Basis of Presentation (Policies)	3 Months Ended
Mar. 31, 2021
Accounting Policies [Abstract]
Basis of Presentation	The unaudited Condensed Consolidated Financial Statements for the interim periods ended March 31, 2021 and 2020 include all adjustments which are, in the opinion of management, necessary for a fair presentation of the results for the interim periods. This includes all normal and recurring adjustments and elimination of significant intercompany transactions. Financial results for the Company and airlines in general can be seasonal in nature. In many years, the Company's revenues, as well as its Operating income and Net income, have been better in its second and third fiscal quarters than in its first and fourth fiscal quarters. However, beginning in early 2020, as a result of the COVID-19 pandemic, the Company's results have not been in line with such historical trends. See Note 2 for further information. Air travel is also significantly impacted by general economic conditions, the amount of disposable income available to consumers and changes in consumer behavior, unemployment levels, corporate travel budgets, global pandemics such as COVID-19, extreme or severe weather and natural disasters, fears of terrorism or war, governmental actions, and other factors beyond the Company's control. These and other factors, such as the price of jet fuel in some periods, the nature of the Company's fuel hedging program, and the periodic volatility of commodities used by the Company for hedging jet fuel, have created, and may continue to create, significant volatility in the Company's financial results. See Note 4 for further information on fuel and the Company's hedging program. Operating results for the three months ended March 31, 2021, are not necessarily indicative of the results that may be expected for future quarters or for the year ended December 31, 2021. For further information, refer to the Consolidated Financial Statements and footnotes thereto included in the Company's Annual Report on Form 10-K for the year ended December 31, 2020.
Derivatives	Upon proper qualification, the Company accounts for its fuel derivative instruments as cash flow hedges. Qualification is re-evaluated quarterly, and all periodic changes in fair value of the derivatives designated as hedges are recorded in AOCI until the underlying jet fuel is consumed. See Note 5.If a derivative ceases to qualify for hedge accounting, any change in the fair value of derivative instruments since the last reporting period would be recorded in Other (gains) losses, net, in the unaudited Condensed Consolidated Statement of Comprehensive Income (Loss) in the period of the change; however, any amounts previously recorded to AOCI would remain there until such time as the original forecasted transaction occurs, at which time these amounts would be reclassified to Fuel and oil expense. Factors that have and may continue to lead to the loss of hedge accounting include: significant fluctuation in energy prices, significant weather events affecting refinery capacity and the production of refined products, and the volatility of the different types of products the Company uses in hedging. Increased volatility in these commodity markets for an extended period of time, especially if such volatility were to worsen, could cause the Company to lose hedge accounting altogether for the commodities used in its fuel hedging program, which would create further volatility in the Company’s GAAP financial results. However, even though derivatives may not qualify for hedge accounting, the Company continues to hold the instruments as management believes derivative instruments continue to afford the Company the opportunity to stabilize jet fuel costs. When the Company has sold derivative positions in order to effectively "close" or offset a derivative already held as part of its fuel derivative instrument portfolio, any subsequent changes in fair value of those positions are marked to market through earnings. Likewise, any changes in fair value of those positions that were offset by entering into the sold positions and were de-designated as hedges are concurrently marked to market through earnings. However, any changes in value related to hedges that were deferred as part of AOCI while designated as a hedge would remain until the originally forecasted transaction occurs. In a situation where it becomes probable that a fuel hedged forecasted transaction will not occur, any gains and/or losses that have been recorded to AOCI would be required to be immediately reclassified into earnings.The Company is party to certain interest rate swap agreements that are accounted for as cash flow hedges, and has in the past held interest rate swap agreements that have qualified as fair value hedges, as defined in the applicable accounting guidance for derivative instruments and hedging. Several of the Company's interest rate swap agreements qualify for the "shortcut" method of accounting for hedges, which dictated that the hedges were assumed to be perfectly effective, and, thus, there was no ineffectiveness to be recorded in earnings.
Fair Value of Financial Instruments	Accounting standards pertaining to fair value measurements establish a three-tier fair value hierarchy, which prioritizes the inputs used in measuring fair value. These tiers include: Level 1, defined as observable inputs such as quoted prices in active markets; Level 2, defined as inputs other than quoted prices in active markets that are either directly or indirectly observable; and Level 3, defined as unobservable inputs in which little or no market data exists, therefore requiring an entity to develop its own assumptions. As of March 31, 2021, the Company held certain items that are required to be measured at fair value on a recurring basis. These included cash equivalents, short-term investments (primarily treasury bills and certificates of deposit), interest rate derivative contracts, fuel derivative contracts, and available-for-sale securities. The majority of the Company’s short-term investments consist of instruments classified as Level 1. However, the Company has certificates of deposit, commercial paper, and time deposits that are classified as Level 2, due to the fact that the fair value for these instruments is determined utilizing observable inputs in non-active markets. Other available-for-sale securities primarily consist of investments in equity securities with readily determinable market values associated with the Company’s excess benefit plan. The Company’s fuel and interest rate derivative instruments consist of over-the-counter contracts, which are not traded on a public exchange. Fuel derivative instruments currently consist solely of option contracts, whereas interest rate derivatives consist solely of swap agreements. See Note 4 for further information on the Company’s derivative instruments and hedging activities. The fair