Commitments and Contingencies - Narrative (Details) $ in Thousands	1 Months Ended		3 Months Ended			6 Months Ended
	Mar. 31, 2018 patent	Nov. 30, 2017 patent	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Jun. 30, 2019 USD ($) party	Jun. 30, 2018 USD ($)
Other Commitments [Line Items]
Operating leases, rent expense | $			$ 1,600	$ 1,100		$ 2,900	$ 2,200
Number of parties with patent license agreement | party						4
Royalty expense | $			$ 1,100	$ 445		$ 1,700	$ 617
Percentage of precision oncology testing revenue			3.00%	2.00%		2.00%	2.00%
Personal Genome Diagnostics, Inc. and Foundation Medicine, Inc, vs. Guardant Health, Inc. Subsequent Filing
Other Commitments [Line Items]
Number of asserted patents		4
Personal Genome Diagnostics, Inc. vs. Guardant Health, Inc.
Other Commitments [Line Items]
Number of asserted patents	2
Number of petitions filed	2
Personal Genome Diagnostics, Inc. vs. Guardant Health, Inc. Subsequent Filing
Other Commitments [Line Items]
Number of asserted patents	4
Number of petitions filed	6
Other Income
Other Commitments [Line Items]
Amount awarded from other party | $					$ 4,250