Spirit Airlines (SAVEQ) 10-K 10 Feb 21 2020 FY Annual report Financial data

Cover

Cover - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2020	Feb. 03, 2021	Jun. 30, 2020
Entity Information [Line Items]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
Current Fiscal Year End Date	--12-31
Document Transition Report	false
Entity File Number	001-35186
Entity Registrant Name	Spirit Airlines, Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	38-1747023
Entity Address, Address Line One	2800 Executive Way
Entity Address, City or Town	Miramar
Entity Address, State or Province	FL
Entity Address, Postal Zip Code	33025
City Area Code	954
Local Phone Number	447-7920
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
ICFR Auditor Attestation Flag	true
Entity Public Float			$ 1.6
Entity Common Stock, Shares Outstanding (in shares)		97,783,282
Documents Incorporated by Reference	Portions of the registrant's Proxy Statement for the registrant's 2021 Annual Meeting of Stockholders are incorporated by reference into Part III of this Form 10-K to the extent stated herein. The Proxy Statement will be filed within 120 days of the registrant's fiscal year ended December 31, 2020.
Entity Central Index Key	0001498710
Amendment Flag	false
Document Fiscal Period Focus	FY
Document Fiscal Year Focus	2020
Common Stock, $0.0001 par value
Entity Information [Line Items]
Title of 12(b) Security	Common Stock, $0.0001 par value
Trading Symbol	SAVE
Security Exchange Name	NYSE
Series A Preferred Stock Purchase Rights
Entity Information [Line Items]
Title of 12(b) Security	Series A Preferred Stock Purchase Rights
Trading Symbol	SAVE
Security Exchange Name	NYSE

Consolidated Statements Of Oper

Consolidated Statements Of Operations - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating revenues:
Total operating revenues	$ 1,810,022,000	$ 3,830,536,000	$ 3,323,034,000
Operating expenses:
Salaries, wages and benefits	909,834,000	865,019,000	719,635,000
Aircraft fuel	431,000,000	993,478,000	939,324,000
Depreciation and amortization	278,588,000	225,264,000	176,727,000
Landing fees and other rents	251,028,000	256,275,000	214,677,000
Aircraft rent	196,359,000	182,609,000	177,641,000
Maintenance, materials and repairs	111,227,000	143,575,000	129,078,000
Distribution	85,059,000	153,770,000	137,001,000
Loss on disposal of assets	2,264,000	17,350,000	9,580,000
Special charges (credits)	(302,761,000)	717,000	88,921,000
Other operating	355,186,000	491,432,000	379,536,000
Total operating expenses	2,317,784,000	3,329,489,000	2,972,120,000
Operating income (loss)	(507,762,000)	501,047,000	350,914,000
Other (income) expense:
Interest expense	134,520,000	101,350,000	83,777,000
Capitalized interest	(15,995,000)	(12,471,000)	(9,841,000)
Interest income	(6,314,000)	(25,133,000)	(19,107,000)
Other (income) expense	211,000	875,000	752,000
Special charges, non-operating	0	0	90,357,000
Total other (income) expense	112,422,000	64,621,000	145,938,000
Income (loss) before income taxes	(620,184,000)	436,426,000	204,976,000
Provision (benefit) for income taxes	(191,484,000)	101,171,000	49,227,000
Net income (loss)	$ (428,700,000)	$ 335,255,000	$ 155,749,000
Basic earnings (loss) per share ( in dollars per share)	$ (5.06)	$ 4.90	$ 2.28
Diluted earnings (loss) per share ( in dollars per share)	$ (5.06)	$ 4.89	$ 2.28
Passenger
Operating revenues:
Total operating revenues	$ 1,765,533,000	$ 3,757,605,000	$ 3,260,015,000
Other
Operating revenues:
Total operating revenues	$ 44,489,000	$ 72,931,000	$ 63,019,000

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ (428,700)	$ 335,255	$ 155,749
Unrealized gain (loss) on short-term investment securities and cash and cash equivalents, net of deferred taxes of $(1), $38 and $44	(20)	167	30
Interest rate derivative loss reclassified into earnings, net of taxes of $63, $76 and $75	189	239	241
Other comprehensive income (loss)	169	406	271
Comprehensive income (loss)	$ (428,531)	$ 335,661	$ 156,020

Consolidated Statements of Co_2

Consolidated Statements of Comprehensive Income (Loss) (Parentheticals) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Comprehensive Income [Abstract]
Deferred taxes on unrealized loss on investment securities	$ (1)	$ 38	$ 44
Loss reclassified from AOCI into income, tax	$ 63	$ 76	$ 75

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 1,789,723	$ 978,957
Restricted cash	71,401	0
Short-term investment securities	106,339	105,321
Accounts receivable, net	42,940	73,807
Aircraft maintenance deposits, net	73,134	102,906
Income tax receivable	147,460	21,013
Prepaid expenses and other current assets	124,983	103,439
Total current assets	2,355,980	1,385,443
Property and equipment:
Flight equipment	4,177,631	3,730,751
Ground property and equipment	334,167	291,998
Less accumulated depreciation	(680,230)	(492,447)
Total property and equipment	3,831,568	3,530,302
Operating lease right-of-use assets	1,417,823	1,369,555
Pre-delivery deposits on flight equipment	356,262	291,930
Long-term aircraft maintenance deposits	53,158	67,682
Deferred heavy maintenance, net	347,907	361,603
Other long-term assets	36,127	36,897
Total assets	8,398,825	7,043,412
Current liabilities:
Accounts payable	28,454	43,601
Air traffic liability	401,966	315,408
Current maturities of long-term debt and finance leases	384,197	258,852
Current maturities of operating leases	133,791	120,662
Other current liabilities	393,614	373,521
Total current liabilities	1,342,022	1,112,044
Long-term debt and finance leases, less current maturities	3,066,635	1,960,453
Operating leases, less current maturities	1,248,519	1,218,014
Deferred income taxes	439,894	469,292
Deferred gains and other long-term liabilities	52,060	22,277
Shareholders’ equity:
Common stock: Common stock, $0.0001 par value, 240,000,000 shares authorized at December 31, 2020 and 2019, respectively; 99,427,203 and 70,148,386 issued and 97,689,583 and 68,455,011 outstanding as of December 31, 2020 and 2019, respectively	10	7
Additional paid-in-capital	799,549	379,380
Treasury stock, at cost: 1,737,620 and 1,693,375 as of December 31, 2020 and 2019, respectively	(74,124)	(72,455)
Retained earnings	1,524,878	1,955,187
Accumulated other comprehensive income (loss)	(618)	(787)
Total shareholders’ equity	2,249,695	2,261,332
Total liabilities and shareholders’ equity	$ 8,398,825	$ 7,043,412

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized (in shares)	240,000,000	240,000,000
Common stock, shares issued (in shares)	99,427,203	70,148,386
Common stock, shares outstanding (in shares)	97,689,583	68,455,011
Treasury stock (in shares)	1,737,620	1,693,375

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($)		12 Months Ended
		Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018
Operating activities:
Net income (loss)		$ (428,700,000)		$ 335,255,000		$ 155,749,000
Adjustments to reconcile net income (loss) to net cash provided by operations:
Losses reclassified from other comprehensive income		252,000		315,000		315,000
Share-based compensation		11,575,000		8,154,000		11,021,000
Allowance for doubtful accounts (recoveries)		(249,000)		0		(11,000)
Amortization of debt issuance costs		10,752,000		8,654,000		8,819,000
Depreciation and amortization		278,588,000		225,264,000		176,727,000
Accretion expense		10,138,000		0		0
Deferred income tax expense (benefit)		(46,086,000)		115,689,000		46,303,000
Loss on disposal of assets		2,264,000		17,350,000		9,580,000
Special charges, non-operating		0		0		90,357,000
Changes in operating assets and liabilities:
Accounts receivable, net		30,486,000		(26,147,000)		1,674,000
Aircraft maintenance deposits, net		23,732,000		22,453,000		14,019,000
Deposits and other assets		(12,944,000)		14,999,000		(4,803,000)
Deferred heavy maintenance, net		(75,230,000)		(175,957,000)		(190,381,000)
Income tax receivable		(126,447,000)		(21,013,000)		69,844,000
Prepaid income taxes		(223,000)		1,431,000		0
Accounts payable		(17,052,000)		569,000		15,317,000
Air traffic liability		86,558,000		23,429,000		28,270,000
Other liabilities		27,194,000		1,698,000		74,038,000
Other		118,000		(822,000)		(375,000)
Net cash provided (used) by operating activities		(225,274,000)		551,321,000		506,463,000
Investing activities:
Purchase of available-for-sale investment securities		(118,893,000)		(122,410,000)		(124,430,000)
Proceeds from the maturity and sale of available-for-sale investment securities		117,665,000		120,830,000		122,947,000
Proceeds from sale of property and equipment		0		0		11,400,000
Pre-delivery deposits on flight equipment, net of refunds		(143,220,000)		(102,102,000)		(177,424,000)
Capitalized interest		(12,233,000)		(10,774,000)		(8,729,000)
Assets under construction for others		(3,944,000)		(7,936,000)		(501,000)
Purchase of property and equipment		(393,375,000)		(334,537,000)		(606,971,000)
Net cash used in investing activities		(554,000,000)		(456,929,000)		(783,708,000)
Financing activities:
Proceeds from issuance of long-term debt		1,612,391,000		225,891,000		832,099,000
Proceeds from issuance of common stock and warrants		366,783,000		0		0
Proceeds from stock options exercised		39,000		1,000		51,000
Payments on debt obligations		(254,304,000)		(246,783,000)		(137,275,000)
Payments on finance lease obligations		(25,401,000)		(96,547,000)		(205,720,000)
Reimbursement for assets under construction for others		4,153,000		5,618,000		501,000
Repurchase of common stock		(1,669,000)		(5,439,000)		(1,162,000)
Debt and equity issuance costs		(40,551,000)		(2,909,000)		(7,365,000)
Net cash provided (used) by financing activities		1,661,441,000		(120,168,000)		481,129,000
Net increase (decrease) in cash, cash equivalents, and restricted cash		882,167,000		(25,776,000)		203,884,000
Cash and cash equivalents at beginning of period		978,957,000	[1]	1,004,733,000	[1]	800,849,000
Cash and cash equivalents at the end of period	[1]	1,861,124,000		978,957,000		1,004,733,000
Cash payments for:
Interest, net of capitalized interest		80,837,000		80,254,000		65,123,000
Income taxes paid (received), net		(17,790,000)		5,843,000		(73,489,000)
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows for operating leases	[2]	180,805,000		191,004,000
Financing cash flows for finance leases	[2]	194,000		674,000
Non-cash transactions:
Capital expenditures funded by finance lease borrowings		565,000		45,608,000		987,000
Capital expenditures funded by operating lease borrowings	[2]	$ 168,526,000		$ 569,948,000		$ 0
[1]	The sum of cash and cash equivalents and restricted cash on our consolidated balance sheets equals cash, cash equivalents, and restricted cash in our statement of cash flows.
[2]	The Company adopted ASU No. 2016-02, "Leases (Topic 842)," utilizing the modified retrospective adoption method with an effective date of January 1, 2019. Therefore, the consolidated financial statements for 2020 and 2019 are presented under the new standard, while 2018 is not adjusted and continues to be reported in accordance with the Company's historical accounting policy.

Consolidated Statements of Ca_2

Consolidated Statements of Cash Flows (Parenthetical)	Sep. 17, 2020
8.00% Senior Secured Notes Due 2025 | 8.00% senior secured notes due in 2025
Stated interest rate percentage	8.00%

Consolidated Statements of Shar

Consolidated Statements of Shareholders’ Equity - USD ($) $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment	Common Stock	Additional Paid-In Capital	Treasury Stock	Retained Earnings	Retained EarningsCumulative Effect, Period of Adoption, Adjustment	Accumulated Other Comprehensive Income (Loss)
Beginning balance at Dec. 31, 2017	$ 1,762,574		$ 7	$ 360,153	$ (65,854)	$ 1,469,732		$ (1,464)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201602Member
Share-based compensation	$ 11,021			11,021
Repurchase of common stock	(1,162)				(1,162)
Proceeds from options exercised	51			51
Changes in comprehensive income	271							271
Net income (loss)	155,749					155,749
Ending balance at Dec. 31, 2018	$ 1,928,504	$ (5,549)	7	371,225	(67,016)	1,625,481	$ (5,549)	(1,193)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Share-based compensation	$ 8,154			8,154
Repurchase of common stock	(5,439)				(5,439)
Proceeds from options exercised	1			1
Changes in comprehensive income	406							406
Net income (loss)	335,255					335,255
Ending balance at Dec. 31, 2019	2,261,332		7	379,380	(72,455)	1,955,187		(787)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	11,575			11,575
Repurchase of common stock	(1,669)				(1,669)
Proceeds from options exercised	39			39
Changes in comprehensive income	169							169
Issuance of Common Stock and Warrants, net	352,968		3	352,965
Equity component value of convertible debt issuance, net	55,590			55,590
Net income (loss)	(428,700)					(428,700)
Ending balance at Dec. 31, 2020	$ 2,249,695	$ (1,609)	$ 10	$ 799,549	$ (74,124)	$ 1,524,878	$ (1,609)	$ (618)