values of swap contracts are determined based on inputs that are readily available in public markets or can be derived from information available in publicly quoted markets. Therefore, the Company has categorized these swap contracts as Level 2. The Company’s Treasury Department, which reports to the Chief Financial Officer, determines the value of option contracts utilizing an option pricing model based on inputs that are either readily available in public markets, can be derived from information available in publicly quoted markets, or are provided by financial institutions that trade these contracts. The option pricing model used by the Company is an industry standard model for valuing options and is a similar model used by the broker/dealer community (i.e., the Company’s counterparties). The inputs to this option pricing model are the option strike price, underlying price, risk free rate of interest, time to expiration, and volatility. Because certain inputs used to determine the fair value of option contracts are unobservable (principally implied volatility), the Company has categorized these option contracts as Level 3. Volatility information is obtained from external sources, but is analyzed by the Company for reasonableness and compared to similar information received from other external sources. The fair value of option contracts considers both the intrinsic value and any remaining time value associated with those derivatives that have not yet settled. The Company also considers counterparty credit risk and its own credit risk in its determination of all estimated fair values. To validate the reasonableness of the Company’s option pricing model, on a monthly basis, the Company compares its option valuations to third party valuations. If any significant differences were to be noted, they would be researched in order to determine the reason. However, historically, no significant differences have been noted. The Company has consistently applied these valuation techniques in all periods presented and believes it has obtained the most accurate information available for the types of derivative contracts it holds.
New Accounting Pronouncements	On January 7, 2021, the Financial Accounting Standards Board (the "FASB") issued ASU No. 2021-01, Reference Rate Reform (Topic 848). This new standard provides optional temporary guidance for entities transitioning away from LIBOR to new reference rates so that derivatives affected by the discounting transition are explicitly eligible for certain optional expedients and exceptions with Topic 848. These amendments do not apply to any contract modifications made after December 31, 2022, any new hedging relationships entered into after December 31, 2022, or to existing hedging relationships evaluated for effectiveness existing as of December 31, 2022, that apply certain optional practical expedients. This standard is effective immediately and may be applied (i) on a full retrospective basis as of any date from the beginning of an interim period that includes or is subsequent to March 12, 2020, or (ii) on a prospective basis to new modifications from any date within an interim period that includes or is subsequent to the date of the issuance of a final update, up to the date that financial statements are available to be issued. The Company is currently evaluating its contracts that reference LIBOR and the potential impacts of applying the optional temporary guidance under this standard. There were no LIBOR-related contract modifications during first quarter 2021, and the Company will provide additional information about the transition to new reference rates for affected contracts and adoption of this standard at a future date, if material.On August 5, 2020, the FASB issued ASU No. 2020-06, Debt—Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging—Contracts in Entity’s Own Equity (Subtopic 815-40): Accounting for Convertible Instruments and Contracts in an Entity’s Own Equity. This new standard reduces the number of accounting models for convertible debt instruments and convertible preferred stock, enhances information transparency by making targeted improvements to the disclosures for convertible instruments and earnings-per-share (EPS) guidance, and amends the guidance for the derivatives scope exception for contracts in an entity’s own equity to reduce form-over-substance-based accounting conclusions. This standard is effective for fiscal years beginning after December 15, 2021. Companies may elect early adoption for periods beginning no earlier than December 15, 2020, including interim periods within those fiscal years. The FASB specified that an entity should adopt the guidance as of the beginning of its annual fiscal year. The Company plans to adopt this standard as of January 1, 2022. Upon adoption, the Company will reclassify the remaining equity component from Additional paid-in capital to Long-term debt associated with its convertible notes, and no longer record amortization of the debt discount to Interest expense. The computation of diluted net income (loss) per share will be affected in the numerator as the Company will no longer record the debt discount amortization in Interest expense and may have to add back Interest expense to the numerator. The denominator could also be affected as the Company will be required to use the if-converted method to calculate diluted shares.
Revenue Recognition	Spoilage estimates are based on the Company's Customers' historical travel behavior, as well as assumptions about the Customers' future travel behavior. Assumptions used to generate spoilage estimates can be impacted by several factors including, but not limited to: fare increases, fare sales, changes to the Company's ticketing policies, changes to the Company’s refund, exchange, and unused funds policies, seat availability, and economic factors. Given the unprecedented amount of 2020 Customer flight cancellations and the amount of travel funds provided, the Company expects additional variability in the amount of spoilage revenue recorded in future periods, as the estimates of the portion of sold tickets that will expire unused may differ from historical experience.The Company has a co-branded credit card agreement (the “Agreement”) with Chase Bank USA, N.A. (“Chase”), through which the Company sells loyalty points and certain marketing components, which consist of the use of the brand and access to Rapid Rewards Member lists, licensing and advertising elements, and the use of the Company’s resource team. In 2018, Chase and Southwest executed a multi-year extension of the Agreement, extending the decades-long relationship between the parties. The Company recognized revenue related to the marketing, advertising, and other travel-related benefits of the revenue associated with various loyalty partner agreements including, but not limited to, the Agreement with Chase, within Other operating revenues.

