
Laboratory partners
Disaggregation of Revenue [Line Items]
Total revenues									59,876	44,062	36,001
Patients
Disaggregation of Revenue [Line Items]
Total revenues									31,533	19,697	9,763
Product
Disaggregation of Revenue [Line Items]
Total revenues									269,881	240,366	203,777
Upfront license and constellation and paternity
Disaggregation of Revenue [Line Items]
Total revenues									5,089	10,344	4,229
License and development services
Disaggregation of Revenue [Line Items]
Total revenues									16,379	294
Other, licensing and other
Disaggregation of Revenue [Line Items]
Total revenues									10,979	6,650	1,619
Licensing and other
Disaggregation of Revenue [Line Items]
Total revenues									$ 32,447	$ 17,288	$ 5,848