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies Basis of Presentation The accompanying unaudited consolidated financial statements include the accounts of Spirit Airlines, Inc. ("Spirit") and its consolidated subsidiaries (the "Company"). Spirit is an ultra low-cost, low-fare airline that provides affordable travel opportunities principally throughout the domestic United States, the Caribbean and Latin America and is headquartered in Miramar, Florida. Spirit manages operations on a system-wide basis due to the interdependence of its route structure in the various markets served. As only one service is offered (i.e., air transportation), management has concluded there is only one reportable segment. In August 2020, Spirit formed several new subsidiaries; Spirit Finance Cayman 1 Ltd. (“HoldCo 1”), Spirit Finance Cayman 2 Ltd. (“HoldCo 2), Spirit IP Cayman Ltd. (“Spirit IP”) and Spirit Loyalty Cayman Ltd. (“Spirit Loyalty”). Each are Cayman Islands exempted companies incorporated with limited liability. Spirit IP and Spirit Loyalty are wholly-owned subsidiaries of HoldCo 2 (other than the special share issued to the special shareholder, who granted a proxy to vote such share to the collateral agent for the 8.00% senior secured notes (as defined herein)). HoldCo 1 and HoldCo 2 are special purpose holding companies. HoldCo 2 is a wholly-owned direct subsidiary of HoldCo 1 (other than the special share issued to the special shareholder, who granted a proxy to vote such share to the collateral agent for the 8.00% senior secured notes). HoldCo 1 is a wholly-owned subsidiary of Spirit (other than the special share issued to the special shareholder, who granted a proxy to vote such share to the collateral agent for the 8.00% senior secured notes). As a result, the Company's financial statements are presented on a consolidated basis. Use of Estimates The preparation of financial statements in accordance with generally accepted accounting principles in the United States of America requires the Company's management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. The Company's estimates and assumptions are based on historical experience and changes in the business environment. However, actual results may differ from estimates under different conditions, sometimes materially. Critical accounting policies and estimates are defined as those that both (i) are most important to the portrayal of the Company's financial condition and results and (ii) require management's most subjective judgments. The Company's most critical accounting policies and estimates are described below. Cash and Cash Equivalents The Company considers all highly liquid investments with maturities of less than three months at the date of acquisition to be cash equivalents. Investments included in this category primarily consist of cash and money market funds. Cash and cash equivalents are stated at cost, which approximates fair value. Restricted Cash The Company's restricted cash is comprised of cash held in account subject to account control agreements to be used for the payment of interest and fees on the Company's 8.00% senior secured notes and cash pledged as collateral against the Company's secured letters of credit. Short-term Investment Securities The Company's short-term investment securities are classified as available-for-sale and generally consist of U.S. Treasury and U.S. government agency securities with contractual maturities of twelve months or less. These securities are stated at fair value within current assets on the Company's consolidated balance sheet. For all short-term investments, at each reset period or upon reinvestment, the Company accounts for the transaction as proceeds from the maturity of short-term investment securities for the security relinquished, and purchase of short-term investment securities for the security purchased, in the Company's consolidated statements of cash flows. Realized gains and losses on sales of investments, if any, are reflected in non-operating income (expense) in the consolidated statements of operations. Unrealized gains and losses on investment securities are reflected as a component of accumulated other comprehensive income. Accounts Receivable Accounts receivable primarily consist of amounts due from credit card processors associated with the sales of tickets and amounts due from the Internal Revenue Service related to federal excise fuel tax. The Company records an allowance for amounts not expected to be collected. The Company estimates the allowance based on historical write-offs and aging trends as well as an estimate of the expected lifetime credit losses. The allowance for doubtful accounts was immaterial as of December 31, 2020 and 2019. In addition, the provision for doubtful accounts and write-offs for 2020, 2019 and 2018 were each immaterial. Income Tax Receivable Income tax receivable consists of amounts due from tax authorities for recovery of income taxes paid in prior periods. Property and Equipment Property and equipment is stated at cost, less accumulated depreciation and amortization. Depreciation of operating property and equipment is computed using the straight-line method applied to each unit of property. Residual values for new aircraft, new engines, major spare rotable parts, avionics and assemblies are generally estimated to be 10%. Property under finance leases and related obligations are initially recorded at an amount equal to the present value of future minimum lease payments computed using the Company's incremental borrowing rate or, when known, the interest rate implicit in the lease. Amortization of property under finance leases is recorded on a straight-line basis over the lease term and is included in depreciation and amortization expense. The depreciable lives used for the principal depreciable asset classifications are: Estimated Useful Life Aircraft, engines and flight simulators 25 Spare rotables and flight assemblies 7 to 25 years Other equipment and vehicles 5 to 7 years Internal use software 3 to 10 years Finance leases Lease term or estimated useful life of the asset Leasehold improvements Lesser of lease term or estimated useful life of the improvement Buildings Lesser of lease term or 30 years As of December 31, 2020, the Company had 101 aircraft, 16 spare engines and 1 flight simulator capitalized within flight equipment with depreciable lives of 25 years. As of December 31, 2020, the Company had 56 aircraft financed through operating leases with lease terms from 8 to 18 years. In addition, the Company had 8 spare engines financed through operating leases with lease terms from 12 to 16 years. The following table illustrates the components of depreciation and amortization expense: Year Ended December 31, 2020 2019 2018 (in thousands) Depreciation $ 179,470 $ 155,326 $ 129,412 Amortization of heavy maintenance 88,927 63,364 41,286 Amortization of capitalized software 10,191 6,574 6,029 Total depreciation and amortization $ 278,588 $ 225,264 $ 176,727 The Company capitalizes certain internal and external costs associated with the acquisition and development of internal-use software for new products, and enhancements to existing products, that have reached the application development stage and meet recoverability tests. Capitalized costs include external direct costs of materials and services utilized in developing or obtaining internal-use software, and labor cost for employees who are directly associated with, and devote time, to internal-use software projects. Capitalized computer software, included as a component of ground and other equipment in the accompanying consolidated balance sheets, net of amortization, was $24.3 million and $13.0 million at December 31, 2020 and 2019, respectively. The Company records amortization of capitalized software on a straight-line basis within depreciation and amortization expense in the accompanying consolidated statements of operations. The Company placed in service internal-use software of $21.5 million, $5.9 million and $12.0 million, during the years ended 2020, 2019 and 2018, respectively. Operating Lease Right-of-Use Asset and Liabilities The Company adopted Topic 842 utilizing the modified retrospective adoption method with an effective date of January 1, 2019. 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. Right-of-use assets represent the Company's right to use an underlying asset for the lease term and lease liabilities represent the Company's obligation to make lease payments arising from the lease. Right-of-use assets and liabilities are recognized at the lease commencement date based on the estimated present value of lease payments over the lease term. When available, the Company uses the rate implicit in the lease to discount lease payments to present value. However, the Company's leases generally do not provide a readily determinable implicit rate. Therefore, the Company estimates the incremental borrowing rate to discount lease payments based on information available at lease commencement. The Company uses publicly available data for instruments with similar characteristics when calculating its incremental borrowing rates. The Company has options to extend certain of its operating leases for an additional period of time and options to early terminate several of its operating leases. The lease term consists of the noncancellable period of the lease, periods covered by options to extend the lease if the Company is reasonably certain to exercise the option, periods covered by an option to terminate the lease if the Company is reasonably certain not to exercise the option and periods covered by an option to extend or not terminate the lease in which the exercise of the option is controlled by the lessor. The Company's lease agreements do not contain any residual value guarantees. The Company elected to not separate non-lease components from the associated lease component for all underlying classes of assets with lease and non-lease components. The Company elected not to apply the recognition requirements in Topic 842 to short-term leases (i.e., leases of 12 months or less) but instead recognize these lease payments in income on a straight-line basis over the lease term. The Company elected this accounting policy for all classes of underlying assets. In addition, in accordance with Topic 842, variable lease payments in the period in which the obligation for those payments is incurred are not included in the recognition of a lease liability or right-of-use asset. 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 consolidated statements of operations. Pre-Delivery Deposits on Flight Equipment The Company is required to make pre-delivery deposit payments ("PDPs") towards the purchase price of each new aircraft and engine prior to the scheduled delivery date. These deposits are initially classified as pre-delivery deposits on flight equipment on the Company's consolidated balance sheets until the aircraft or engine is delivered, at which time the related PDPs are deducted from the final purchase price of the aircraft or engine and are reclassified to flight equipment on the Company's consolidated balance sheets. In addition, the Company capitalizes the interest that is attributable to the outstanding PDP balances as a percentage of the related debt on which interest is incurred. Capitalized interest represents interest cost incurred during the acquisition period of a long-term asset, and is the amount which theoretically could have been avoided had the Company not paid PDPs for the related aircraft or engines. Related interest is capitalized and included within pre-delivery deposits on flight equipment through the acquisition period until delivery is taken of the aircraft or engine and the asset is ready for service. Once the aircraft or engine is delivered, the capitalized interest is also reclassified into flight equipment on the Company's consolidated balance sheets along with the related PDPs as they are included in the cost of the aircraft or engine. Capitalized interest for 2020, 2019 and 2018 is primarily related to the interest incurred on long-term debt. Measurement of Asset Impairments The Company records impairment charges on long-lived assets used in operations when events and circumstances indicate that the assets may be impaired, the undiscounted cash flows estimated to be generated by those assets are less than the carrying amount of those assets, and the net book value of the assets exceeds their estimated fair value. In making these determinations, the Company uses certain assumptions, including, but not limited to: (i) estimated fair value of the assets; and (ii) estimated, undiscounted future cash flows expected to be generated by these assets, which are based on additional assumptions such as asset utilization, length of service the asset will be used in the Company’s operations, and estimated salvage values. The Company has assessed whether any impairment of its long-lived assets existed and has determined that no charges were deemed necessary under applicable accounting standards as of December 31, 2020. The Company's assumptions about future conditions important to its assessment of potential impairment of its long-lived assets, including the impact of the COVID-19 pandemic to its business, are subject to uncertainty, and the Company will continue to monitor these conditions in future periods as new information becomes available, and will updated its analyses accordingly. Passenger Revenues Fare revenues. Tickets sold are initially deferred within air traffic liability on the Company's consolidated balance sheet. Passenger fare revenues are recognized at time of departure when transportation is provided. Generally, all tickets sold by the Company are nonrefundable. An unused ticket expires at the date of scheduled travel and is recognized as revenue at the date of scheduled travel. As of December 31, 2020 and 2019, the Company had air traffic liability ("ATL") balances of $402.0 million and $315.4 million, respectively. As of December 31, 2020, substantially all of the ATL balance as of December 31, 2019 has been recognized. Substantially all of the Company's ATL balance as of December 31, 2020 is expected to be recognized within 12 months. Non-fare revenues. The adoption of ASU 2014-09 on January 1, 2018 impacted the classification of certain ancillary items such as bags, seats and other travel-related fees, since they are deemed part of the single performance obligation of providing passenger transportation. These ancillary items are now recognized in non-fare revenues within passenger revenues, at the time of departure, in the Company's disaggregated revenue table within Note 4, Revenue Disaggregation. The following table summarizes the primary components of the Company's non-fare revenue and the revenue recognition method utilized for each service or product: Year Ended December 31, Non-fare revenue Recognition method 2020 2019 2018 (in thousands) Baggage Time of departure $ 404,896 $ 734,243 $ 620,154 Passenger usage fee Time of departure 358,561 669,177 531,459 Advance seat selection Time of departure 125,213 228,876 180,012 Other 120,638 238,454 224,283 Non-fare revenue $ 1,009,308 $ 1,870,750 $ 1,555,908 Changes and cancellations. Customers may elect to change or cancel their itinerary prior to the date of departure. For changes, a service charge is recognized at time of departure of newly scheduled travel and is deducted from the face value of the original purchase price of the ticket, and the original ticket becomes invalid. For cancellations, a service charge is assessed and the amount remaining after deducting the service charge is called a credit shell which prior to 2020 had an expiration of 60 days from the date the credit shell was created. During 2020, in response to the COVID-19 pandemic, the Company increased the expiration period on some of its credit shells from 60 days to up to 12 months and waived change and cancellation fees for the Guests who booked travel to occur by February 28, 2021. As the COVID-19 pandemic continues to evolve, the Company will evaluate any continued impact to travel plans and may decide to further extend credit shell expiration dates and/or waive change and cancellation fees in the future. Credit shells can be used towards the purchase of a new ticket and the Company’s other service offerings. Both service charge and credit shell amounts are recorded as deferred revenue and amounts expected to expire unused are estimated based on historical experience. Estimating the amount of credits that will go unused involves some level of subjectivity and judgment. Assumptions used to generate breakage estimates can be impacted by several factors including, but not limited to, changes to the Company's ticketing policies, changes to the Company’s refund, exchange, and credit shell policies, and economic factors. Given the unprecedented amount of cancellations in the current year and the related increase in credit shells provided, the Company expects additional variability in the amount of breakage revenue recorded in future periods, as the estimates of the portion of those funds that will expire unused may differ from historical experience. Other Revenues Other revenues primarily consist of the marketing component of the sale of frequent flyer miles to the Company's credit card partner and commissions revenue from the sale of various items such as hotels and rental cars. Frequent Flyer Program The Company's frequent flyer program generates customer loyalty by rewarding customers with mileage credits to travel on Spirit. When traveling, customers earn redeemable mileage credits for each mile flown on Spirit. Customers can also earn mileage credits through participating companies such as the co-branded Spirit credit card. Mileage credits are redeemable by customers in future periods for air travel on Spirit. To reflect the mileage credits earned, the program includes two types of transactions that are considered revenue arrangements with multiple performance obligations: (1) mileage credits earned with travel and (2) mileage credits sold to co-branded credit card partner. Passenger ticket sales earning mileage credits. Passenger ticket sales earning mileage credits provide customers with (1) mileage credits earned and (2) air transportation. The Company values each performance obligation on a standalone basis. To value the mileage credits earned, the Company considers the quantitative value a passenger receives by redeeming miles for a ticket rather than paying cash, which is referred to as equivalent ticket value ("ETV"). The Company defers revenue for the mileage credits when earned and recognizes loyalty travel awards in passenger revenue as the miles are redeemed and services are provided. The Company records the air transportation portion of the passenger ticket sales in air traffic liability and recognizes passenger revenue when transportation is provided or if the ticket goes unused, at the date of scheduled travel. Sale of mileage credits. Customers may earn mileage credits based on their spending with the Company's co-branded credit card company with which the Company has an agreement to sell mileage credits. The contract to sell mileage credits under this agreement has multiple performance obligations, as discussed below. The Company's co-branded credit card agreement provides for joint marketing where cardholders earn mileage credits for making purchases using co-branded cards. During 2020, the Company extended its agreement with the administrator of the FREE SPIRIT affinity credit card program to extend through March 31, 2024. In connection with its extension of the agreement, in January 2021, the Company launched a new loyalty program with extended mileage expiration, additional benefits based on status tiers, and other changes. The Company accounts for this agreement consistently with the accounting method that allocates the consideration received to the individual products and services delivered. The value is allocated based on the relative selling prices of those products and services, which generally consists of (i) travel miles to be awarded, (ii) licensing of brand and access to member lists and (iii) advertising and marketing efforts. The Company determined the best estimate of the selling prices by considering discounted cash flow analysis using multiple inputs and assumptions, including: (1) the expected number of miles awarded and number of miles redeemed, (2) ETV for the award travel obligation, (3) licensing of brand and access to member lists and (4) advertising and marketing efforts. The new program terms will require updated estimates of the allocation of future revenues to the performance obligations described above. The Company defers the amount for award travel obligation as part of loyalty deferred revenue within air traffic liability on the consolidated balance sheet and recognizes loyalty travel awards in passenger revenue as the mileage credits are used for travel. Revenue allocated to the remaining performance obligations, primarily marketing components, is recorded in other revenue over time as miles are delivered. Total unrecognized revenue from future FREE SPIRIT award redemptions and the sale of mileage credits was $31.6 million and $29.8 million at December 31, 2020 and 2019, respectively. The current portion of this balance is recorded within air traffic liability and the long-term portion of this balance is recorded within deferred gains and other long-term liabilities in the accompanying consolidated balance sheets. The following table illustrates total cash proceeds received from the sale of mileage credits and the portion of such proceeds recognized in non-ticket revenue immediately as marketing component: Consideration received from credit card mile programs Portion of proceeds recognized immediately as marketing component Year Ended (in thousands) December 31, 2020 $ 33,201 $ 25,918 December 31, 2019 48,136 37,151 December 31, 2018 39,194 30,353 Mileage breakage. For mileage credits that the Company estimates are not likely to be redeemed ("breakage"), the Company recognizes the associated value proportionally during the period in which the remaining mileage credits are redeemed. Management uses statistical models to estimate breakage based on historical redemption patterns. A change in assumptions as to the period over which mileage credits are expected to be redeemed, the actual redemption activity for mileage credits or the estimated fair value of mileage credits expected to be redeemed could have an impact on revenues in the year in which the change occurs and in future years. Current activity of frequent flyer program. Mileage credits are combined in one homogeneous pool and are not separately identifiable. As such, revenue is comprised of miles that were part of the frequent flyer deferred revenue balance at the beginning of the period as well as miles that were issued during the period. Airframe and Engine Maintenance The Company accounts for heavy maintenance and major overhaul under the deferral method whereby the cost of heavy maintenance and major overhaul is deferred and amortized until the earlier of the end of the useful life of the related asset, the end of the remaining lease term or the next scheduled heavy maintenance event. Amortization of heavy maintenance and major overhaul costs charged to depreciation and amortization expense was $88.9 million, $63.4 million and $41.3 million for the years ended 2020, 2019 and 2018, respectively. During the years ended 2020, 2019 and 2018, the Company deferred $75.2 million, $176.0 million and $190.5 million, respectively, of costs for heavy maintenance, net of reimbursements. At December 31, 2020 and 2019, the Company had deferred heavy maintenance balance of $570.6 million and $504.2 million, and accumulated heavy maintenance amortization of $222.7 million and $142.6 million, respectively. The Company outsources certain routine, non-heavy maintenance functions under contracts that require payment on a utilization basis, such as flight hours. Costs incurred for maintenance and repair under flight hour maintenance contracts, where labor and materials price risks have been transferred to the service provider, are expensed based on contractual payment terms. All other costs for routine maintenance of the airframes and engines are charged to expense as performed. The table below summarizes the components of the Company’s maintenance cost: Year Ended December 31, 2020 2019 2018 (in thousands) Flight hour-based maintenance expense $ 52,092 $ 78,253 $ 68,039 Non-flight hour-based maintenance expense 59,135 65,322 61,039 Total maintenance, materials and repairs $ 111,227 $ 143,575 $ 129,078 Leased Aircraft Return Costs The Company's aircraft lease agreements often contain provisions that require the Company to return aircraft airframes, engines and other aircraft components to the lessor in a certain condition or pay an amount to the lessor based on the airframe and engine's actual return condition. Lease return costs include all costs that would be incurred at the return of the aircraft, including costs incurred to repair the airframe and engines to the required condition as stipulated by the lease. Lease return costs are recognized beginning when it is probable that such costs will be incurred and they can be estimated. When determining the need to accrue lease return costs, there are various probability and estimated cost, there are various factors which need to be considered such as the contractual terms of the lease agreement, current condition of the aircraft, the age of the aircraft at lease expiration, projected number of hours run on the engine at the time of return, and the number of projected cycles run on the airframe at the time of return, among others. Management assesses the need to accrue lease return costs periodically throughout the year or whenever facts and circumstances warrant an assessment. Lease return costs will generally be estimable closer to the end of the lease term but may be estimable earlier in the lease term depending on the contractual terms of the lease agreement and the timing of maintenance events for a particular aircraft. Aircraft Maintenance Deposits Some of the Company's aircraft and engine master lease agreements provide that the Company pay maintenance reserves to aircraft lessors to be held as collateral in advance of the Company's required performance of major maintenance activities. A majority of these maintenance reserve payments are calculated based on a utilization measure, such as flight hours or cycles, while some maintenance reserve payments are fixed, time-based contractual amounts. These lease agreements generally provide that maintenance reserves are reimbursable to the Company upon completion of the maintenance event. Some of the master lease agreements do not require that the Company pay maintenance reserves so long as the Company's cash balance does not fall below a certain level. As of December 31, 2020, the Company is in full compliance with such requirements and does not anticipate having to pay reserves related to these master leases in the future. Maintenance reserve payments are reflected as aircraft maintenance deposits in the accompanying consolidated balance sheets. The Company makes certain assumptions to determine the recoverability of maintenance deposits. These assumptions are based on various factors such as the estimated time between the maintenance events, the date the aircraft is due to be returned to the lessor, the cost of future maintenance events and the utilization of the aircraft is estimated before it is returned to the lessor. When it is not probable the Company will recover amounts currently on deposit with a lessor, such amounts are expensed as supplemental rent. Aircraft Fuel Aircraft fuel expense includes jet fuel and associated into-plane costs, taxes, and oil, and realized and unrealized gains and losses associated with fuel derivative contracts, if any. Advertising The Company expenses advertising and the production costs of advertising as incurred. Marketing and advertising expenses of $5.5 million, $6.3 million and $6.3 million for the years ended 2020, 2019 and 2018, respectively, were recorded within distribution expense in the consolidated statements of operations. Income Taxes The Company accounts for income taxes using the asset and liability method. The Company records a valuation allowance to reduce the deferred tax assets reported if, based on the weight of the evidence, it is more likely than not that some portion or all of the deferred tax assets will be not realized. As of December 31, 2020 and 2019, the Company recorded a valuation allowance of $2.9 million and $1.7 million, respectively. For additional information, refer to Note 17, Income Taxes. Stock-Based Compensation The Company recognizes cost of employee services received in exchange for awards of equity instruments based on the fair value of each instrument at the date of grant. For the majority of awards, compensation expense is recognized on a straight-line basis over the period during which an employee is required to provide service in exchange for an award. Certain awards have performance conditions that must be achieved prior to vesting and are expensed based on the expected achievement at each reporting period. The Company has issued and outstanding restricted stock awards, stock option awards and performance share awards. Restricted stock awards are valued at the fair value of the shares on the date of grant. The fair value of share option awards is estimated on the date of grant using the Black-Scholes valuation model. The fair value of performance share awards based on a market condition is estimated through the use of a Monte Carlo simulation model. The fair value of performance share awards based on a performance condition is based on the fair value of the shares on the date of grant. The performance share awards based on a performance condition are evaluated at each report date and adjustments are made to stock-based compensation expense based on the number of shares deemed probable of issuance upon vesting. For additional information, refer to Note 12, Stock-Based Compensation. Concentrations of Risk The Company’s business may be adversely affected by increases in the price of aircraft fuel, the volatility of the price of aircraft fuel, or both. Aircraft fuel, one of the Company’s largest expenditures, represented approximately 19%, 30% and 32% of total operating expenses in 2020, 2019 and 2018, respectively. The Company’s operations are largely concentrated in the southeast United States with Fort Lauderdale being the highest volume fueling point in the system. Gulf Coast Jet indexed fuel is the basis for a substantial majority of the Company’s fuel consumption. Any disruption to the oil production or refinery capacity in the Gulf Coast, as a result of weather or any other disaster, or disruptions in supply of jet fuel, dramatic escalations in the costs of jet fuel and/or the failure of fuel providers to perform under fuel arrangements for other reasons could have a material adverse effect on the Company’s financial condition and results of operations. The Company’s operations will continue to be vulnerable to weather conditions (including hurricane season or snow and severe winter weather), which could disrupt service or create air traffic control problems. These events may result in decreased revenue and/or increased costs. Due to the relatively small size of the Company's fleet and high utilization rate, the unavailability of aircraft and resulting reduced capacity could have a material adverse effect on the Company’s business, results of operations and financial condition. As of December 31, 2020, the Company had five union-represented employee groups that together represented approximately 82% of all employees. A strike or other significant labor dispute with the Company’

