Basis of Presentation (Policies)	9 Months Ended
Basis of Presentation (Policies)	Sep. 30, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of accounting	The accompanying unaudited condensed financial statements include the accounts of Spirit Airlines, Inc. (the "Company"). These unaudited condensed financial statements reflect all normal recurring adjustments which management believes are necessary to fairly present the financial position, results of operations and cash flows of the Company for the respective periods presented. Certain information and footnote disclosures normally included in the audited annual financial statements prepared in accordance with U.S. generally accepted accounting principles ("GAAP") have been condensed or omitted pursuant to the rules and regulations of the Securities and Exchange Commission for Form 10-Q. These unaudited interim condensed financial statements should be read in conjunction with the audited financial statements of the Company and notes thereto included in the Company's Annual Report on Form 10-K for the year ended December 31, 2018 filed with the Securities and Exchange Commission on February 13, 2019.
Use of estimates	The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect both the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results may differ from these estimates.
Recent Accounting Developments	Recently Adopted Accounting Pronouncements Leases The Company adopted ASU No. 2016-02, "Leases (Topic 842)," effective January 1, 2019. The Company adopted Topic 842 utilizing the modified retrospective adoption method with an effective date of January 1, 2019 and elected the package of transition practical expedients for expired or existing contracts, which does not require reassessment of: (1) whether any of the Company's contracts are or contain leases, (2) lease classification and (3) initial direct costs. Therefore, the condensed financial statements for 2019 are presented under the new standard, while the comparative periods presented are not adjusted and continue to be reported in accordance with the Company's historical accounting policy. This standard requires all lessees to recognize a right-of-use asset and a lease liability, initially measured at the present value of the lease payments, for all leases with a term greater than 12 months. The adoption of the new lease standard had a significant impact on the Company's condensed balance sheets due to the recognition of $1.0 billion of right-of-use assets for operating leases, $128.1 million of current maturities of operating leases and $895.1 million of operating leases, less current maturities. In addition, the Company recognized a $5.5 million cumulative effect adjustment, net of tax, to retained earnings. This adjustment was driven by the recognition of unamortized deferred gains and losses related to aircraft sale-leaseback transactions entered into in prior periods. Prior to the adoption of Topic 842, gains and losses on sale-leaseback transactions were generally deferred and recognized in income over the lease term. The accounting for finance leases is substantially unchanged. The adoption of Topic 842 did not have a significant impact on the Company's lease classification or a material impact on its statements of operations and liquidity. Additionally, the adoption of Topic 842 did not have a material impact on the Company’s debt-covenant compliance under its current agreements. Refer to Note 10, Leases for information regarding the Company's adoption of Topic 842 and the Company's undiscounted future lease payments and the timing of those payments. Recently Issued Accounting Pronouncements Not Yet Adopted Cloud Computing Arrangements In August 2018, the FASB issued ASU No. 2018-15, "Intangibles - Goodwill and Other - Internal-Use Software." This new standard requires a customer in a cloud computing arrangement that is a service contract to follow the internal-use software guidance in Accounting Standards Codification ("ASC") 350-40, "Accounting for Internal-Use Software," to determine which implementation costs to capitalize as assets and amortize over the term of the hosting arrangement or expense as incurred. This standard is effective for the Company for fiscal years, and interim periods within those years, beginning January 1, 2020. Early adoption is permitted, including during an interim period. The Company has elected not to early adopt. Entities have the option to apply this standard prospectively to all implementation costs incurred after the date of adoption or retrospectively. The Company is evaluating this new standard, but does not expect it to have a significant impact on its financial statement presentation or results. Accounting for Credit Losses In June 2016, the FASB issued ASU No. 2016-13, "Financial Instruments - Credit Losses." The standard requires the use of an "expected loss" model on certain types of financial instruments. The standard also amends the impairment model for available-for-sale securities and requires estimated credit losses to be recorded as allowances rather than as reductions to the amortized cost of the securities. This standard is effective for the Company for fiscal years, and interim periods within those years, beginning January 1, 2020, with early adoption permitted. The Company has elected not to early adopt. The Company is evaluating the new guidance, but does not expect it to have a material impact on its financial statement presentation or results.
Sale Leaseback Transactions	Prior to the adoption of Topic 842, gains and losses on sale-leaseback transactions were generally deferred and recognized in income over the lease term. Under Topic 842, gains and losses on sale-leaseback transactions, subject to adjustment for off-market terms, are recognized immediately and recorded within loss on disposal of assets on the Company's condensed statements of operations.
Leases	Aircraft rent expense consists of monthly lease rents for aircraft and spare engines under the terms of the Company's aircraft and spare engine lease agreements recognized on a straight-line basis. Aircraft rent expense also includes maintenance reserves paid to aircraft lessors in advance of the performance of major maintenance activities that are not probable of being reimbursed and probable lease return condition obligations.