Stockholders' Equity - Additional Information (Details) - USD ($)					1 Months Ended						10 Months Ended	12 Months Ended				58 Months Ended
	Nov. 09, 2023	Jul. 14, 2023	May 23, 2023	Oct. 29, 2019	Dec. 31, 2023	Feb. 28, 2023	Sep. 30, 2022	Nov. 30, 2020	Oct. 31, 2019	Mar. 31, 2019	Nov. 30, 2023	Dec. 31, 2023		Dec. 31, 2022	Dec. 31, 2019	Dec. 31, 2023	Jan. 01, 2023
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Preferred stock, shares authorized					40,000,000							40,000,000		40,000,000		40,000,000
Preferred stock, shares issued					0							0		0		0
Preferred stock, shares outstanding					0							0		0		0
Stock options, granted												1,474,910		1,741,931
Stock-based compensation cost												$ 45,025,000		$ 24,133,000
Common stock initially reserved for issuance					9,378,875							9,378,875				9,378,875
Unrecognized stock-based compensation expense					$ 13,200,000							$ 13,200,000				$ 13,200,000
Defined contribution plan employer contribution liabilities expense												$ 1,900,000		$ 1,300,000
Defined contribution plan, employer matching contribution, percentage												75.00%
Employer discretionary match number of shares settled												135,956		101,540
Unrecognized stock-based compensation expense, weighted-average period for recognition												2 years 4 months 24 days
Dividend yield												0.00%		0.00%
Total intrinsic value of stock options exercised												$ 100,000
Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Dividend yield												0.00%
Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation expense					$ 26,600,000							$ 26,600,000				$ 26,600,000
Unrecognized stock-based compensation expense, weighted-average period for recognition												2 years 4 months 24 days
Total fair value of vested												$ 5,700,000		$ 900,000
2019 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares authorized for issuance															2,231,739
Number of shares available for issuance									0
Stock options, granted															1,400,528
2019 Equity Incentive Plan | Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock awards, granted															16,260
2019 Incentive Award Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares available for issuance		2,406,622
Stock options, granted		1,203,341
Share-based compensation incremental expense												$ 2,200,000
Annual increase to shares available for issuance percentage of outstanding common stock									5.00%
Equity plan, description												The number of shares initially available for issuance will be increased by (i) the number of shares subject to stock options or similar awards granted under the Existing Incentive Plan that expire or otherwise terminate without having been exercised in full after the effective date of the 2019 Plan and unvested shares issued pursuant to awards granted under the Existing Incentive Plan that are forfeited to or repurchased by the Company after the effective date of the 2019 Plan, with the maximum number of shares to be added to the 2019 Plan pursuant to clause (i) above or equal to 1,416,788 shares, and (ii) an annual increase on January 1 of each calendar year beginning in 2020 and ending in 2029, equal to the lesser of (a) 5% of the shares of common stock outstanding on the final day of the immediately preceding calendar year and (b) such smaller number of shares as determined by the Board of Directors. On July 14, 2023, the Company completed a voluntary, one-time stock option exchange program, or the Option Exchange, pursuant to which eligible employees were able to exchange certain outstanding stock options granted under the 2019 Plan for a lesser amount of new RSUs issued under the 2019 Plan. Participants in the Option Exchange received one RSU for every two shares of Phathom common stock underlying the eligible options surrendered. This exchange ratio was applied on a grant by grant basis. The Option Exchange resulted in 2,406,622 options being exchanged for 1,203,341 RSUs. The Company is recognizing an additional $2.2 million of incremental expense related to the Option Exchange to be recognized over a three-year vesting period. As of December 31, 2023, 1,110,376 shares remain available for issuance, which reflects 4,492,336 stock options, performance-based units, and restricted stock units, or RSUs, awards granted, and 2,558,662 of awards cancelled or forfeited, during the year ended December 31, 2023 as well as an annual increase of 2,086,165 shares authorized on January 1, 2023.