Impact of COVID-19

Impact of COVID-19	12 Months Ended
	Dec. 31, 2020
Unusual or Infrequent Items, or Both [Abstract]
Impact of COVID-19	Impact of COVID-19 Since its initial onset in early 2020, the COVID-19 pandemic has evolved throughout the year and continues to be fluid. Therefore, the Company's financial and operational outlook still remains subject to change and fluctuation. The Company continues to monitor the impacts of the pandemic on its operations and financial condition, and to implement and adapt mitigation strategies while working to preserve cash and protect the long-term sustainability of the Company. Capacity Reductions At the onset of the COVID-19 pandemic in March 2020, in response to government restrictions on travel and drastically reduced consumer demand, the Company began to significantly reduce capacity each month with the largest capacity reduction in May 2020 at approximately 94%, year over year. In response to modest demand recovery, the Company strategically added back capacity during certain peak travel periods. During the holiday months of November and December, capacity was reduced to a lesser extent with reductions of 20.8% and 20.1%, year over year. The Company continues to closely monitor demand and will make adjustments to the flight schedule as appropriate. The COVID-19 pandemic and its effects continue to evolve with recent developments including the uptick in the rate of infections following the 2020 holiday season, the emergency use authorization issued by the U.S. Food and Drug Administration for certain COVID-19 vaccines in late 2020, and the requirement, effective January 26, 2021, that all U.S. inbound international travelers provide a negative COVID-19 test prior to flying. The Company currently estimates that air travel demand will continue to be volatile and will fluctuate in the upcoming months as the lingering effects of COVID-19 continue to develop. The Company expects that air travel demand will continue to gradually recover in 2021. However, the situation continues to be fluid and actual capacity adjustments may be different than what the Company currently expects. Refer to Note 4, Revenue Disaggregation, for discussion of the impact of COVID-19 on the Company's air traffic liability, credit shells and refunds. COVID-19 Legislation On March 27, 2020, President Donald Trump signed the Coronavirus Aid, Relief, and Economic Security Act ("CARES Act") into law. The CARES Act was a relief package intended to assist many aspects of the American economy, including providing the airline industry with up to $25 billion in grants to be used for employee salaries, wages and benefits and up to $25 billion in secured loans. In April 2020, the Company entered into a Payroll Support Program ("PSP") Agreement with the United States Department of the Treasury ("Treasury"), pursuant to which the Company received a total of $344.4 million, used exclusively to pay for salaries, wages and benefits for the Company’s Team Members through September 30, 2020. Of that amount, $73.3 million is in the form of a low-interest 10-year loan. In addition, in connection with its participation in the PSP, the Company issued to Treasury warrants pursuant to a warrant agreement to purchase up to 520,797 shares of the Company’s common stock at a strike price of $14.08 per share (the closing price for the shares of the Company's common stock on April 9, 2020) with a fair value of $3.9 million. The remaining amount of $267.2 million is in the form of a grant and was recognized in special charges and credits, net of related costs, in the Company's consolidated statement of operations. Refer to Note 5, Special Charges and Credits, for additional information. Pursuant to the warrant agreement with the Treasury, the Company registered the resale of the warrants and the 520,797 shares of common stock issuable upon exercise of such warrants in September and October 2020. Total warrants issued represent less than 1% of the outstanding shares of the Company's common stock as of December 31, 2020. Refer to Note 14, Debt and Other Obligations, for additional information on the notes issued and Note 11, Common Stock and Preferred Stock, for additional information on the warrants. In connection with the Company's participation in the PSP, the Company was, and continues to be, subject to certain restrictions and limitations, including, but not limited to: • Restrictions on payment of dividends and stock buybacks through September 30, 2021; • Limits on certain executive compensation including limiting pay increases and severance pay or other benefits upon terminations, through March 24, 2022; • Requirements to maintain certain levels of scheduled services (including to destinations where there may currently be significantly reduced or no demand) through September 30, 2020; • A prohibition on involuntary terminations or furloughs of the Company's employees (except for health, disability, cause, or certain disciplinary reasons) through September 30, 2020; • A prohibition on reducing the salaries, wages, or benefits of the Company's employees (other than the Company's executive officers or independent contractors, or as otherwise permitted under the terms of the PSP) through September 30, 2020; • Limitations on the use of the grant funds exclusively for the continuation of payment of employee wages, salaries and benefits; and • Additional reporting and recordkeeping requirements relating to the CARES Act funds. On April 29, 2020, the Company applied for additional funds under the Treasury's loan program under the CARES Act (“Loan Program”). On July 1, 2020, the Company executed a non-binding letter of intent with the Treasury which summarized the principal terms of the financing request submitted by the Company to the Treasury. In September 2020, the Company decided that it would not participate in the Treasury's loan program as it was able to secure other forms of financing described below. On December 27, 2020, the Consolidated Appropriations Act, 2021 was signed into law. This new legislation provides an extension or additional benefits designed to address the continuing economic fallout from the COVID-19 pandemic. The bill extends the PSP program of the CARES Act through March 31, 2021 ("PSP2") and provides an additional $15 billion to fund the PSP2 program for employees of passenger air carriers. In late December, the Company notified the Treasury of its intent to participate in the PSP2 agreement. The Company entered into a new payroll support program agreement with the Treasury on January 15, 2021. The Company expects to receive approximately $184.5 million pursuant to its participation in the PSP2 program. In January 2021, the Company received the first installment of $92.2 million in the form of a grant. Of the remaining amount, the Company expects that approximately $25 million will be in the form of a low-interest 10-year loan. In addition, in connection with its participation in the PSP2, the Company expects to issue to Treasury warrants to purchase up to 103,761 shares of the Company’s common stock at a strike price of $24.42 per share (the closing price of the shares of the Company's common stock on December 24, 2020). In connection with the Company's participation in the PSP2, the Company is subject to certain restrictions and limitations, including, but not limited to: • Restrictions on payment of dividends and stock buybacks through March 31, 2022; • Limits on executive compensation through October 1, 2022; • Restrictions from conducting involuntary furloughs or reducing pay rates and benefits until March 31, 2021; • Requirements to maintain certain levels of scheduled services through March 1, 2022; • Reporting requirements; and • A recall of all employees that were involuntarily furloughed or terminated between October 1, 2020 and the date the carrier enters into the new payroll support agreement with the Treasury. Such employees, if returning to work, must be compensated for lost pay and benefits between December 1, 2020 and the date of such new payroll support agreement. The CARES Act also provided an employee retention credit (“CARES Employee Retention credit”) which is a refundable tax credit against certain employment taxes of up to $5,000 per employee for eligible employers. The credit is equal to 50% of qualified wages paid to employees during a quarter, capped at $10,000 of qualified wages through year end. The Company qualified for the credit beginning on April 1, 2020 and received additional credits for qualified wages through December 31, 2020. During the twelve months ended December 31, 2020 , the Company recorded $38.5 million related to the CARES Employee Retention credit within special charges (credits) on the Company’s consolidated statements of operations. Refer to Note 5, Special Charges and Credits, for additional information. The Consolidated Appropriations Act, 2021 also extends and expands the availability of the CARES Employee Retention credit through June 30, 2021, however, certain provisions apply only after December 31, 2020. This new legislation amends the employee retention credit to be equal to 70% of qualified wages paid to employees after December 31, 2020, and before July 1, 2021. During the first two quarters of 2021, a maximum of $10,000 in qualified wages for each employee per calendar quarter may be counted in determining the 70% credit. Therefore, the maximum tax credit that can be claimed by an eligible employer in 2021 is $7,000 per employee per calendar quarter for the first and second quarters of 2021. The CARES Act also provides for certain tax loss carry backs and a waiver on federal fuel taxes through December 31, 2020. As of December 31, 2020 , the Company had recognized $142.0 million in related federal tax loss carrybacks and $6.5 million in federal fuel tax savings reflected within aircraft fuel in the Company’s statements of operations. Finally, the CARES Act also provides for deferred payment of the employer portion of social security taxes through the end of 2020, with 50% of the deferred amount due December 31, 2021 and the remaining 50% due December 31, 2022. As of December 31, 2020 , the Company had deferred $23.2 million in social security tax payments. The deferred amounts are recorded within other current liabilities and within deferred gains and other long-term liabilities on the Company’s consolidated balance sheet. Income Taxes The Company's effective tax rate for the twelve months ended December 31, 2020 was 30.9% compared to 23.2% for the twelve months ended December 31, 2019. The increase in tax rate, as compared to the prior year period, is primarily due to a $56.1 million discrete federal tax benefit recorded during the twelve months ended December 31, 2020 related to the passage of the CARES Act. The CARES Act allows for carryback of net operating losses generated at a 21% tax rate to recover taxes paid at a 35% tax rate. Excluding this discrete tax benefit, the Company's effective tax rate for the twelve months ended December 31, 2020 would have been 21.8%. While the Company expects its tax rate to be fairly consistent in the near term, it will tend to vary depending on recurring items such as the amount of income we earn in each state and the state tax rate applicable to such income. Discrete items particular to a given year may also affect our effective tax rates. Refer to Note 17, Income Taxes, for additional information. Balance Sheet, Cash Flow and Liquidity Since the onset of the spread of COVID-19 in the U.S. in the first quarter of 2020, the Company has taken several actions to increase liquidity and strengthen its financial position. As a result of these actions, as of December 31, 2020 , the Company had unrestricted cash and cash equivalents and short-term investment securities of $1,896.1 million. In March 2020, the Company entered into a senior secured revolving credit facility (the "2022 revolving credit facility") for an initial commitment amount of $110.0 million , and subsequently, in the second quarter of 2020, increased its commitment amount to $180.0 million. As of December 31, 2020 , the Company had fully drawn the available amount of $180.0 million under the 2022 revolving credit facility. The 2022 revolving credit facility matures on March 30, 2022. Refer to Note 14, Debt and Other Obligations, for additional information about the 2022 revolving credit facility. On May 12, 2020, the Company completed the public offering of $175.0 million aggregate principal amount of 4.75% convertible senior notes due 2025 (the “convertible notes”). The convertible notes will bear interest at the rate of 4.75% per year and will mature on May 15, 2025. Interest on the convertible notes is payable semi-annually in arrears on May 15 and November 15 of each year, beginning on Nove mber 15, 2020. The Company received proceeds of $168.3 million, net of total issuance costs of $6.7 million and recorded $95.6 million in long-term debt and finance leases, net of debt issuance costs of $3.8 million on its consolidated balance sheets, related to the debt component of the convertible notes, and $72.7 million in additional paid-in-capital ("APIC"), net of issuance costs of $2.9 million on its consolidated balance sheets, related to the equity component of the convertible notes. Refer to Note 14, Debt and Other Obligations for additional information about the Company’s convertible debt. Also on May 12, 2020, the Company completed the public offering of 20,125,000 shares of its voting common stock, which includes full exercise of the underwriters’ option to purchase an additional 2,625,000 shares of common stock, at a public offering price of $10.00 per share (the “common stock offering”). The Company received proceeds of $192.4 million , net of issuance costs of $8.9 million . Refer to Note 11, Common Stock and Preferred Stock, for further information about the Company’s common stock offering. In June 2020, the Company entered into an agreement to amend its revolving credit facility entered into in 2018 to finance aircraft pre-delivery payments. The agreement amends the revolving credit facility to extend the final maturity date from December 30, 2020 to March 31, 2021. Upon execution of the amended agreement, the maximum borrowing capacity decreased from $160.0 million to $111.2 million. This facility is secured by the collateral assignment of certain of the Company’s rights under the purchase agreement with Airbus. As of December 31, 2020, collateralized amounts were related to 11 Airbus A320neo aircraft scheduled to be delivered between June 2021 and April 2022. The maximum borrowing capacity of $95.1 million, as of December 31, 2020, decreased from $111.2 million due to the delivery of aircraft during the third and fourth quarters of 2020 and will continue to decrease as the Company takes delivery of the related aircraft. The amendment provides approximately $54 million in additional liquidity through March 2021. Refer to Note 14, Debt and Other Obligations, for further information. Also, in June 2020, the Company entered into an agreement to defer certain aircraft deliveries originally scheduled in 2020 and 2021, as well as the related pre-delivery deposit payments. The Company may elect to supplement these deliveries by additional acquisitions from the manufacturer or in the open market if demand conditions merit. The Company also may adjust or defer deliveries, or change models of aircraft in the delivery stream, from time to time, as a means to match future capacity with anticipated demand and growth trends. During the twelve months ended December 31, 2020 , the Company took delivery of 12 aircraft. In addition, the Company has 16 aircraft scheduled for delivery in 2021. Refer to Note 18, Commitments and Contingencies, for further information about the Company’s future aircraft deliveries. On July 22, 2020, the Company entered into an equity distribution agreement relating to the issuance and sale from time to time by the Company of up to 9,000,000 shares of the Company's common stock in sales deemed to be "at-the-market offerings" as defined in Rule 415 under the Securities Act of 1933, as amended (the "Securities Act"). During the third quarter of 2020, the Company completed the sale of all 9,000,000 shares under its "at-the-market offering" program ("ATM program") and had received proceeds of $156.7 million , net of $5.0 million in related issuance costs. Refer to Note 11, Common Stock and Preferred Stock, for further information. On September 17, 2020, the Company completed a private offering by Spirit IP Cayman Ltd., an indirect wholly-owned subsidiary of the Company and Spirit Loyalty Cayman Ltd., an indirect wholly-owned subsidiary of the Company, of an aggregate of $850 million principal amount of 8.00% senior secured notes. The 8.00% senior secured notes will be secured by, among other things, a first priority lien on the core assets of the Company’s loyalty programs, comprised of cash proceeds from its Free Spirit co-branded credit card programs, its $9 Fare Club TM program membership fees, and certain intellectual property required or necessary to operate the loyalty programs, as well as the Company’s brand intellectual property. Refer to Note 4, Revenue Disaggregation, for further information on the Company's loyalty programs. The 8.00% senior secured notes will mature on September 20, 2025. The Company received proceeds of $823.9 million, net of issuance costs of $17.4 million and original issue discount of $8.7 million, related to this private offering. Refer to Note 14, Debt and Other Obligations, for further information. For purposes of assessing its liquidity needs, the Company estimates that demand will continue to be volatile as it recovers through 2021, but remain well below 2019 levels. The Company believes the actions described above, along with the expected funds of the PSP2 program, sufficiently address its future liquidity needs, yet anticipates it may implement further discretionary changes and other cost reduction and liquidity preservation and/or enhancement measures, as needed, to address the volatility and changing dynamics of passenger demand and the impact of revenue changes, regulatory and public health directives and prevailing government policy and financial market conditions. Workforce Actions In July 2020, the Company distributed a letter to employees, including approximately 2,500 U.S.-based union represented employees, regarding the possibility of a workforce reduction at their work location. Throughout the second and third quarters of 2020, the Company worked with unionized employees and the related unions to create voluntary leave programs for pilots, flight attendants and other unionized employee groups. The Company also created voluntary leave programs for certain non-unionized employee groups. In August 2020, the Company announced a voluntary separation program for non-unionized employees. Due to the high level of support and acceptance of the voluntary programs offered, no unionized employees were involuntarily furloughed and the total number of non-unionized employees involuntarily separated as of October 1, 2020 was reduced by more than 95%. In the year ended December 31, 2020 , the Company recorded $2.5 million in expenses related to the voluntary and involuntary employee separations. These expenses were recorded within special credits on the Company’s consolidated statement of operations. Expenses related to voluntary leave programs were recorded within salaries, wages and benefits on the Company’s consolidated statement of operations. With the Company's expected participation in the PSP2 program, the Company will comply with any related restrictions and limitations on any workforce actions.

