Commitments and Contingencies - Narrative (Details) $ in Thousands	1 Months Ended		3 Months Ended
	Mar. 31, 2018 patent	Nov. 30, 2017 patent	Mar. 31, 2019 USD ($) party	Mar. 31, 2018 USD ($)
Other Commitments [Line Items]
Operating leases, rent expense | $			$ 1,300	$ 1,100
Number of parties with patent license agreement | party			4
Royalty expense | $			$ 633	$ 172
Percentage of precision oncology testing revenue			2.00%	1.00%
Personal Genome Diagnostics, Inc. and Foundation Medicine, Inc, vs. Guardant Health, Inc. Subsequent Filing
Other Commitments [Line Items]
Number of asserted patents		3
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