Summary of Significant Accounting Policies (Policies)	6 Months Ended
Jun. 30, 2023
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The unaudited consolidated financial statements of the Company have been prepared in conformity with accounting principles generally accepted in the United States ("U.S. GAAP") and in accordance with applicable rules and regulations of the Securities Exchange Commission (the "SEC") for interim financial reporting and include the accounts of the Company and its consolidated subsidiaries. Cullinan considers consolidation of entities over which control is achieved by means other than voting rights. Intercompany balances and transactions have been eliminated in consolidation. The Company operates as one segment, which is developing early-stage cancer therapeutics. In the opinion of Cullinan’s management, the unaudited consolidated financial statements reflect all adjustments, which are normal and recurring in nature, and necessary for fair financial statement presentation. The preparation of these unaudited consolidated financial statements and accompanying notes in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the amounts reported. Actual results could differ materially from those estimates. These unaudited consolidated financial statements and accompanying notes should be read in conjunction with the Company’s annual audited consolidated financial statements and accompanying notes included in the 2022 10-K.
Equity-based Compensation	Equity-Based Compensation The fair value of equity-based awards is measured at the grant date and is recognized as expense over the requisite service period, which is generally the vesting period. Forfeitures are recognized as they occur. Cullinan classifies equity-based compensation in its consolidated statements of operations and comprehensive income (loss) in the same manner in which the award recipient’s payroll costs or service payments are classified. The fair value of service-based restricted stock units ("RSUs") is the closing market price of the Company's common stock on the grant date. Cullinan measures the fair value of market-based RSUs on the grant date using a Monte Carlo simulation model. The Company estimates the fair value of stock options using the Black-Scholes option pricing model. Both the Monte Carlo simulation model and the Black-Scholes option pricing model require the input of objective and subjective assumptions. Certain assumptions used, including Cullinan's expected stock price volatility, involve inherent uncertainties and the application of management’s judgment. As a result, if factors change and management uses different assumptions, equity-based compensation expense could be materially different for future awards. Prior to 2023, the expected volatility used in the Black-Scholes option pricing model for new options was based on historical volatilities of the stock prices of similar entities within the Company’s industry over a period of time commensurate with the expected term assumption. In 2023, Cullinan determined that a sufficient amount of historical information was available regarding the volatility of its stock price to begin using a blended rate that combines the Company's historical volatility with the historical volatilities of the stock prices of similar entities within the Company’s industry over a period of time commensurate with the expected term assumption.
Recently Adopted Accounting Pronouncements	Recently Issued Accounting Pronouncements There are no recently issued accounting pronouncements that will have a material impact on Cullinan's consolidated financial statements.

Summary of Significant Accounting Policies (Policies)

6 Months Ended

Jun. 30, 2023

The unaudited consolidated financial statements of the Company have been prepared in conformity with accounting principles generally accepted in the United States ("U.S. GAAP") and in accordance with applicable rules and regulations of the Securities Exchange Commission (the "SEC") for interim financial reporting and include the accounts of the Company and its consolidated subsidiaries. Cullinan considers consolidation of entities over which control is achieved by means other than voting rights. Intercompany balances and transactions have been eliminated in consolidation. The Company operates as one segment, which is developing early-stage cancer therapeutics. In the opinion of Cullinan’s management, the unaudited consolidated financial statements reflect all adjustments, which are normal and recurring in nature, and necessary for fair financial statement presentation. The preparation of these unaudited consolidated financial statements and accompanying notes in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the amounts reported. Actual results could differ materially from those estimates. These unaudited consolidated financial statements and accompanying notes should be read in conjunction with the Company’s annual audited consolidated financial statements and accompanying notes included in the 2022 10-K.

Equity-based Compensation

Equity-Based Compensation

The fair value of equity-based awards is measured at the grant date and is recognized as expense over the requisite service period, which is generally the vesting period. Forfeitures are recognized as they occur. Cullinan classifies equity-based compensation in its consolidated statements of operations and comprehensive income (loss) in the same manner in which the award recipient’s payroll costs or service payments are classified.

The fair value of service-based restricted stock units ("RSUs") is the closing market price of the Company's common stock on the grant date. Cullinan measures the fair value of market-based RSUs on the grant date using a Monte Carlo simulation model. The Company estimates the fair value of stock options using the Black-Scholes option pricing model. Both the Monte Carlo simulation model and the Black-Scholes option pricing model require the input of objective and subjective assumptions. Certain assumptions used, including Cullinan's expected stock price volatility, involve inherent uncertainties and the application of management’s judgment. As a result, if factors change and management uses different assumptions, equity-based compensation expense could be materially different for future awards.

Prior to 2023, the expected volatility used in the Black-Scholes option pricing model for new options was based on historical volatilities of the stock prices of similar entities within the Company’s industry over a period of time commensurate with the expected term assumption. In 2023, Cullinan determined that a sufficient amount of historical information was available regarding the volatility of its stock price to begin using a blended rate that combines the Company's historical volatility with the historical volatilities of the stock prices of similar entities within the Company’s industry over a period of time commensurate with the expected term assumption.

Recently Adopted Accounting Pronouncements

Recently Issued Accounting Pronouncements

There are no recently issued accounting pronouncements that will have a material impact on Cullinan's consolidated financial statements.

- Definition

Disclosure of accounting policy for award under share-based payment arrangement. Includes, but is not limited to, methodology and assumption used in measuring cost.

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