Basis of Presentation (Policies)

3 Months Ended

Mar. 31, 2021

The unaudited Condensed Consolidated Financial Statements for the interim periods ended March 31, 2021 and 2020 include all adjustments which are, in the opinion of management, necessary for a fair presentation of the results for the interim periods. This includes all normal and recurring adjustments and elimination of significant intercompany transactions. Financial results for the Company and airlines in general can be seasonal in nature. In many years, the Company's revenues, as well as its Operating income and Net income, have been better in its second and third fiscal quarters than in its first and fourth fiscal quarters. However, beginning in early 2020, as a result of the COVID-19 pandemic, the Company's results have not been in line with such historical trends. See Note 2 for further information. Air travel is also significantly impacted by general economic conditions, the amount of disposable income available to consumers and changes in consumer behavior, unemployment levels, corporate travel budgets, global pandemics such as COVID-19, extreme or severe weather and natural disasters, fears of terrorism or war, governmental actions, and other factors beyond the Company's control. These and other factors, such as the price of jet fuel in some periods, the nature of the Company's fuel hedging program, and the periodic volatility of commodities used by the Company for hedging jet fuel, have created, and may continue to create, significant volatility in the Company's financial results. See Note 4 for further information on fuel and the Company's hedging program. Operating results for the three months ended March 31, 2021, are not necessarily indicative of the results that may be expected for future quarters or for the year ended December 31, 2021. For further information, refer to the Consolidated Financial Statements and footnotes thereto included in the Company's Annual Report on Form 10-K for the year ended December 31, 2020.