Recent Accounting Developments

Recent Accounting Developments	12 Months Ended
	Dec. 31, 2020
Accounting Standards Update and Change in Accounting Principle [Abstract]
Recent Accounting Developments	Recent Accounting Developments Recently Adopted Accounting Pronouncements 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. For accounts receivables, aircraft maintenance deposits and security deposits (recorded within other long-term assets on the Company's consolidated balance sheets) the Company is required to estimate lifetime expected credit losses. 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. As such, the Company is required to recognize an allowance for credit losses for its short-term available-for-sale investment securities, with the exception of U.S. Treasury securities which do not require an allowance for credit losses. The Company adopted this standard effective January 1, 2020. In connection with the adoption of this standard, the Company recognized a cumulative effect adjustment, net of tax, of $1.6 million to retained earnings on the Company's consolidated balance sheets with corresponding reserves against certain of our outstanding financial instruments. These amounts were not material to the Company's consolidated financial statements individually or in the aggregate. 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. The Company adopted this standard effective January 1, 2020 and is applying the standard prospectively to all implementation costs incurred after the date of adoption. This adoption has not had a material impact on the Company's consolidated financial statement presentation or results. Recently Issued Accounting Pronouncements Not Yet Adopted Convertible Instruments and Contracts In August 2020, the FASB issued ASU No. 2020-06, "Accounting for Convertible Instruments and Contracts in an Entity's Own Equity." This new standard simplifies and adds disclosure requirements for the accounting and measurement of convertible instruments. It eliminates the treasury stock method for convertible instruments and requires application of the “if-converted” method for certain agreements. This standard is effective for the Company for fiscal years, and interim periods within those years, beginning January 1, 2022. Early adoption is permitted, but no earlier than fiscal years beginning January 1,

Revenue Disaggregation

Revenue Disaggregation	12 Months Ended
	Dec. 31, 2020
Revenue from Contract with Customer [Abstract]
Revenue Disaggregation	Revenue Disaggregation Operating revenues is comprised of passenger revenues, which includes fare and non-fare revenues, and other revenues. The following table shows disaggregated operating revenues for the twelve months ended December 31, 2020, 2019 and 2018. Twelve Months Ended December 31, 2020 2019 2018 (in thousands) Operating revenues: Fare $ 756,225 $ 1,886,855 $ 1,704,107 Non-fare 1,009,308 1,870,750 1,555,908 Total passenger revenues 1,765,533 3,757,605 3,260,015 Other 44,489 72,931 63,019 Total operating revenues $ 1,810,022 $ 3,830,536 $ 3,323,034 The Company defers the amount for award travel obligation as part of loyalty deferred revenue within air traffic liability ("ATL") on the Company's consolidated balance sheets and recognizes loyalty travel awards in passenger revenues as the mileage credits are used for travel or expire unused. As a result of the COVID-19 pandemic, the Company experienced significantly increased customer requests for credit shells, or customer travel funds held by the Company that can be redeemed for future travel, and refunds beginning in the second half of March 2020 and continuing to varying degrees through the remainder of the year primarily due to flight cancellations and a change in the Company's flight cancellation and refund policy. The total value of refunds issued during the twelve months ended December 31, 2020 was $183.6 million. The Company expects that the level of requests for credit shells and refunds will continue to fluctuate and vary as the effects of COVID-19 continue to develop. In addition, in response to COVID-19, the Company increased the expiration period on some of its credit shells from 60 days to up to 12 months and waived change and cancellation fees for the Guests who booked travel to occur by February 28, 2021. As a result, the outstanding balance of the unused credit shells (which is recorded within ATL on the Company's consolidated balance sheets), as of December 31, 2020, significantly exceeds the balance in the prior year period. As of December 31, 2020 and December 31, 2019, the Company had ATL balances of $402.0 million and $315.4 million, respectively. Substantially all of the Company's ATL, including the balance of credit shells, is expected to be recognized within 12 months of the respective balance sheet date. Refer to Note 2, Impact of COVID-19, for further information on COVID-19's impact to the Company. For credit shells that the Company estimates are not likely to be used prior to expiration (“breakage”), the Company recognizes the associated value proportionally during the period over which the remaining credit shells may be used. Breakage estimates are based on the Company's historical information about customer behavior as well as assumptions about customers' future travel behavior. Assumptions used to generate breakage estimates can be impacted by several factors including, but not limited to, changes to the Company's ticketing policies, changes to the Company’s refund, exchange, and credit shell policies, and economic factors. Given the unprecedented amount of cancellations in the current year and the related increase in credit shells provided, the Company expects additional variability in the amount of breakage revenue recorded in future periods, as the estimates of the portion of those funds that will expire unused may differ from historical experience. Loyalty Programs The Company operates the $9 Fare Club TM which is a subscription-based loyalty program that allows members access to unpublished, extra-low fares as well as discounted prices on bags, exclusive offers on hotels, rental cars and other travel necessities. The Company also operates the Free Spirit loyalty program (the “Free Spirit Program”), which attracts members and partners and builds customer loyalty for the Company by offering a variety of awards, benefits and services. Free Spirit Program members earn and accrue miles for taking our flights and services from non-air partners such as retail merchants, hotels or car rental companies or by making purchases with credit cards issued by partner banks and financial services providers. Miles earned and accrued by Free Spirit Program members can be redeemed for travel awards such as free (other than taxes and government-imposed fees), discounted or upgraded travel. The Company launched a more expansive Free Spirit Program with extended mileage expiration, additional benefits based on status tiers, and other changes in January 2021. The new program terms will require updated estimates of the allocation of future revenues to the performance obligations. Refer to Note 1, Summary of Significant Accounting Policies, for further information. In addition, starting in January 2021, the benefits of the $9 Fare Club TM , now known as the Spirit Saver$ Club TM , were expanded to include discounts on seats, shortcut boarding and security, and "Flight Flex" flight modification product.

Special Charges and Credits

Special Charges and Credits	12 Months Ended
	Dec. 31, 2020
Special Charges and Credits [Abstract]
Special Charges and Credits	Special Charges and Credits Special Charges and Credits, Operating During the twelve months ended December 31, 2020, the Company recorded a $266.8 million credit, net of the related costs, within special charges (credits) on the Company’s consolidated statements of operations related to the grant component of the PSP with the Treasury. These funds were used exclusively to pay for salaries, wages and benefits for the Company's Team Members through September 30, 2020. In addition, during the twelve months ended December 31, 2020, the Company recorded a credit of $38.5 million related to the CARES Act Employee Retention credit within special charges (credits) on the Company’s consolidated statements of operation. These special credits were partially offset by $2.5 million in special charges recorded in the third and fourth quarters of 2020 related to the Company's voluntary and involuntary employee separation programs. Refer to Note 2, Impact of COVID-19, for further information on the CARES Act and the Company 's workforce actions. During the twelve months ended December 31, 2019, the Company recorded $0.7 million within special charges (credits) on the Company's consolidated statement of operations related to the write-off of aircraft related credits resulting from the exchange of credits negotiated under the new purchase agreement with Airbus S.A.S. ("Airbus") executed during the fourth quarter of 2019. For additional information on the new purchase agreement with Airbus, refer to Note 18, Commitments and Contingencies. During the twelve months ended December 31, 2018, the Company negotiated and amended the collective bargaining agreement with the Air Line Pilots Association, International ("ALPA"), under the guidance of the National Mediation Board ("NMB"). In connection with the new agreement, the Company incurred a one-time ratification incentive of $80.2 million, including payroll taxes, and an $8.5 million adjustment related to other contractual provisions. As a result, the Company recorded $88.7 million within special charges (credits) on the Company's consolidated statement of operations for the twelve months ended December 31, 2018. Special Charges, Non-Operating During the twelve months ended December 31, 2020 and December 31, 2019, the Company had no special charges, non-operating within other (income) expense in the consolidated statement of operations.

Loss on Disposal of Assets

Loss on Disposal of Assets	12 Months Ended
	Dec. 31, 2020
Discontinued Operations and Disposal Groups [Abstract]
Loss on Disposal of Assets	Loss on Disposal of Assets During the twelve months ended December 31, 2020, the Company recorded $2.3 million in loss on disposal of assets in the consolidated statement of operations. This loss on disposal of assets mainly consists of $1.5 million related to the write-off of certain unrecoverable costs previously capitalized with a project to upgrade the Company's enterprise accounting software which was subsequently suspended and $0.8 million related to the disposal of excess and obsolete inventory. During the twelve months ended December 31, 2019, the Company recorded $17.4 million in loss on disposal of assets in the statement of operations. This loss on disposal of assets consisted of $13.4 million related to the disposal of excess and obsolete inventory, $3.1 million related to the write-down of certain held-for-sale assets to fair value less cost to sell and $2.4 million related to the write-off of certain unrecoverable costs previously capitalized with a project to upgrade the Company's enterprise accounting software which was suspended as the Company pursued alternative solutions. Refer to Note 19, Fair Value Measurements for information regarding the Company's held-for-sale assets. These losses on disposal were partially offset by a $1.5 million gain on sale-leaseback transactions for 6 aircraft delivered during the twelve months ended December 31, 2019. Refer to Note 15, Leases and Prepaid Maintenance Deposits for information regarding the Company's accounting policy on sale-leaseback transactions. During the twelve months ended December 31, 2018, the Company recorded $9.6 million in loss on disposal of assets in the consolidated statement of operations. During the twelve months ended December 31, 2018, the Company sold 6 used engines for $11.4 million at a loss of $5.2 million. In addition, the Company wrote off $4.4 million related to the disposal of excess and obsolete inventory.

Letters of Credit

Letters of Credit	12 Months Ended
	Dec. 31, 2020
Financial Instruments Pledged as Collateral [Abstract]
Letters of Credit	Letters of Credit As of December 31, 2020, the Company had a $30.0 million standby letter of credit secured by restricted cash, of which $23.6 million had been drawn upon for issued letters of credit. As of December 31, 2019, the Company had a $35.0 million unsecured standby letter of credit facility, of which $23.3 million

Credit Card Processing Arrangem

Credit Card Processing Arrangements	12 Months Ended
	Dec. 31, 2020
Credit Card Processing Arrangements [Abstract]
Credit Card Processing Arrangements	Credit Card Processing Arrangements The Company has agreements with organizations that process credit card transactions arising from the purchase of air travel, baggage charges and other ancillary services by customers. As it is standard in the airline industry, the Company's contractual arrangements with credit card processors permit them, under certain circumstances, to retain a holdback or other collateral, which the Company records as restricted cash, when future air travel and other future services are purchased via credit card transactions. The required holdback is the percentage of the Company's overall credit card sales that its credit card processors hold to cover refunds to customers if the Company fails to fulfill its flight obligations. The Company's credit card processors do not require the Company to maintain cash collateral provided that the Company satisfies certain liquidity and other financial covenants. Failure to meet these covenants would provide the processors the right to place a holdback, resulting in a commensurate reduction of unrestricted cash. As of December 31, 2020 and 2019, the Company was in compliance with such liquidity and other financial covenants in its credit card processing agreements, and the processors were holding back no remittances. The maximum potential exposure to cash holdbacks by the Company's credit card processors, based upon advance ticket sales and $9 Fare Club TM memberships as of December 31, 2020 and 2019, was $423.7 million and $342.3 million, respectively.

Short-term Investment Securitie

Short-term Investment Securities	12 Months Ended
	Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Short-term Investment Securities	Short-term Investment Securities The Company's short-term investment securities are classified as available-for-sale and generally consist of U.S. Treasury and U.S. government agency securities with contractual maturities of twelve months or less. These securities are stated at fair value within current assets on the Company's consolidated balance sheet. Realized gains and losses on sales of investments, if any, are reflected in non-operating income (expense) in the consolidated statements of operations. Unrealized gains and losses on investment securities are reflected as a component of accumulated other comprehensive income, ("AOCI"). As of December 31, 2020 and December 31, 2019, the Company had $106.3 million and $105.3 million in short-term available-for-sale investment securities, respectively. During the twelve months ended December 31, 2020, 2019 and 2018, these investments earned interest income at a weighted-average fixed rate of approximately 1.1%, 2.3% and 1.6% respectively. For the twelve months ended December 31, 2020 and December 31, 2019, an unrealized loss of $73 thousand and an unrealized gain of $104 thousand, net of deferred taxes of $21 thousand and $31 thousand, respectively, were recorded within AOCI related to these investment securities. For the twelve months ended December 31, 2020 and 2019, a realized gain of $3 thousand and $5 thousand were recorded within non-operating income (expense) in the consolidated statements of operations. For the twelve months ended December 31, 2018, the Company did not recognize any realized gains or losses related to these securities as the Company did not transact any sales of these securities during these periods. As of December 31, 2020 and December 31, 2019, $31 thousand and $104 thousand, net of tax, respectively, remained in AOCI, related to these instruments.

