Stock-Based Compensation - Narrative (Details) $ / shares in Units, $ in Thousands			1 Months Ended						12 Months Ended
	Mar. 02, 2023 shares	Jan. 01, 2020 shares	Jul. 31, 2022 USD ($) grantee	Jun. 30, 2022 $ / shares	Nov. 30, 2020 USD ($)	May 31, 2020 segment $ / shares shares	Oct. 31, 2018 shares	Sep. 30, 2018 shares	Dec. 31, 2023 USD ($) $ / shares shares	Dec. 31, 2022 USD ($) $ / shares shares	Dec. 31, 2021 USD ($) $ / shares shares	Dec. 31, 2020 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares of common stock available for issuance (in shares)									12,003,394	5,438,296	5,231,624	1,819,223
Weighted average grant date fair value, grants in period (in usd per share) | $ / shares									$ 19.90	$ 28.61	$ 70.25
Stock based compensation not recognized | $									$ 39,800
Stock based compensation not recognized, period for recognition (in years)									2 years 4 months 24 days
Total stock-based compensation expense | $									$ 90,759	$ 94,685	$ 151,449
Granted (in usd per share) | $ / shares									$ 30.80	$ 44.86	$ 119.82
Shares underlying outstanding stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercises in period, intrinsic value | $									$ 1,000	$ 12,200	$ 83,500
Expected dividend yield									0.00%	0.00%	0.00%
Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock based compensation not recognized, period for recognition (in years)									2 years 4 months 24 days
Stock based compensation not recognized, restricted stock | $									$ 151,900
Weighted-average grant date fair value, granted (in usd per share) | $ / shares									$ 26.62	$ 45.04	$ 123.36
Granted (in shares)									2,436,947	2,902,217	873,916
Canceled (in shares)									1,049,447	490,525	315,988
Vested and released (in shares)									728,603	315,673	178,030
PSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock based compensation not recognized, period for recognition (in years)									1 year 9 months 18 days
Stock based compensation not recognized, restricted stock | $									$ 5,300
Service period (in years)					4 years
Additional service period (in months)					2 years
Total stock-based compensation expense | $					$ 0				$ 2,600	$ 1,300	$ 1,300
Total market-based restricted stock units approved and granted (in shares)									126,041	26,935	52,917
Weighted-average grant date fair value, granted (in usd per share) | $ / shares									$ 32.84	$ 37.50	$ 135.94
Granted (in shares)									126,041	26,935	52,917
Canceled (in shares)									51,829	59,818	56,243
Vested and released (in shares)									3,435
PSUs | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service period (in years)					3 months 18 days
Vesting period (in years)					3 months 18 days
PSUs | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service period (in years)					4 years
Vesting period (in years)					4 years 6 months
Shares underlying unvested performance-based restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service period (in years)					4 years
Vesting period (in years)					6 months
Forfeitures value | $					$ 25,700
MSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period (in years)						7 years
Total stock-based compensation expense | $										$ 16,100	$ 99,200
Market-based restricted stock share price goal (in days)						30 days
Weighted-average grant date fair value, granted (in usd per share) | $ / shares									$ 67.00
Granted (in shares)									0	0
Canceled (in shares)									0	0	558,254
Vested and released (in shares)									0	0	572,130
MSUs | Tranche 1 - $120 per share
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Price goal (in dollars per share) | $ / shares						$ 120			$ 120
MSUs | Chief Executive Officer
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total market-based restricted stock units approved and granted (in shares)						1,695,574
Number of tranches | segment						3
MSUs | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service period (in years)									9 months 29 days
Market-based stock units holding period during vesting (in months)						6 months
MSUs | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service period (in years)									2 years 25 days
Market-based stock units holding period during vesting (in months)						12 months
ESPP obligation
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional amount of shares available (in shares)	1,026,194	942,614
Stock based compensation not recognized, period for recognition (in years)									4 months 24 days
Stock based compensation not recognized, restricted stock | $									$ 2,100
Total stock-based compensation expense | $									$ 5,100	$ 4,600	$ 3,500
Expected dividend yield									0.00%	0.00%	0.00%
Shares authorized (in shares)	1,026,194	942,614
Employee stock purchase plan, maximum employee subscription rate (as a percent)									10.00%
Employee stock purchase plan, purchase price of common stock (as a percent)									85.00%
Purchase period (in months)									6 months
Common stock issued under employee stock purchase plan (in shares)									464,870	307,953	110,227
2018 Incentive Award Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional amount of shares available (in shares)								3,689,000
Percentage of common stock outstanding								4.00%
Shares authorized (in shares)								3,689,000
2012 Stock Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares canceled (in shares)							508,847
AMEA 2020 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)										0
AMEA 2020 Plan | Common Class B
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares of common stock available for issuance (in shares)										0	340,928	542,220
Weighted average grant date fair value, grants in period (in usd per share) | $ / shares											$ 0.33
Total market-based restricted stock units approved and granted (in shares)											826,667
Granted (in usd per share) | $ / shares											$ 0.58
AMEA 2020 Plan | Guardant Health AMEA, Inc | Common Class B
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock based compensation not recognized | $			$ 4,100
Stock based compensation not recognized, period for recognition (in years)			2 years 2 months 12 days
Granted (in usd per share) | $ / shares				$ 4.44
Issuance of exercise of vested stock option granted | grantee			39
Settled of tender, amount | $			$ 13,700
2018 Employee Stock Purchase Plan | ESPP obligation
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional amount of shares available (in shares)								1,106,700
Percentage of common stock outstanding								1.00%
Common stock, shares reserved for future issuance (in shares)								922,250
Shares authorized (in shares)								1,106,700