Derivatives

Upon proper qualification, the Company accounts for its fuel derivative instruments as cash flow hedges. Qualification is re-evaluated quarterly, and all periodic changes in fair value of the derivatives designated as hedges are recorded in AOCI until the underlying jet fuel is consumed. See Note 5.If a derivative ceases to qualify for hedge accounting, any change in the fair value of derivative instruments since the last reporting period would be recorded in Other (gains) losses, net, in the unaudited Condensed Consolidated Statement of Comprehensive Income (Loss) in the period of the change; however, any amounts previously recorded to AOCI would remain there until such time as the original forecasted transaction occurs, at which time these amounts would be reclassified to Fuel and oil expense. Factors that have and may continue to lead to the loss of hedge accounting include: significant fluctuation in energy prices, significant weather events affecting refinery capacity and the production of refined products, and the volatility of the different types of products the Company uses in hedging. Increased volatility in these commodity markets for an extended period of time, especially if such volatility were to worsen, could cause the Company to lose hedge accounting altogether for the commodities used in its fuel hedging program, which would create further volatility in the Company’s GAAP financial results. However, even though derivatives may not qualify for hedge accounting, the Company continues to hold the instruments as management believes derivative instruments continue to afford the Company the opportunity to stabilize jet fuel costs. When the Company has sold derivative positions in order to effectively "close" or offset a derivative already held as part of its fuel derivative instrument portfolio, any subsequent changes in fair value of those positions are marked to market through earnings. Likewise, any changes in fair value of those positions that were offset by entering into the sold positions and were de-designated as hedges are concurrently marked to market through earnings. However, any changes in value related to hedges that were deferred as part of AOCI while designated as a hedge would remain until the originally forecasted transaction occurs. In a situation where it becomes probable that a fuel hedged forecasted transaction will not occur, any gains and/or losses that have been recorded to AOCI would be required to be immediately reclassified into earnings.The Company is party to certain interest rate swap agreements that are accounted for as cash flow hedges, and has in the past held interest rate swap agreements that have qualified as fair value hedges, as defined in the applicable accounting guidance for derivative instruments and hedging. Several of the Company's interest rate swap agreements qualify for the "shortcut" method of accounting for hedges, which dictated that the hedges were assumed to be perfectly effective, and, thus, there was no ineffectiveness to be recorded in earnings.

Fair Value of Financial Instruments

Accounting standards pertaining to fair value measurements establish a three-tier fair value hierarchy, which prioritizes the inputs used in measuring fair value. These tiers include: Level 1, defined as observable inputs such as quoted prices in active markets; Level 2, defined as inputs other than quoted prices in active markets that are either directly or indirectly observable; and Level 3, defined as unobservable inputs in which little or no market data exists, therefore requiring an entity to develop its own assumptions.

As of March 31, 2021, the Company held certain items that are required to be measured at fair value on a recurring basis. These included cash equivalents, short-term investments (primarily treasury bills and certificates of deposit), interest rate derivative contracts, fuel derivative contracts, and available-for-sale securities. The majority of the Company’s short-term investments consist of instruments classified as Level 1. However, the Company has certificates of deposit, commercial paper, and time deposits that are classified as Level 2, due to the fact that the fair value for these instruments is determined utilizing observable inputs in non-active markets. Other available-for-sale securities primarily consist of investments in equity securities with readily determinable market values associated with the Company’s excess benefit plan.

The Company’s fuel and interest rate derivative instruments consist of over-the-counter contracts, which are not traded on a public exchange. Fuel derivative instruments currently consist solely of option contracts, whereas interest rate derivatives consist solely of swap agreements. See Note 4 for further information on the Company’s

derivative instruments and hedging activities. The fair values of swap contracts are determined based on inputs that are readily available in public markets or can be derived from information available in publicly quoted markets. Therefore, the Company has categorized these swap contracts as Level 2. The Company’s Treasury Department, which reports to the Chief Financial Officer, determines the value of option contracts utilizing an option pricing model based on inputs that are either readily available in public markets, can be derived from information available in publicly quoted markets, or are provided by financial institutions that trade these contracts. The option pricing model used by the Company is an industry standard model for valuing options and is a similar model used by the broker/dealer community (i.e., the Company’s counterparties). The inputs to this option pricing model are the option strike price, underlying price, risk free rate of interest, time to expiration, and volatility. Because certain inputs used to determine the fair value of option contracts are unobservable (principally implied volatility), the Company has categorized these option contracts as Level 3. Volatility information is obtained from external sources, but is analyzed by the Company for reasonableness and compared to similar information received from other external sources. The fair value of option contracts considers both the intrinsic value and any remaining time value associated with those derivatives that have not yet settled. The Company also considers counterparty credit risk and its own credit risk in its determination of all estimated fair values. To validate the reasonableness of the Company’s option pricing model, on a monthly basis, the Company compares its option valuations to third party valuations. If any significant differences were to be noted, they would be researched in order to determine the reason. However, historically, no significant differences have been noted. The Company has consistently applied these valuation techniques in all periods presented and believes it has obtained the most accurate information available for the types of derivative contracts it holds.