Accrued Liabilities

Accrued Liabilities	12 Months Ended
	Dec. 31, 2020
Payables and Accruals [Abstract]
Accrued Liabilities	Accrued Liabilities Accrued liabilities included in other current liabilities as of December 31, 2020 and 2019 consist of the following: As of December 31, 2020 2019 (in thousands) Salaries, wages and benefits $ 112,838 $ 89,163 Airport obligations 68,677 80,134 Aircraft and facility lease obligations 67,374 20,656 Interest payable 37,202 16,941 Federal excise and other passenger taxes and fees payable 36,884 65,312 Aircraft maintenance 27,466 38,099 Fuel 11,704 28,510 Other 31,469 34,706 Other current liabilities $ 393,614 $ 373,521

Common Stock and Preferred Stoc

Common Stock and Preferred Stock	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Common Stock and Preferred Stock	Common Stock and Preferred Stock The Company’s amended and restated certificate of incorporation dated June 1, 2011, authorizes the Company to issue up to 240,000,000 shares of common stock, $0.0001 par value per share, 50,000,000 shares of non-voting common stock, $0.0001 par value per share and 10,000,000 shares of preferred stock, $0.0001 par value per share. All of the Company’s issued and outstanding shares of common stock and preferred stock, if any, are duly authorized, validly issued, fully paid and non-assessable. The Company’s shares of common stock and non-voting common stock are not redeemable and do not have preemptive rights. Common Stock Dividend Rights . Holders of the Company’s common stock are entitled to receive dividends, if any, as may be declared from time to time by the Company’s board of directors out of legally available funds ratably with shares of the Company’s non-voting common stock, subject to preferences that may be applicable to any then outstanding preferred stock and limitations under Delaware law. Voting Rights . Each holder of the Company’s common stock is entitled to one vote for each share on all matters submitted to a vote of the stockholders, including the election of directors. The Company’s stockholders do not have cumulative voting rights in the election of directors. Accordingly, holders of a majority of the voting shares are able to elect all of the directors properly up for election at any given stockholders’ meeting. Liquidation . In the event of the Company’s liquidation, dissolution or winding up, holders of the Company's common stock will be entitled to share ratably with shares of the Company’s non-voting common stock in the net assets legally available for distribution to stockholders after the payment of all of the Company’s debts and other liabilities and the satisfaction of any liquidation preference granted to the holders of any then outstanding shares of preferred stock. Rights and Preferences . Other than under the Rights Agreement (as defined below), holders of the Company’s common stock have no preemptive, conversion, subscription or other rights and there are no redemption or sinking fund provisions applicable to the Company’s common stock. The rights, preferences and privileges of the holders of the Company’s common stock are subject to and may be adversely affected by, the rights of the holders of shares of any series of the Company’s preferred stock that the Company may designate in the future. Non-Voting Common Stock Dividend Rights. Holders of the Company’s non-voting common stock are entitled to receive dividends, if any, as may be declared from time to time by the Company’s board of directors out of legally available funds ratably with shares of the Company’s common stock, subject to preferences that may be applicable to any then outstanding preferred stock and limitations under Delaware law. Voting Rights. Shares of the Company’s non-voting common stock are not entitled to vote on any matters submitted to a vote of the stockholders, including the election of directors, except to the extent required under Delaware law. Conversion Rights . Shares of the Company’s non-voting common stock will be convertible on a share-for-share basis into common stock at the election of the holder subject to the Company remaining in compliance with applicable foreign ownership limitations. Liquidation. In the event of the Company’s liquidation, dissolution or winding up, holders of the Company’s non-voting common stock will be entitled to share ratably with shares of the Company’s common stock in the net assets legally available for distribution to stockholders after the payment of all of the Company’s debts and other liabilities and the satisfaction of any liquidation preference granted to the holders of any then outstanding shares of preferred stock. Rights and Preferences. Other than under the Rights Agreement (as defined below), holders of the Company’s non-voting common stock have no preemptive, subscription or other rights, and there are no redemption or sinking fund provisions applicable to the Company’s non-voting common stock. The rights, preferences and privileges of the holders of the Company’s non-voting common stock are subject to and may be adversely affected by, the rights of the holders of shares of any series of the Company’s preferred stock that the Company may designate in the future. As of December 31, 2020 and 2019, there were no shares of non-voting common stock outstanding. Preferred Stock The Company’s Board of Directors has the authority, without further action by the Company’s stockholders, to issue up to 10,000,000 shares of preferred stock in one or more series and to fix the rights, preferences, privileges and restrictions thereof. These rights, preferences and privileges could include dividend rights, conversion rights, voting rights, terms of redemption, liquidation preferences, sinking fund terms and the number of shares constituting any series or the designation of such series, any or all of which may be greater than the rights of common stock. The Company’s issuance of preferred stock could adversely affect the voting power of holders of common stock and the likelihood that such holders will receive dividend payments and payments upon liquidation. In addition, the issuance of preferred stock could have the effect of delaying, deferring or preventing a change of control of the Company or other corporate action. As of December 31, 2020 and 2019, there were no shares of preferred stock outstanding. Series A Preferred Stock Purchase Rights On March 29, 2020, the Board of Directors of the Company declared a dividend of one preferred stock purchase right (a “Right”) for each outstanding share of common stock of the Company. The dividend was paid on April 9, 2020 (the “Record Date”) to holders of record as of the close of business on that date. Pursuant to a Rights Agreement (the "Rights Agreement") between the Company and Equiniti Trust Company, as Rights Agent. The Rights will initially trade with, and will be inseparable from, the Company's common stock, and the registered holders of the Company's common stock will be deemed to be the registered holders of the Rights. In addition, each share issued upon conversion of the convertible notes and any shares of common stock issued through March 29, 2021 will have such Right. The Board of Directors has adopted the Rights Agreement to reduce the likelihood that a potential acquirer would gain (or seek to influence or change) control of the Company by open market accumulation or other tactics without paying an appropriate premium for the Company’s shares. In general terms and subject to certain exceptions, it works by imposing a significant penalty upon any person or group (including a group of persons that are acting in concert with each other) that acquires 10% or more of the outstanding common stock of the Company without the approval of the Board of Directors. The Rights will not be exercisable until after the Distribution Date (as defined below). After the Distribution Date, each Right will be exercisable to purchase, for $60.00 (the “Purchase Price”), one one-thousandth of a share of Series A Participating Cumulative Preferred Stock, par value $0.0001 per share (the “Preferred Stock”). This portion of a share of Preferred Stock will give the stockholder approximately the same dividend, voting or liquidation rights as would one share of the Company’s common stock. Prior to exercise, Rights holders in their capacity as such have no rights as a stockholder of the Company, including the right to vote and to receive dividends. The Rights will expire on March 29, 2021, unless earlier exercised, exchanged, amended or redeemed. The Board of Directors may redeem all of the Rights at a price of $0.001 per Right at any time before any person has become an Acquiring Person. If the Board of Directors redeems any Rights, it must redeem all of the Rights. Once the Rights are redeemed, the only right of the holders of Rights will be to receive the redemption price per Right. The redemption price will be subject to adjustment. The “Distribution Date” generally means the earlier of: • the close of business on the 10 th business day after the date of the first public announcement that a person or any of its affiliates and associates has become an “Acquiring Person,” as defined below, and • the close of business on the 10 th business day (or such later day as may be designated by the Board of Directors before any person has become an Acquiring Person) after the date of the commencement of a tender or exchange offer by any person which would, if consummated, result in such person becoming an Acquiring Person. An “Acquiring Person” generally means any person who or which, together with all affiliates and associates of such person obtains beneficial ownership of 10% or more of shares of the Company’s common stock, with certain exceptions, including that an Acquiring Person does not include the Company, any subsidiary of the Company, any employee benefit plan of the Company or any subsidiary of the Company, any entity or trustee holding the Company’s common stock for or pursuant to the terms of any such plan or for the purpose of funding any such plan or other benefits for employees of the Company or of any subsidiary of the Company or any passive investor. A passive investor generally means any person beneficially owning shares of the Company’s common stock without a plan or an intent to seek control of or influence the Company. The Rights Agreement also provides that any person that would otherwise be deemed an Acquiring Person as of the date of the adoption of the Rights Agreement will be exempted but only for so long as it does not acquire, without the prior approval of the Board, beneficial ownership of any additional common stock of the Company following the adoption of the Rights Agreement. The value of one one-thousandth interest in a share of Preferred Stock should approximate the value of one share of Common Stock, subject to adjustment. Each one one-thousandth of a share of Preferred Stock, if issued: • will not be redeemable, • will entitle holders to quarterly dividend payments of $0.01 per share, or an amount equal to the dividend paid on one share of Common Stock, whichever is greater, • will entitle holders upon liquidation either to receive $1.00 per share or an amount equal to the payment made on one share of Common Stock, whichever is greater, • will have the same voting power as one share of Common Stock, • if shares of the Common Stock are exchanged via merger, consolidation, or a similar transaction, will entitle holders to a per share payment equal to the payment made on one share of Common Stock. Consequences of a Person or Group Becoming an Acquiring Person Flip in. Subject to the Company’s exchange rights, described below, at any time after any person has become an Acquiring Person, each holder of a Right (other than an Acquiring Person, its affiliates and associates) will be entitled to purchase for each Right held, at the Purchase Price, a number of shares of the Company’s common stock having a market value of twice the Purchase Price. Exchange. At any time on or after any person has become an Acquiring Person (but before any person becomes the beneficial owner of 50% or more of the outstanding shares of the Company's common stock or the occurrence of any of the events described in the next paragraph), the Board of Directors may exchange all or part of the Rights (other than Rights beneficially owned by an Acquiring Person, its affiliates and associates) for shares of the Company’s common stock at an exchange ratio of one share of the Company’s common stock per Right. Flip over. If, after any person has become an Acquiring Person, (1) the Company is involved in a merger or other business combination in which the Company is not the surviving corporation or its common stock is exchanged for other securities or assets or (2) the Company and/or one or more of its subsidiaries sell or otherwise transfer assets or earning power aggregating more than 50% of the assets or earning power of the Company and its subsidiaries, taken as a whole, then each Right (other than Rights beneficially owned by an Acquiring Person, its affiliates and associates) will entitle the holder to purchase for each Right held, for the Purchase Price, a number of shares of common stock of the other party to such business combination or sale (or in certain circumstances, an affiliate) having a market value of twice the Purchase Price. Warrants In connection with the Company's participation in the PSP agreement with the Treasury, the Company issued to the Treasury warrants pursuant to a warrant agreement to purchase up to 520,797 shares of the Company's common stock at a strike price of $14.08 per share (the closing price for the shares of the Company's common stock on April 9, 2020). The warrant agreement sets out the Company’s obligations to issue warrants in connection with disbursements under the PSP and to file a resale shelf registration statement for the warrants and the underlying shares of common stock; prospectus supplements for which were filed on September 30, 2020 and October 8, 2020. The Company has also granted the Treasury certain demand and piggyback registration rights with respect to the warrants and the underlying common stock. The warrants include adjustments for below market issuances, payment of dividends and other customary anti-dilution provisions. The warrants are transferable and have no voting rights. The warrants expire in five years from the date of issuance and at the Company's option, may be settled on a "net cash" or "net shares" basis. Refer to Note 2, Impact of COVID-19, for further information on the PSP agreement with Treasury. The 520,797 warrants issued in connection with the PSP agreement represent less than 1% of the outstanding shares of the Company's common stock as of December 31, 2020. The Company concluded that the PSP warrants agreement is a derivative contract classified within equity, at fair value upon issuance, within the Company’s consolidated balance sheet. Equity-classified contracts are initially measured at fair value and subsequent changes in fair value are not recognized as long as the contract continues to be classified in equity. As of December 31, 2020 , the Company had recorded $3.9 million, net of issuance costs, in APIC related to the fair value of the warrants issued. In connection with its participation in the PSP2 program, the Company expects to issue to the Treasury warrants to purchase approximately 103,761 shares of the Company's common stock. Refer to Note 2, Impact of COVID for further information. Common Stock Offering On May 12, 2020, the Company completed the public offering of 20,125,000 shares of its voting common stock, which includes full exercise of the underwriters’ option to purchase an additional 2,625,000 shares of common stock, at a public offering price of $10.00 per share (the “Common Stock Offering”). In connection with the common stock offering, the Company received proceeds of $192.4 million , net of $8.9 million in related issuance costs. The Company recorded $2.0 thousand of common stock at par and the excess proceeds within APIC. At-the-Market Offering Program On July 22, 2020, the Company entered into an equity distribution agreement relating to the issuance and sale from time to time by the Company of up to 9,000,000 shares of the Company's common stock in sales deemed to be "at-the-market offerings" as defined in Rule 415 under the Securities Act. As of December 31, 2020 , the Company had completed the sale of all 9,000,000 shares under its ATM program and had received proceeds of $156.7 million , net of $5.0 million in related issuance costs. The Company recorded $900 of common stock at par and the excess proceeds, net of related issuance costs, within APIC on the Company's consolidated balance sheets.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2020
Share-based Payment Arrangement [Abstract]
Stock-Based Compensation	Stock-Based CompensationThe Company has stock plans under which directors, officers, key employees and consultants of the Company may be granted restricted stock awards, stock options, performance share awards and other equity-based instruments as a means of promoting the Company’s long-term growth and profitability. The plans are intended to encourage participants to contribute to, and participate in the success of the Company. On December 16, 2014, the Company's Board of Directors approved the 2015 Incentive Award Plan, or 2015 Plan, which was subsequently approved by the Company's stockholders on June 16, 2015. As of December 31, 2020 and December 31, 2019, 1,618,417 and 1,897,809 shares of the Company’s common stock, respectively, remained available for future issuance under the 2015 Plan. Stock-based compensation cost amounted to $11.6 million, $8.2 million and $11.0 million for 2020, 2019 and 2018, respectively. During 2020, 2019 and 2018 there was a $3.6 million, $1.9 million and $2.6 million tax benefit recognized in income related to stock-based compensation. Restricted Stock and Restricted Stock Units Restricted stock and restricted stock unit awards are valued at the fair value of the shares on the date of grant. Generally, granted shares and units vest over a three four A summary of the status of the Company’s restricted stock shares (restricted stock awards and restricted stock unit awards) as of December 31, 2020 and changes during the year ended December 31, 2020 is presented below: Number of Shares Weighted-Average Outstanding at December 31, 2019 332,966 49.84 Granted 222,401 35.48 Vested (146,096) 50.31 Forfeited (8,795) 47.86 Outstanding at December 31, 2020 400,476 41.74 There were 222,401 and 148,120 restricted stock shares granted during the years ended December 31, 2020 and December 31, 2019, respectively. As of December 31, 2020 and December 31, 2019, there was $10.4 million and $10.0 million, respectively, of total unrecognized compensation cost related to nonvested restricted stock to be recognized over 2.0 years and 2.7 years, respectively. The weighted-average fair value of restricted stock granted during the years ended December 31, 2020, 2019 and 2018 was $35.48, $53.41 and $46.90, respectively. The total fair value of restricted stock shares vested during the years ended December 31, 2020, 2019 and 2018 was $5.5 million, $5.4 million and $6.5 million, respectively. Performance Share Awards The Company grants certain senior-level executives performance stock units that vest based on either market and time-based service conditions or performance and time-based service conditions as part of a long-term incentive plan, which are referred to herein as performance share awards. The number of shares of common stock underlying each award is determined at the end of a three-year performance period. In order to vest, the senior level executive must still be employed by the Company, with certain contractual exclusions, at the end of the performance period. Depending on the type of performance stock unit, at the end of the performance period, the percentage of the stock units that will vest will be determined by ranking the Company’s total shareholder return compared to the total shareholder return of the peer companies identified in the plan or by ranking the Company's adjusted operating margin percentage compared to the adjusted operating margin percentage of the peer company's identified in the plan. Based on the level of performance, between 0% and 200% of the award may vest. Within 60 days after vesting, the shares underlying the award will be issued to the participant. In the event of a change in control of the Company or the death or permanent disability of a participant, the payout of any award is limited to a pro-rated portion of such award based upon a performance assessment prior to the change-in-control date or date of death or permanent disability. The grant date fair value of the performance share awards based on total shareholder return (market condition) is determined through the use of a Monte Carlo simulation model. The market condition requirements are reflected in the grant date fair value of the award, and the compensation expense, net of forfeitures, for the award is recognized assuming that the requisite service is rendered regardless of whether the market conditions are achieved. Compensation expense is recognized on a straight-line basis over the requisite service period. The Monte Carlo simulation model used for valuation of these awards utilizes multiple input variables that determine the probability of satisfying the market condition requirements applicable to each award. The inputs utilized for the performance share awards based on total shareholder return are as follows: Weighted-Average at Grant Date for Twelve Months Ended December 31, 2020 Weighted-Average at Grant Date for Twelve Months Ended December 31, 2019 Expected volatility factor 0.40 0.38 Risk free interest rate 1.58 % 2.50 % Expected term (in years) 2.96 2.97 Expected dividend yield — % — % For grants awarded in 2020, 2019 and 2018, the volatility was based upon a weighted average historical volatility for the Company. The Company chose to use historical volatility to value these awards because historical prices were used to develop the correlation coefficients between the Company and each of the peer companies within the peer group in order to model stock price movements. The volatilities used were calculated as the remaining term of the performance period at the date of grant. The risk-free interest rate was based on the implied yield available on U.S. Treasury zero-coupon issues with remaining terms equivalent to the remaining performance period. The Company does not intend to pay dividends on its common stock in the foreseeable future. Accordingly, the Company used a dividend yield of zero in its model. The following table summarizes the Company’s market condition performance share awards for the year ended December 31, 2020: Number of Awards Weighted-Average Fair Value at Grant Date ($) Outstanding at December 31, 2019 96,159 61.58 Granted 36,328 47.43 Vested (47,240) 52.07 Forfeited (336) 47.43 Outstanding at December 31, 2020 84,911 60.88 The grant date fair value of the performance share awards based on operating margin (performance condition) is based on grant date stock price, in accordance with the valuation of performance conditions applicable to this award type. The probability of payout for these awards is evaluated at each report date and adjustments are made to stock-based compensation expense based on the number of shares deemed probable of issuance upon vesting. The following table summarizes the Company’s performance condition performance share awards for the year ended December 31, 2020: Number of Awards Weighted-Average Fair Value at Grant Date ($) Outstanding at December 31, 2019 47,794 53.24 Granted 72,593 40.11 Vested (23,620) 46.21 Forfeited (669) 40.11 Outstanding at December 31, 2020 96,098 45.14 As of December 31, 2020 and 2019, there was $4.8 million and $4.4 million, respectively, of total unrecognized compensation cost related to performance share awards expected to be recognized over 1.66 years and 1.73 years, respectively. Stock Appreciation Rights During 2018, the Company issued stock appreciation awards to certain senior-level executives. These awards had a four-year service requisite period from January 1, 2018 through December 31, 2021 and a two-year performance period from January 1, 2018 through December 31, 2019. These market-condition performance awards were based on the appreciation of the Company's stock price over the two-year performance period. Issuance of the award on January 1, 2018 represented a right to receive shares of the Company's common stock upon achievement of certain performance goals by the grant date of December 31, 2019. As performance goals stipulated by the award were not achieved, these shares were not granted on December 31, 2019. During the twelve months ended December 31, 2019 and 2018, the Company recognized $0.7 million and $1.2 million of stock-based compensation cost related to the stock appreciation awards issued during 2018, respectively. On December 31, 2019, the Company reversed the total expense of $1.9 million related to these awards as these awards were not granted. No further expense was recognized related to these awards. Treasury Stock During the year ended December 31, 2020, 2019 and 2018, the Company repurchased 44 thousand, 91 thousand and 28 thousand shares, respectively, for $1.7 million, $5.4 million and $1.2 million, respectively. Repurchases made during the twelve months ended December 31, 2020, 2019 and 2018 include repurchases made from employees who received restricted stock. During the year ended December 31, 2020, 2019 and 2018, the Company did not retire any treasury shares.