Number of shares remain available for issuance, annual increase					2,086,165							2,086,165				2,086,165
Common stock initially reserved for issuance					1,110,376							1,110,376				1,110,376
2019 Incentive Award Plan | Performance-Based Stock Units (PSU)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock awards, granted												597,650		37,500
Stock-based compensation cost												$ 19,300,000
2019 Incentive Award Plan | Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock awards, granted												2,419,776	[1]	1,010,437
2019 Incentive Award Plan | Stock Option Performance-based Unit and Restricted Stock Unit [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares, granted												4,492,336
Number of shares, cancelled or forfeited												2,558,662
Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares available for issuance														973,298
Annual increase to shares available for issuance percentage of outstanding common stock									1.00%
Equity plan, description												In addition, the number of shares available for issuance under the ESPP will be annually increased on January 1 of each calendar year beginning in 2020 and ending in 2029, by an amount equal to the lesser of: (i) 1% of the shares outstanding on the final day of the immediately preceding calendar year and (ii) such smaller number of shares as is determined by the Board of Directors. As of December 31, 2023, 973,298 shares of common stock remain available for issuance, which includes the 196,873 shares sold to employees during the year ended December 31, 2023 as well as an annual increase of 417,233 shares authorized on January 1, 2023.
Number of shares remain available for issuance, annual increase																	417,233
Stock-based compensation cost												$ 600,000		$ 500,000
Common stock initially reserved for issuance					973,298				270,000			973,298				973,298
Number of shares issued												196,873
Estimated weighted-average fair value												$ 3.64		$ 3.98
Unrecognized stock-based compensation expense					$ 100,000							$ 100,000				$ 100,000
Unrecognized stock-based compensation expense, weighted-average period for recognition												6 months
Dividend yield												0.00%		0.00%
Underwritten Public Offering
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares issued			12,793,750
Common stock, price per share			$ 11.75
Proceeds from issuance initial public offering gross			$ 150,300,000
Purchase price per share after deducting underwriting discounts and commissions			$ 11.08
Net proceeds after deducting underwriters commission			$ 141,800,000
Purchase of additional offering expenses			$ 400,000
At-the-Market Offering Program
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares issued						1,514,219						3,929,116
Proceeds from issuance of shares					$ 38,700,000	$ 14,100,000						$ 14,073,000		$ 24,596,000
Stock issuance costs						400,000
Amount utilized of equity financing											$ 39,900,000	0
Available equity financing amount						$ 125,000,000						$ 150,000,000
2020 At-the-Market Offering Program
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares issued							2,414,897
Proceeds from issuance of shares							$ 24,600,000
Stock issuance costs							$ 800,000
Employee and Nonemployee Director
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Estimated weighted-average fair value												$ 5.34		$ 8.4
Open Market Sale Agreement with Jefferies LLC | At-the-Market Offering Program
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Sales commission payable as a percentage of sale of gross sales price per share								3.00%
Open Market Sale Agreement with Jefferies LLC | At-the-Market Offering Program | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate offering price through equity financing	$ 150,000,000											$ 150,000,000
Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares authorized to repurchase										3,373,408
Stock repurchase program, number of shares right lapse										843,352
Remaining number of shares to be repurchased										2,530,056
Stock repurchase program, period in force										48 months
Stock repurchase program expiration period										2023-03
Employer discretionary match number of shares settled												135,956		101,540
Common Stock | 2019 Incentive Award Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Additional number of shares available for issuance									1,416,788
Common Stock | Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Maximum percentage of eligible compensation contributed by participants									20.00%
Common Stock | IPO
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares issued				10,997,630												26,041,380
Proceeds from Initial Public Offering (IPO)				$ 191,500,000												$ 421,500,000
Common Stock | Underwritten Public Offering
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares issued			1,668,750									12,793,750
Common Stock | At-the-Market Offering Program
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares issued												1,514,219		2,414,897
Common Stock | Founders
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Remaining number of shares to be repurchased					0							0		79,064		0

[1] The number of RSUs granted includes those exchanged in the Option Exchange (as defined above).