Stock Plans and Share-Based Compensation - Fair Value Assumptions Used for Stock Options (Details) - Stock Options - $ / shares	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-Average Grant Date Fair Value (in dollars per share)	$ 9.18	$ 5.92	$ 7.89	$ 6.29
Weighted-Average Assumptions Used:
Annual Dividend Yield	1.10%	1.50%	1.20%	1.20%
Risk-free Interest Rate	1.00%	0.50%	0.70%	1.40%
Annualized Volatility	31.00%	30.10%	31.20%	26.00%
Expected Life (in years)	6 years	6 years 6 months	6 years	6 years
Other Pricing Model Inputs:
Weighted-Average Grant Date Market Price of CSX Stock (strike price) (in dollars per share)	$ 33.35	$ 23.62	$ 29.53	$ 26.49

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- Definition

The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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- Definition

Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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