Earnings (Loss) per Share

Earnings (Loss) per Share	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
Earnings per Share	Earnings (Loss) per Share The following table sets forth the computation of basic and diluted earnings per common share: Year Ended December 31, 2020 2019 2018 (in thousands, except per-share amounts) Numerator: Net income (loss) $ (428,700) $ 335,255 $ 155,749 Denominator: Weighted-average shares outstanding, basic 84,692 68,429 68,249 Effect of dilutive stock awards — 130 182 Adjusted weighted-average shares outstanding, diluted 84,692 68,559 68,431 Earnings (Loss) per Share: Basic earnings (loss) per common share $ (5.06) $ 4.90 $ 2.28 Diluted earnings (loss) per common share $ (5.06) $ 4.89 $ 2.28 Anti-dilutive weighted-average shares 441 143 145

Debt and Other Obligations

Debt and Other Obligations	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
Debt and Other Obligations	Debt and Other Obligations Long-term debt As of December 31, 2020, the Company had outstanding public and non-public debt instruments. During 2020, the Company acquired additional debt through fixed-rate secured and unsecured term loans, secured a new revolving credit facility and issued convertible notes and secured notes described below. Fixed-rate term loans During 2020, the Company acquired additional debt under facility agreements, which as of December 31, 2020 provided $333.0 million of debt financing for 8 Airbus A320 aircraft delivered during 2020. Each loan extended under the facility agreements was funded on or near the delivery date of each aircraft and is secured by a first-priority security interest on the individual aircraft. Each loan has a term life of 10 to 12 years and amortizes on a mortgage-style basis, which requires quarterly principal and interest payments. Loans bear interest on a fixed-rate basis with interest rates ranging between 1.90% and 3.32%. As of December 31, 2020, the Company has taken delivery of all 8 Airbus A320 aircraft financed through these facility agreements. Fixed-rate unsecured term loans Pursuant to the Company's PSP agreement with the Treasury, the Company received a total of $334.7 million through July 31, 2020, used exclusively to pay for salaries, wages and benefits for the Company’s Team Members through September 30, 2020. Of that amount, $70.4 million is in the form of a low-interest 10-year unsecured term loan. In September 2020, the Company was notified by the Treasury of additional funds available under the PSP agreement. The Company received the additional installment of $9.7 million of which $2.9 million is in the form of a low-interest 10-year unsecured term loan. Interest on these loans is payable semi-annually at a rate of 1.0% in years 1 through 5 and a rate of the Secured Overnight Financing Rate plus 2.0% in years 6 through 10. The notes are prepayable at any time, without penalty, at the Company’s option and have principal due at maturity in 2030. The Company has concluded that no terms exist within the contract that would require a short-term classification of the debt instrument within the Company’s consolidated balance sheet at the inception of the loan. Therefore, the debt has been recorded at face value and classified within long-term debt and finance leases in the Company’s consolidated balance sheets. As of December 31, 2020, the Company had recorded $73.3 million in long-term debt on its consolidated balance sheets, related to the PSP. In connection with its participation in the PSP2 program, the Company expects to incur an additional approximately $25 million in fixed-rate unsecured term loans. Revolving credit facility due in 2022 On March 30, 2020, the Company entered into the 2022 revolving credit facility for $110.0 million, with an option to increase the overall commitment amount up to $350 million with the consent of any participating lenders and subject to borrowing base availability. In the second quarter of 2020, the commitment was increased to $180.0 million. As of December 31, 2020 , the Company had fully drawn the $180.0 million available. Any amounts drawn on this facility are included in long-term debt and finance leases, less current maturities on the Company's consolidated balance sheets. The final maturity of the facility is March 30, 2022. The Company may pledge the following types of assets as collateral to secure its obligations under the revolving credit facility: (i) certain take-off and landing rights of the Company at LaGuardia Airport, (ii) certain eligible aircraft spare parts and ground support equipment, (iii) aircraft, spare engines and flight simulators, (v) real property assets and (vi) cash and cash equivalents. The revolving credit facility bears variable interest based on LIBOR, plus a 2.00% margin per annum, or another rate, at the Company's election, based on certain market interest rates, plus a 1.00% margin per annum, in each case with a floor of 0%. The 2022 revolving credit facility requires the Company to maintain (i) so long as any loans or letters of credit are outstanding under the 2022 revolving credit facility, unrestricted cash, cash equivalents, short-term investment securities and unused commitments available under all revolving credit facilities (including the 2022 revolving credit facility) aggregating not less than $400 million, of which no more than $200 million may be derived from unused commitments under the 2022 revolving credit facility, (ii) a minimum ratio of the borrowing base of the collateral described above (determined as the sum of a specified percentage of the appraised value of each type of such collateral) to outstanding obligations under the 2022 revolving credit facility of not less than 1.0 to 1.0 (if the Company does not meet the minimum collateral coverage ratio, it must either provide additional collateral to secure its obligations under the 2022 revolving credit facility or repay the loans under the 2022 revolving credit facility by an amount necessary to maintain compliance with the collateral coverage ratio), and (iii) at any time following the date that is one month after the effective date of the 2022 revolving credit facility, the pledged take-off and landing rights of the Company at LaGuardia Airport and a specified number of spare engines in the collateral described above so long as any loans or letters of credit are outstanding under the 2022 revolving credit facility. Revolving credit facility due in 2021 During the fourth quarter of 2018, the Company entered into a revolving credit facility for up to $160.0 million secured by the collateral assignment of certain of the Company's rights under the purchase agreement with Airbus, related to 43 Airbus A320neo aircraft scheduled to be delivered between August 2019 and December 2021. In June 2020, the Company entered into an agreement to amend the revolving credit facility originally maturing in December 2020. The agreement extended the final maturity date of the revolving credit facility from December 30, 2020 to March 31, 2021. Upon execution of the amended agreement, the maximum borrowing capacity decreased from $160.0 million to $111.2 million. This facility is secured by the collateral assignment of certain of the Company’s rights under the purchase agreement with Airbus. As of December 31, 2020, collateralized amounts were related to 11 Airbus A320neo aircraft scheduled to be delivered between June 2021 and April 2022. The maximum borrowing capacity of $95.1 million, as of December 31, 2020, decreased from $111.2 million due to the delivery of aircraft during the third and fourth quarters of 2020 and will continue to decrease as the Company takes delivery of the related aircraft. As of December 31, 2020, the Company had drawn $95.1 million on the facility, which is included in current maturities of long-term debt and finance leases on the Company's consolidated balance sheets. As of December 31, 2019, the Company had drawn $160.0 million on the facility. The revolving credit facility bears variable interest based on LIBOR. Convertible debt On May 12, 2020, the Company completed the public offering of $175.0 million aggregate principal amount of 4.75% convertible senior notes due 2025. The convertible notes bear interest at the rate of 4.75% per year and will mature on May 15, 2025. Interest on the notes is payable semi-annually in arrears on May 15 and November 15 of each year, beginning on November 15, 2020. To allocate the proceeds of the convertible notes, the Company first determined the fair value of the debt component of the convertible notes based on a similar liability with no conversion feature and utilized a discounted cash flow method whereby the contractual cash flows have been discounted at a risk-adjusted yield reflective of both the time value of money and the credit risk inherent in the convertible notes, as well as certain observable inputs. The Company allocated the remaining proceeds of the convertible notes to the equity component within APIC. The Company will accrete the resulting discount on the debt component through interest expense, using the effective interest method, over the 5-year life of the instrument. The Company received proceeds of $168.3 million as a result of the offering, net of total issuance costs of $6.7 million. The Company recorded $95.6 million in long-term debt and finance leases, net of debt issuance costs of $3.8 million, on its consolidated balance sheets, related to the debt component of the convertible notes, and $72.7 million in APIC, net of issuance costs of $2.9 million, on its consolidated balance sheets, related to the equity component of the convertible notes. As of December 31, 2020, the if-converted value exceeds the principal amount of the convertible notes by $75.2 million, using the average stock price for the twelve months ended December 31, 2020. Noteholders may convert their notes at their option only in the following circumstances: (1) during any calendar quarter commencing after the calendar quarter ending on June 30, 2020 (and only during such calendar quarter), if the last reported sale price per share of the Company’s common stock exceeds 130% of the conversion price for each of at least 20 trading days (whether or not consecutive) during the 30 consecutive trading days ending on, and including, the last trading day of the immediately preceding calendar quarter; (2) during the five five five December 31, 2020, the Company had $109.0 million recorded within c urrent maturities of long-term debt and finance leases on its consolidated balance sheets related to its convertible debt. As of December 31, 2020, the notes may be converted by noteholders through March 31, 2021. No notes were converted during the year ended December 31, 2020. Upon conversion, the Company will pay or deliver, as the case may be, cash, shares of the Company’s common stock or a combination of cash and shares of common stock, at the Company’s election. The initial conversion rate is 78.4314 shares of voting common stock per $1,000 principal amount of convertible notes (equivalent to an initial conversion price of approximately $12.75 per share of common stock). The conversion rate will be subject to adjustment in some events but will not be adjusted for any accrued and unpaid interest. In addition, following certain corporate events that occur prior to the maturity date, the Company will increase the conversion rate for a holder who elects to convert its convertible notes in connection with such a corporate event in certain circumstances. In the event of a “Fundamental Change,” as defined in the indenture governing the convertible notes, the holders may require the Company to purchase for cash all or a portion of their notes at a purchase price equal to the principal amount of the notes, plus accrued and unpaid interest, if any. The Company may not redeem the notes at its option prior to the maturity date. The Company intends to settle conversions in cash up to the principal amount of the convertible notes, with any excess conversion value settled in shares of the Company's common stock. The convertible notes are being accounted for using the treasury stock method for the purposes of net income (loss) per share. Using this method, the denominator will be affected when the average share price of the Company's common stock for a given period is greater than the initial conversion price of approximately $12.75 per share. 8.00% Senior Secured Notes due 2025 On September 17, 2020, the Company completed the private offering by Spirit IP Cayman Ltd., an indirect wholly-owned subsidiary of the Company (the “Brand Issuer”), and Spirit Loyalty Cayman Ltd., an indirect wholly-owned subsidiary of the Company (the “Loyalty Issuer” and, together with the Brand Issuer, the “Issuers”) of an aggregate of $850 million principal amount of 8.00% senior secured notes due 2025. The 8.00% senior secured notes are guaranteed by the Company, HoldCo 1, a direct wholly owned subsidiary of the Company and HoldCo 2, a direct subsidiary of HoldCo 1 and indirect wholly owned subsidiary of the Company. HoldCo 1 and HoldCo 2 are referred to together as the "Cayman Guarantors." The 8.00% senior secured notes will be secured by, among other things, a first priority lien on the core assets of the Company’s loyalty programs (comprised of cash proceeds from its Free Spirit co-branded credit card programs, cash proceeds from its $9 Fare Club TM program membership fees, and certain intellectual property required or necessary to operate the loyalty programs) as well as the Company’s brand intellectual property. Refer to Note 4, Revenue Disaggregation, for further information on the Company's loyalty programs. The 8.00% senior secured notes will mature on September 20, 2025. The 8.00% senior secured notes bear interest at a rate of 8.00% per annum, payable in quarterly installments on January 20, April 20, July 20 and October 20 of each year, beginning January 20, 2021. In the twelve months ended December 31, 2020, the Company received proceeds of $823.9 million, net of issuance costs of $17.4 million and original issue discount of $8.7 million, related to this private offering. The 8.00% senior secured notes will be secured on a senior basis by first-priority security interests in substantially all of the assets of the Issuers, other than excluded property and subject to certain permitted liens. The note guarantee of the Company will be secured by (i) a first-priority security interest in 100% of the equity interests in HoldCo 1, with certain exceptions, and (ii) certain other collateral owned by the Company, including, to the extent permitted by such agreements or otherwise by operation of law, any of the Company’s rights under the Free Spirit Agreements and the IP Agreements (each of which are defined in the indenture governing the 8.00% senior secured notes). The note guarantees of the Cayman Guarantors will be secured by first- priority security interests in substantially all of the assets of the Cayman Guarantors, other than excluded property and subject to certain permitted liens, including pledges of the equity of their respective subsidiaries. The Issuers, at their option, may redeem some or all of the 8.00% senior secured n otes on or after September 20, 2023 at pre-determined redemption prices set forth in the indenture governing the 8.00% senior secured n otes. Prior to September 20, 2023, the Issuers may redeem some or all of the 8.00% senior secured notes at a redemption price equal to 100% of the principal amount of the 8.00% senior secured notes, plus a “make-whole” premium set forth in the indenture governing the 8.00% senior secured n otes. Upon the occurrence of certain mandatory prepayment events and mandatory repurchase offer events, the Issuers will be required to make a prepayment on the 8.00% senior secured notes, or offer to repurchase them, pro rata to the extent of any net cash proceeds received in connection with such events, at a price equal to 100% of the principal amount to be prepaid, plus, in some cases, an applicable premium. In addition, upon a change of control of the Company, the Issuers may be required to make an offer to prepay the 8.00% senior secured n otes at a price equal to 101% of the respective principal amounts thereof, plus accrued and unpaid interest, if any, to, but not including, the purchase date. The indenture governing the 8.00% senior secured notes contains certain covenants that limit the ability of the Issuers, the Cayman Guarantors and, in certain circumstances, the Company to, among other things: (i) make restricted payments; (ii) incur certain additional indebtedness, including with respect to sales of pre-paid miles in excess of $25 million during any fiscal year; (iii) create or incur certain liens on the collateral securing the 8.00% senior secured notes and the guarantees; (iv) merge, consolidate or sell assets; (v) engage in certain business activities; (vi) sell, transfer or otherwise convey the collateral securing the 8.00% senior secured notes and the guarantees; (vii) exit from, terminate or substantially reduce the Free Spirit Program business or modify the terms of the Free Spirit Program, except in certain circumstances; and (viii) terminate, amend, waive, supplement or modify any IP Agreement, except under certain circumstances. The Indenture also requires the Issuers and, in certain circumstances, the Company, to comply with certain affirmative covenants, including depositing the Transaction Revenues (as defined in the indenture governing the 8.00% senior secured notes) in collection accounts, with amounts to be distributed for the payment of fees, principal and interest on the 8.00% senior secured notes pursuant to a payment waterfall described in the indenture, and certain financial reporting requirements. In addition, the Company is required to maintain minimum liquidity at the end of any business day of at least $400 million. Long-term debt is comprised of the following: As of December 31, 2020 December 31, 2019 December 31, 2020 December 31, 2019 (in millions) (weighted-average interest rates) 8.00% senior secured notes due in 2025 $ 850.0 $ — 8.00 % N/A Fixed-rate term loans due through 2032 1,301.9 1,074.3 3.36 % 3.79 % Unsecured term loans due in 2030 73.3 — 1.00 % N/A Fixed-rate class A 2015-1 EETC due through 2028 322.6 348.6 4.10 % 4.10 % Fixed-rate class B 2015-1 EETC due through 2024 64.0 72.0 4.45 % 4.45 % Fixed-rate class C 2015-1 EETC due through 2023 86.6 98.1 4.93 % 4.93 % Fixed-rate class AA 2017-1 EETC due through 2030 214.4 228.4 3.38 % 3.38 % Fixed-rate class A 2017-1 EETC due through 2030 71.5 76.1 3.65 % 3.65 % Fixed-rate class B 2017-1 EETC due through 2026 60.6 70.6 3.80 % 3.80 % Fixed-rate class C 2017-1 EETC due through 2023 85.5 85.5 5.11 % 5.11 % Convertible debt due in 2025 175.0 — 4.75 % N/A Revolving credit facility due in 2021 95.1 160.0 1.55 % 3.12 % Revolving credit facility due in 2022 180.0 — 2.15 % N/A Long-term debt $ 3,580.5 $ 2,213.6 Less current maturities 383.5 214.0 Less unamortized discount, net 131.4 40.4 Total $ 3,065.6 $ 1,959.2 The Company's debt financings entered into solely to finance aircraft acquisition costs are collateralized by first priority security interest in the individual aircraft being financed. During the year ended December 31, 2020 and 2019, the Company made principal payments of $254.3 million and $246.8 million on its outstanding debt obligations, respectively. At December 31, 2020, long-term debt principal payments for the next five years and thereafter are as follows: December 31, 2020 (in millions) 2021 $ 290.0 2022 372.0 2023 335.5 2024 221.0 2025 1,212.2 2026 and beyond 1,149.8 Total debt principal payments $ 3,580.5 Interest Expense Interest expense related to long-term debt and finance leases consists of the following: Year Ended December 31, 2020 2019 (in thousands) 8.00% senior secured notes (1) $ 19,953 $ — Fixed-rate senior term loans 43,591 41,053 Fixed-rate junior term loans — 1,811 Unsecured term loans 409 — Class A 2015-1 EETC 13,730 14,894 Class B 2015-1 EETC 3,027 3,377 Class C 2015-1 EETC 4,565 5,117 Class AA 2017-1 EETC 7,412 7,887 Class A 2017-1 EETC 2,672 2,843 Class B 2017-1 EETC 2,445 2,870 Class C 2017-1 EETC 4,379 4,367 Convertible debt (2) 14,905 — Revolving credit facilities 5,380 5,792 Finance leases 195 408 Commitment and other fees 1,106 2,217 Amortization of deferred financing costs 10,751 8,714 Total $ 134,520 $ 101,350 (1) Includes $0.5 million of accretion and $19.5 million of interest expense for the twelve months ended December 31, 2020. (2) Includes $9.6 million of accretion and $5.3 million of interest expense for the twelve months ended December 31, 2020. As of December 31, 2020 and 2019, the Company had a line of credit for $3.1 million and $33.6 million related to corporate credit cards. Respectively, the Company had drawn $0.6 million and $4.6 million as of December 31, 2020 and 2019, which is included in accounts payable. As of December 31, 2020 and 2019, the Company had lines of credit with counterparties for derivatives and physical fuel delivery in the amount of $41.5 million. As of December 31, 2020 and 2019, the Company had drawn $3.7 million and $25.3 million, respectively, on these lines of credit for physical fuel delivery, which is included within other current liabilities in the Company's consolidated balance sheets. The Company is required to post collateral for any excess above the lines of credit if the fuel derivatives, if any, are in a net liability position and make periodic payments in order to maintain an adequate undrawn