New Accounting Pronouncements

On January 7, 2021, the Financial Accounting Standards Board (the "FASB") issued ASU No. 2021-01, Reference Rate Reform (Topic 848). This new standard provides optional temporary guidance for entities transitioning away from LIBOR to new reference rates so that derivatives affected by the discounting transition are explicitly eligible for certain optional expedients and exceptions with Topic 848. These amendments do not apply to any contract modifications made after December 31, 2022, any new hedging relationships entered into after December 31, 2022, or to existing hedging relationships evaluated for effectiveness existing as of December 31, 2022, that apply certain optional practical expedients. This standard is effective immediately and may be applied (i) on a full retrospective basis as of any date from the beginning of an interim period that includes or is subsequent to March 12, 2020, or (ii) on a prospective basis to new modifications from any date within an interim period that includes or is subsequent to the date of the issuance of a final update, up to the date that financial statements are available to be issued. The Company is currently evaluating its contracts that reference LIBOR and the potential impacts of applying the optional temporary guidance under this standard. There were no LIBOR-related contract modifications during first quarter 2021, and the Company will provide additional information about the transition to new reference rates for affected contracts and adoption of this standard at a future date, if material.On August 5, 2020, the FASB issued ASU No. 2020-06, Debt—Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging—Contracts in Entity’s Own Equity (Subtopic 815-40): Accounting for Convertible Instruments and Contracts in an Entity’s Own Equity. This new standard reduces the number of accounting models for convertible debt instruments and convertible preferred stock, enhances information transparency by making targeted improvements to the disclosures for convertible instruments and earnings-per-share (EPS) guidance, and amends the guidance for the derivatives scope exception for contracts in an entity’s own equity to reduce form-over-substance-based accounting conclusions. This standard is effective for fiscal years beginning after December 15, 2021. Companies may elect early adoption for periods beginning no earlier than December 15, 2020, including interim periods within those fiscal years. The FASB specified that an entity should adopt the guidance as of the beginning of its annual fiscal year. The Company plans to adopt this standard as of January 1, 2022. Upon adoption, the Company will reclassify the remaining equity component from Additional paid-in capital to Long-term debt associated with its convertible notes, and no longer record amortization of the debt discount to Interest expense. The computation of diluted net income (loss) per share will be affected in the numerator as the Company will no longer record the debt discount amortization in Interest expense and may have to add back Interest expense to the numerator. The denominator could also be affected as the Company will be required to use the if-converted method to calculate diluted shares.

Revenue Recognition

Spoilage estimates are based on the Company's Customers' historical travel behavior, as well as assumptions about the Customers' future travel behavior. Assumptions used to generate spoilage estimates can be impacted by several factors including, but not limited to: fare increases, fare sales, changes to the Company's ticketing policies, changes to the Company’s refund, exchange, and unused funds policies, seat availability, and economic factors. Given the unprecedented amount of 2020 Customer flight cancellations and the amount of travel funds provided, the Company expects additional variability in the amount of spoilage revenue recorded in future periods, as the estimates of the portion of sold tickets that will expire unused may differ from historical experience.The Company has a co-branded credit card agreement (the “Agreement”) with Chase Bank USA, N.A. (“Chase”), through which the Company sells loyalty points and certain marketing components, which consist of the use of the brand and access to Rapid Rewards Member lists, licensing and advertising elements, and the use of the Company’s resource team. In 2018, Chase and Southwest executed a multi-year extension of the Agreement, extending the decades-long relationship between the parties. The Company recognized revenue related to the marketing, advertising, and other travel-related benefits of the revenue associated with various loyalty partner agreements including, but not limited to, the Agreement with Chase, within Other operating revenues.