Leases and Aircraft Maintenance

Leases and Aircraft Maintenance Deposits	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Leases and Maintenance Deposits	Leases and Aircraft Maintenance Deposits The Company leases aircraft, engines, airport terminals, maintenance and training facilities, aircraft hangars, commercial real estate and office and computer equipment, among other items. Certain of these leases include provisions for variable lease payments which are based on several factors, including, but not limited to, relative leases square footage, enplaned passengers, and airports' annual operating budgets. Due to the variable nature of the rates, these leases are not recorded on the Company's consolidated balance sheets as a right-of-use asset and lease liability. Lease terms are generally 8 to 18 years for aircraft and up to 99 years for other leased equipment and property. As of December 31, 2020, the Company had a fleet consisting of 157 A320 family aircraft. As of December 31, 2020, the Company had 56 aircraft financed under operating leases with lease term expirations between 2022 and 2038. In addition, the Company owned 101 aircraft of which 29 were purchased off lease and are currently unencumbered. As of December 31, 2020, the Company also had 8 spare engines financed under operating leases with lease term expiration dates ranging from 2023 to 2027 and owned 16 spare engines, all of which as of December 31, 2020, were pledged as collateral under the Company's 2022 revolving credit facility. Total rent expense for all leases charged to operations for the years ended 2020, 2019 and 2018 was $371.6 million, $345.0 million and $312.0 million, respectively. Total rental expense charged to operations for aircraft and engine operating leases for the years ended December 31, 2020, 2019 and 2018 was $196.4 million, $182.6 million and $177.6 million, respectively. Some of the Company’s aircraft and engine master lease agreements provide that the Company pays maintenance reserves to aircraft lessors to be held as collateral in advance of the Company’s required performance of major maintenance activities. Maintenance reserve payments that are substantively and contractually related to the maintenance of the leased asset are accounted for as aircraft maintenance deposits to the extent they are expected to be recoverable. A majority of these maintenance reserve payments are calculated based on a utilization measure, such as flight hours or cycles, while some maintenance reserve payments are fixed, time-based contractual amounts. Fixed maintenance reserve payments that are not probable of being recovered are considered lease payments and are included in the right-of-use asset and lease liability. Maintenance reserve payments that are based on a utilization measure and are not probable of being recovered are considered variable lease payments that are recognized when they are probable of being incurred and are not included in the right-of-use asset and lease liability. These lease agreements generally provide that maintenance reserves are reimbursable to the Company upon completion of the maintenance event. Some of the master lease agreements do not require that the Company pay maintenance reserves so long as the Company's cash balance does not fall below a certain level. As of December 31, 2020, the Company is in full compliance with those requirements and does not anticipate having to pay reserves related to these master leases in the future. Under the terms of the lease agreements, the Company will continue to operate and maintain the aircraft. Payments under the majority of the lease agreements are fixed for the term of the lease. The lease agreements contain standard termination events, including termination upon a breach of the Company's obligations to make rental payments and upon any other material breach of the Company's obligations under the leases, and standard maintenance and return condition provisions. These return provisions are evaluated at inception of the lease and throughout the lease terms and are accounted for as either fixed or variable lease payments (depending on the nature of the lease return condition) when it is probable that such amounts will be incurred. When determining probability and estimated cost of lease return obligations, there are various other factors that need to be considered such as the contractual terms of the lease, the ability to swap engines or other aircraft components, current condition of the aircraft, the age of the aircraft at lease expiration, utilization of engines and other components, the extent of repairs needed at return, return locations, current configuration of the aircraft and cost of repairs and materials at the time of return. Management assesses the factors listed above and the need to accrue lease return costs throughout the lease as facts and circumstances warrant an assessment. As a result of COVID-19, the Company is currently operating its aircraft at lower utilization levels. If the Company continues flying its aircraft at lower utilization levels beyond its current projections, the timing of future maintenance events may change such that the Company will be required to accrue lease return costs and/or record reserves against its maintenance deposits earlier than it would have expected and such amounts could be significant. The Company expects lease return costs and unrecoverable maintenance deposits will increase as individual aircraft lease agreements approach their respective termination dates and the Company begins to accrue the estimated cost of return conditions for the corresponding aircraft. Upon a termination of the lease due to a breach by the Company, the Company would be liable for standard contractual damages, possibly including damages suffered by the lessor in connection with remarketing the aircraft or while the aircraft is not leased to another party. 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. Supplemental rent, recorded within aircraft rent expense, is made up of maintenance reserves paid to aircraft lessors that are not probable of being reimbursed and probable and estimable return condition obligations. The Company expensed $3.3 million, $4.8 million and $3.4 million of supplemental rent recorded within aircraft rent during 2020, 2019 and 2018, respectively. The Company did not expense any paid maintenance reserves as supplemental rent in 2020. During 2019 and 2018, the Company expensed $0.5 million and $1.3 million, respectively, of paid maintenance reserves as supplemental rent. As of December 31, 2020, the Company had $126.3 million of aircraft maintenance deposits ($73.1 million in aircraft maintenance deposits and $53.2 million in long-term aircraft maintenance deposits) on the Company's consolidated balance sheets. During the twelve months ended December 31, 2020, the Company took delivery of eight aircraft under secured debt arrangements, one aircraft under a sale-leaseback transaction and three aircraft under direct operating leases. In addition, the Company purchased two previously leased aircraft. The Company also purchased two new engines and returned one previously leased engine. Prior to the adoption of Topic 842 on January 1, 2019, 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 consolidated statements of operations. On December 31, 2019, the Company entered into an aircraft purchase agreement to acquire two A319 aircraft previously operated by the Company under operating leases. The contract was deemed a lease modification, which resulted in a change of classification from operating leases to finance leases for the two aircraft. The Company recorded a finance lease obligation of $44.1 million calculated as the present value of the remaining lease payments, including the final payment to purchase the aircraft and included within current maturities of long-term debt and finance leases on the Company's consolidated balance sheets as of December 31, 2019. In addition, the Company recorded finance lease assets of $48.4 million which include related amounts previously recorded as maintenance reserves and security deposits and included within flight equipment on the Company's consolidated balance sheets as of December 31, 2019. In January 2020, the purchase of the two aircraft was completed and the aircraft were recorded within flight equipment on the Company's consolidated balance sheets. The remainder of the Company's finance lease obligations relate to the lease of computer equipment used by the Company's flight crew and office equipment. Payments under these finance lease agreements are fixed for terms ranging from 4 to 5 years. Finance lease assets are recorded within property and equipment and the related liabilities are recorded within current maturities of long-term debt and finance leases and long-term debt and finance leases, less current maturities on the Company's consolidated balance sheets. During the fourth quarter of 2019, the Company purchased an 8.5-acre parcel of land for $41.0 million and entered into a 99-year lease agreement for the lease of a 2.6-acre parcel of land, in Dania Beach, Florida, where the Company intends to build a new headquarters campus. In connection with the lease agreement, the Company is expected to build a 200-unit residential building. The 8.5-acre parcel of land is capitalized within ground property and equipment on the Company's consolidated balance sheets. The 99-year lease was determined to be an operating lease and is recorded within operating lease right-of-use asset and operating lease liability on the Company's consolidated balance sheets. Operating lease commitments related to this lease are included in the table below within property facility leases. The following table provides details of the Company's future minimum lease payments under finance lease liabilities and operating lease liabilities recorded on the Company's consolidated balance sheets as of December 31, 2020. The table does not include commitments that are contingent on events or other factors that are currently uncertain and unknown. Finance Leases Operating Leases Total Operating and Finance Lease Obligations Aircraft and Spare Engine Leases Property Facility Leases (in thousands) 2021 (1) $ 753 $ 236,101 $ 4,683 $ 241,537 2022 725 209,911 4,397 215,033 2023 349 197,267 3,383 200,999 2024 98 175,204 2,772 178,074 2025 — 153,146 1,060 154,206 2026 and thereafter — 975,682 143,093 1,118,775 Total minimum lease payments $ 1,925 $ 1,947,311 $ 159,388 $ 2,108,624 Less amount representing interest 137 562,493 134,628 697,258 Present value of minimum lease payments $ 1,788 $ 1,384,818 $ 24,760 $ 1,411,366 Less current portion 671 130,484 3,307 134,462 Long-term portion $ 1,117 $ 1,254,334 $ 21,453 $ 1,276,904 (1) Includes $27.3 million of aircraft and spare engine rent payment deferrals due to COVID-19 which are recorded in other current liabilities within the Company's consolidated balance sheets. Commitments related to the Company's noncancellable short-term operating leases not recorded on the Company's consolidated balance sheets are expected to be $1.4 million for 2021 and none for 2022 and beyond. During 2020, the Company e ntered into agreements to defer payments in 2020 related to facility rents and other airport service contracts at certain locations. Also during 2020, the Company entered into agreements to defer payments related to certain aircraft and engine leases from 2020 into 2021. The Company elected to apply the practical expedient issued by the Financial Accounting Standards Board in April 2020 which allows companies to treat a lease concession related to COVID-19 as though enforceable rights and obligations for the concessions existed regardless of whether those enforceable rights and obligations explicitly exist in the lease agreement. Amounts deferred as of December 31, 2020 are recorded in accrued rent within other current liabilities on the Company's consolidated balance sheet. The table below presents information for lease costs related to the Company's finance and operating leases: Year Ended December 31, 2020 2019 (in thousands) Finance lease cost Amortization of leased assets $ 824 $ 998 Interest of lease liabilities 194 674 Operating lease cost Operating lease cost (1) 201,474 179,959 Short-term lease cost (1) 25,195 5,144 Variable lease cost (1) 125,534 140,417 Total lease cost $ 353,221 $ 327,192 (1) Expenses are classified within aircraft rent and landing fees and other rents on the Company's consolidated statements of operations. The table below presents lease-related terms and discount rates as of December 31, 2020: December 31, 2020 December 31, 2019 Weighted-average remaining lease term Operating leases 12.9 years 13.0 years Finance leases 2.6 years 0.1 years Weighted-average discount rate Operating leases 6.08 % 5.86 % Finance leases 5.54 % 2.46 %
Leases and Maintenance Deposits	Leases and Aircraft Maintenance Deposits The Company leases aircraft, engines, airport terminals, maintenance and training facilities, aircraft hangars, commercial real estate and office and computer equipment, among other items. Certain of these leases include provisions for variable lease payments which are based on several factors, including, but not limited to, relative leases square footage, enplaned passengers, and airports' annual operating budgets. Due to the variable nature of the rates, these leases are not recorded on the Company's consolidated balance sheets as a right-of-use asset and lease liability. Lease terms are generally 8 to 18 years for aircraft and up to 99 years for other leased equipment and property. As of December 31, 2020, the Company had a fleet consisting of 157 A320 family aircraft. As of December 31, 2020, the Company had 56 aircraft financed under operating leases with lease term expirations between 2022 and 2038. In addition, the Company owned 101 aircraft of which 29 were purchased off lease and are currently unencumbered. As of December 31, 2020, the Company also had 8 spare engines financed under operating leases with lease term expiration dates ranging from 2023 to 2027 and owned 16 spare engines, all of which as of December 31, 2020, were pledged as collateral under the Company's 2022 revolving credit facility. Total rent expense for all leases charged to operations for the years ended 2020, 2019 and 2018 was $371.6 million, $345.0 million and $312.0 million, respectively. Total rental expense charged to operations for aircraft and engine operating leases for the years ended December 31, 2020, 2019 and 2018 was $196.4 million, $182.6 million and $177.6 million, respectively. Some of the Company’s aircraft and engine master lease agreements provide that the Company pays maintenance reserves to aircraft lessors to be held as collateral in advance of the Company’s required performance of major maintenance activities. Maintenance reserve payments that are substantively and contractually related to the maintenance of the leased asset are accounted for as aircraft maintenance deposits to the extent they are expected to be recoverable. A majority of these maintenance reserve payments are calculated based on a utilization measure, such as flight hours or cycles, while some maintenance reserve payments are fixed, time-based contractual amounts. Fixed maintenance reserve payments that are not probable of being recovered are considered lease payments and are included in the right-of-use asset and lease liability. Maintenance reserve payments that are based on a utilization measure and are not probable of being recovered are considered variable lease payments that are recognized when they are probable of being incurred and are not included in the right-of-use asset and lease liability. These lease agreements generally provide that maintenance reserves are reimbursable to the Company upon completion of the maintenance event. Some of the master lease agreements do not require that the Company pay maintenance reserves so long as the Company's cash balance does not fall below a certain level. As of December 31, 2020, the Company is in full compliance with those requirements and does not anticipate having to pay reserves related to these master leases in the future. Under the terms of the lease agreements, the Company will continue to operate and maintain the aircraft. Payments under the majority of the lease agreements are fixed for the term of the lease. The lease agreements contain standard termination events, including termination upon a breach of the Company's obligations to make rental payments and upon any other material breach of the Company's obligations under the leases, and standard maintenance and return condition provisions. These return provisions are evaluated at inception of the lease and throughout the lease terms and are accounted for as either fixed or variable lease payments (depending on the nature of the lease return condition) when it is probable that such amounts will be incurred. When determining probability and estimated cost of lease return obligations, there are various other factors that need to be considered such as the contractual terms of the lease, the ability to swap engines or other aircraft components, current condition of the aircraft, the age of the aircraft at lease expiration, utilization of engines and other components, the extent of repairs needed at return, return locations, current configuration of the aircraft and cost of repairs and materials at the time of return. Management assesses the factors listed above and the need to accrue lease return costs throughout the lease as facts and circumstances warrant an assessment. As a result of COVID-19, the Company is currently operating its aircraft at lower utilization levels. If the Company continues flying its aircraft at lower utilization levels beyond its current projections, the timing of future maintenance events may change such that the Company will be required to accrue lease return costs and/or record reserves against its maintenance deposits earlier than it would have expected and such amounts could be significant. The Company expects lease return costs and unrecoverable maintenance deposits will increase as individual aircraft lease agreements approach their respective termination dates and the Company begins to accrue the estimated cost of return conditions for the corresponding aircraft. Upon a termination of the lease due to a breach by the Company, the Company would be liable for standard contractual damages, possibly including damages suffered by the lessor in connection with remarketing the aircraft or while the aircraft is not leased to another party. 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. Supplemental rent, recorded within aircraft rent expense, is made up of maintenance reserves paid to aircraft lessors that are not probable of being reimbursed and probable and estimable return condition obligations. The Company expensed $3.3 million, $4.8 million and $3.4 million of supplemental rent recorded within aircraft rent during 2020, 2019 and 2018, respectively. The Company did not expense any paid maintenance reserves as supplemental rent in 2020. During 2019 and 2018, the Company expensed $0.5 million and $1.3 million, respectively, of paid maintenance reserves as supplemental rent. As of December 31, 2020, the Company had $126.3 million of aircraft maintenance deposits ($73.1 million in aircraft maintenance deposits and $53.2 million in long-term aircraft maintenance deposits) on the Company's consolidated balance sheets. During the twelve months ended December 31, 2020, the Company took delivery of eight aircraft under secured debt arrangements, one aircraft under a sale-leaseback transaction and three aircraft under direct operating leases. In addition, the Company purchased two previously leased aircraft. The Company also purchased two new engines and returned one previously leased engine. Prior to the adoption of Topic 842 on January 1, 2019, 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 consolidated statements of operations. On December 31, 2019, the Company entered into an aircraft purchase agreement to acquire two A319 aircraft previously operated by the Company under operating leases. The contract was deemed a lease modification, which resulted in a change of classification from operating leases to finance leases for the two aircraft. The Company recorded a finance lease obligation of $44.1 million calculated as the present value of the remaining lease payments, including the final payment to purchase the aircraft and included within current maturities of long-term debt and finance leases on the Company's consolidated balance sheets as of December 31, 2019. In addition, the Company recorded finance lease assets of $48.4 million which include related amounts previously recorded as maintenance reserves and security deposits and included within flight equipment on the Company's consolidated balance sheets as of December 31, 2019. In January 2020, the purchase of the two aircraft was completed and the aircraft were recorded within flight equipment on the Company's consolidated balance sheets. The remainder of the Company's finance lease obligations relate to the lease of computer equipment used by the Company's flight crew and office equipment. Payments under these finance lease agreements are fixed for terms ranging from 4 to 5 years. Finance lease assets are recorded within property and equipment and the related liabilities are recorded within current maturities of long-term debt and finance leases and long-term debt and finance leases, less current maturities on the Company's consolidated balance sheets. During the fourth quarter of 2019, the Company purchased an 8.5-acre parcel of land for $41.0 million and entered into a 99-year lease agreement for the lease of a 2.6-acre parcel of land, in Dania Beach, Florida, where the Company intends to build a new headquarters campus. In connection with the lease agreement, the Company is expected to build a 200-unit residential building. The 8.5-acre parcel of land is capitalized within ground property and equipment on the Company's consolidated balance sheets. The 99-year lease was determined to be an operating lease and is recorded within operating lease right-of-use asset and operating lease liability on the Company's consolidated balance sheets. Operating lease commitments related to this lease are included in the table below within property facility leases. The following table provides details of the Company's future minimum lease payments under finance lease liabilities and operating lease liabilities recorded on the Company's consolidated balance sheets as of December 31, 2020. The table does not include commitments that are contingent on events or other factors that are currently uncertain and unknown. Finance Leases Operating Leases Total Operating and Finance Lease Obligations Aircraft and Spare Engine Leases Property Facility Leases (in thousands) 2021 (1) $ 753 $ 236,101 $ 4,683 $ 241,537 2022 725 209,911 4,397 215,033 2023 349 197,267 3,383 200,999 2024 98 175,204 2,772 178,074 2025 — 153,146 1,060 154,206 2026 and thereafter — 975,682 143,093 1,118,775 Total minimum lease payments $ 1,925 $ 1,947,311 $ 159,388 $ 2,108,624 Less amount representing interest 137 562,493 134,628 697,258 Present value of minimum lease payments $ 1,788 $ 1,384,818 $ 24,760 $ 1,411,366 Less current portion 671 130,484 3,307 134,462 Long-term portion $ 1,117 $ 1,254,334 $ 21,453 $ 1,276,904 (1) Includes $27.3 million of aircraft and spare engine rent payment deferrals due to COVID-19 which are recorded in other current liabilities within the Company's consolidated balance sheets. Commitments related to the Company's noncancellable short-term operating leases not recorded on the Company's consolidated balance sheets are expected to be $1.4 million for 2021 and none for 2022 and beyond. During 2020, the Company e ntered into agreements to defer payments in 2020 related to facility rents and other airport service contracts at certain locations. Also during 2020, the Company entered into agreements to defer payments related to certain aircraft and engine leases from 2020 into 2021. The Company elected to apply the practical expedient issued by the Financial Accounting Standards Board in April 2020 which allows companies to treat a lease concession related to COVID-19 as though enforceable rights and obligations for the concessions existed regardless of whether those enforceable rights and obligations explicitly exist in the lease agreement. Amounts deferred as of December 31, 2020 are recorded in accrued rent within other current liabilities on the Company's consolidated balance sheet. The table below presents information for lease costs related to the Company's finance and operating leases: Year Ended December 31, 2020 2019 (in thousands) Finance lease cost Amortization of leased assets $ 824 $ 998 Interest of lease liabilities 194 674 Operating lease cost Operating lease cost (1) 201,474 179,959 Short-term lease cost (1) 25,195 5,144 Variable lease cost (1) 125,534 140,417 Total lease cost $ 353,221 $ 327,192 (1) Expenses are classified within aircraft rent and landing fees and other rents on the Company's consolidated statements of operations. The table below presents lease-related terms and discount rates as of December 31, 2020: December 31, 2020 December 31, 2019 Weighted-average remaining lease term Operating leases 12.9 years 13.0 years Finance leases 2.6 years 0.1 years Weighted-average discount rate Operating leases 6.08 % 5.86 % Finance leases 5.54 % 2.46 %

Defined Contribution 401(k) Pla

Defined Contribution 401(k) Plan	12 Months Ended
	Dec. 31, 2020
Retirement Benefits [Abstract]
Defined Contribution 401(k) Plan	Defined Contribution 401(k) Plan The Company sponsors three defined contribution 401(k) plans, Spirit Airlines, Inc. Employee Retirement Savings Plan (first plan), Spirit Airlines, Inc. Pilots’ Retirement Savings Plan (second plan) and Spirit Airlines, Inc. Puerto Rico Retirement Savings Plan (third plan). The first plan is for all employees that are not covered by the pilots’ collective bargaining agreement, who have at least 60 days of service and have attained the age of 21. The second plan is for the Company’s pilots, and contains the same service requirements as the first plan. Prior to March 1, 2018, the Company matched 100% of the pilot's contribution, up to 9% of the individual pilot's annual compensation. Beginning on March 1, 2018, the Company contributed 11% of the individual pilot's annual compensation, regardless of the pilot's contributions to the plan. The Company's contribution will increase by 1% on an annual basis each March until 2022 at which time the contribution will be 15%. The third plan is for all Company employees residing in Puerto Rico and was adopted on April 16, 2012. It contains the same service requirements as the first and second plans.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes Significant components of the provision for income taxes from continuing operations are as follows: Year Ended December 31, 2020 2019 2018 (in thousands) Current: Federal $ (141,997) $ (22,429) $ (2,178) State and local (1,847) 1,218 410 Foreign (1,554) 6,693 4,692 Total current expense (benefit) (145,398) (14,518) 2,924 Deferred: Federal (33,494) 106,703 42,246 State and local (12,592) 8,986 4,057 Total deferred expense (benefit) (46,086) 115,689 46,303 Total income tax expense (benefit) $ (191,484) $ 101,171 $ 49,227 The income tax provision differs from that computed at the federal statutory corporate tax rate as follows: Year Ended December 31, 2020 2019 2018 Expected provision at federal statutory tax rate 21.0 % 21.0 % 21.0 % State tax expense, net of federal benefit 1.9 % 1.8 % 1.7 % Revaluation of deferred taxes 9.2 % (2.1) % — % Other (1.2) % 2.5 % 1.3 % Total income tax expense (benefit) 30.9 % 23.2 % 24.0 % The Company accounts for income taxes using the asset and liability method. Deferred taxes are recorded based on differences between the consolidated financial statement basis and tax basis of assets and liabilities and available tax loss and credit carryforwards. At December 31, 2020 and 2019, the significant components of the Company's deferred taxes consisted of the following: December 31, 2020 2019 (in thousands) Deferred tax assets: Income tax credits 4,298 9,632 Net operating losses 220,071 13,604 Deferred revenue 11,740 8,824 Nondeductible accruals 21,918 14,133 Deferred manufacturing credits 6,442 2,813 Accrued maintenance 568 1,668 Equity compensation 3,433 2,851 Operating lease liability 313,142 305,161 Other 465 482 Valuation allowance (2,949) (1,746) Deferred tax assets $ 579,128 $ 357,422 Deferred tax liabilities: Convertible debt 14,942 — Prepaid expenses 898 1,120 Property, plant and equipment 603,173 430,523 Deferred financing costs 124 154 Accrued aircraft and engine maintenance 80,916 84,479 Right-of-use asset 318,969 310,438 Deferred tax liabilities 1,019,022 826,714 Net deferred tax assets (liabilities) $ (439,894) $ (469,292) On March 27, 2020, the CARES Act was enacted. The CARES Act allows for a five-year carryback of federal net operating losses generated in tax years 2018 through 2020. The Company filed for a carryback of its adjusted 2018 federal net operating tax loss to tax years 2013 and 2014. The federal net operating loss carryback resulted in a tax benefit of $56.1 million since the federal net operating losses can be benefited at the higher 35% federal tax rate in effect for tax years 2013 and 2014. The federal net operating loss carry back also generated an additional income tax receivable of $142.0 million as of December 31, 2020. In assessing the realizability of the deferred tax assets, management considered whether it is more likely than not that some or all of the deferred tax assets would be realized. In evaluating the Company’s ability to utilize its deferred tax assets, it considered all available evidence, both positive and negative, in determining future taxable income on a jurisdiction by jurisdiction basis. As of December 31, 2020 and 2019, the Company had a valuation allowance of $2.9 million and $1.7 million, respectively, against certain deferred tax assets related to equity compensation for executives due to changes in tax law resulting from the Tax Cuts and Jobs Act ("TCJA") and foreign tax credits. As of December 31, 2020, the Company had $2.8 million of foreign tax credits, $1.5 million of general business tax credits, $956.9 million of federal net operating loss and $360.0 million of state net operating loss available, that may be applied against future tax liabilities. The foreign tax credits will begin to expire in 2025, the state net operating losses will begin to expire in 2027, the general business credits will begin to expire in 2038 and there is no expiration of federal net operating losses. For the twelve months ended December 31, 2020 and 2019, a $0.8 million income tax benefit and a $1.4 million of income tax expense related to share-based compensation were included within income tax expense, respectively. For tax years ended December 31, 2020, 2019 and 2018, the Company did not recognize any liabilities for uncertain tax positions nor any interest and penalties on unrecognized tax benefits. For tax years 2020, 2019 and 2018, all income for the Company is subject to domestic income taxes. The Company files its tax returns as prescribed by the tax laws of the jurisdictions in which it operates. The Company's federal income tax returns for 2017 through 2019 tax years are still subject to examination in the U.S. Various state and foreign jurisdiction tax years also remain open to examination. The Company believes that any potential assessment would be immaterial to its consolidated financial statements.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Aircraft-Related Commitments and Financing Arrangements The Company’s contractual purchase commitments consist primarily of aircraft and engine acquisitions through manufacturers and aircraft leasing companies. As of December 31, 2020, the Company's firm aircraft orders consisted of 126 A320 family aircraft with Airbus, including A319neos, A320neos and A321neos, with deliveries expected through 2027. In addition, the Company had 10 direct operating leases for A320neos with third-party lessors, with deliveries expected through 2021. On December 20, 2019, the Company entered into an A320 NEO Family Purchase Agreement with Airbus for the purchase of 100 new Airbus A320neo family aircraft, with options to purchase up to 50 additional aircraft. This agreement includes a mix of Airbus A319neo, A320neo and A321neo aircraft with such aircraft scheduled for delivery through 2027. The Company also has one spare engine order for a V2500 SelectTwo engine with IAE and two spare engine orders for PurePower PW 1100G-JM engines with Pratt & Whitney. Spare engines are scheduled for delivery from 2021 through 2023. As of December 31, 2020, committed expenditures for these aircraft and spare engines, including estimated amounts for contractual price escalations and pre-delivery payments, are expected to be $415.7 million in 2021, $849.1 million in 2022, $676.0 million in 2023, $1,001.6 million in 2024, $1,209.1 million in 2025, and $2,367.8 million in 2026 and beyond. During the third quarter of 2019, the United States announced its decision to levy tariffs on certain imports from the European Union, including commercial aircraft and related parts. These tariffs include aircraft and other parts that the Company is already contractually obligated to purchase including those reflected above. The imposition of these tariffs may substantially increase the cost of new Airbus aircraft and parts required to service the Company's Airbus fleet. For further discussion on this topic, please refer to "Risk Factors - Risks Related to Our Business - Any tariffs imposed on commercial aircraft and related parts imported from outside the United States may have a material adverse effect on our fleet, business, financial condition and our results of operations." As of December 31, 2020, the Company had secured financing for 10 aircraft to be leased directly from third-party lessors, scheduled for delivery in 2021. The Company did not have financing commitments in place for the 126 Airbus aircraft currently on firm order, which are scheduled for delivery through 2027. However, the Company has signed a financing letter of agreement with Airbus which provides backstop financing for a majority of the aircraft included in the A320 NEO Family Purchase Agreement. The agreement provides a standby credit facility in the form of senior secured mortgage debt financing. As of December 31, 2020, aircraft rent commitments for future aircraft deliveries to be financed under direct leases from third-party lessors are expected to be approximately $18.1 million in 2021, $34.2 million in 2022, $34.2 million in 2023, $34.2 million in 2024, $34.2 million in 2025, and $255.5 million in 2026 and beyond. Interest commitments related to the Company's outstanding debt obligations as of December 31, 2020 are $162.5 million in 2021, $148.7 million in 2022, $138.1 million in 2023, $126.8 million in 2024, $110.0 million in 2025, and $103.0 million in 2026 and beyond. For principal commitments related to the Company's outstanding debt obligations, refer to Note 14, Debt and Other Obligations. The Company is contractually obligated to pay the following minimum guaranteed payments for its reservation system and other miscellaneous subscriptions and services as of December 31, 2020: $17.8 million in 2021, $15.9 million in 2022, $14.1 million in 2023, $14.5 million in 2024, $15.0 million in 2025, and $35.3 million in 2026 and beyond. During the first quarter of 2018, the Company entered into a contract renewal with its reservation system provider which expires in 2028. Litigation The Company is subject to commercial litigation claims and to administrative and regulatory proceedings and reviews that may be asserted or maintained from time to time. The Company believes the ultimate outcome of such lawsuits, proceedings and reviews will not, individually or in the aggregate, have a material adverse effect on its financial position, liquidity or results of operations. Employees The Company has five union-represented employee groups that together represent approximately 82% of all employees at December 31, 2020. The table below sets forth the Company's employee groups and status of the collective bargaining agreements as of December 31, 2020. Employee Groups Representative Amendable Date Percentage of Workforce Pilots Air Line Pilots Association, International (ALPA) February 2023 29% Flight Attendants Association of Flight Attendants (AFA-CWA) May 2021 46% Dispatchers Professional Airline Flight Control Association (PAFCA) October 2023 1% Ramp Service Agents International Association of Machinists and Aerospace Workers (IAMAW) June 2020 3% Passenger Service Agents Transport Workers Union of America (TWU) NA 3% In February 2018, the pilot group voted to approve the current five-year agreement with the Company. The current agreement includes a one-time ratification incentive of $80.2 million, including payroll taxes, and an $8.5 million adjustment related to other contractual provisions which was recorded in special charges (credits) within operating expenses in the consolidated statement of operations for the year ended December 31, 2018. For additional information, refer to Note 5, Special Charges and Credits. In February 2020, the IAMAW notified us, as required by the Railway Labor Act, that it intends to submit proposed changes to the collective bargaining agreement covering our ramp service agents which became amendable in June 2020. The parties expect to schedule meeting dates for negotiations soon. The Company's passenger service agents are represented by the TWU, but the representation only applies to the Company's Fort Lauderdale station where the Company has direct employees in the passenger service classification. The Company and the TWU began meeting in late October 2018 to negotiate an initial collective bargaining agreement. As of December 31, 2020, the Company continued to negotiate with the TWU. The Company is self-insured for health care claims, subject to a stop-loss policy, for eligible participating employees and qualified dependent medical claims, subject to deductibles and limitations. The Company’s liabilities for claims incurred but not reported are determined based on an estimate of the ultimate aggregate liability for claims incurred. The estimate is calculated from actual claim rates and adjusted periodically as necessary. The Company has accrued $7.3 million and $5.2 million, for health care claims as of December 31, 2020, and 2019, respectively, recorded within other current liabilities on the Company's consolidated balance sheet.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements Under ASC 820, Fair Value Measurements and Disclosures , disclosures relating to how fair value is determined for assets and liabilities are required, and a hierarchy for which these assets and liabilities must be grouped is established, based on significant levels of inputs, as follows: Level 1 —Quoted prices in active markets for identical assets or liabilities. Level 2 —Observable inputs other than Level 1 prices such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. Level 3 —Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. The Company utilizes several valuation techniques in order to assess the fair value of the Company’s financial assets and liabilities. Long-term Debt The estimated fair value of the Company's secured notes, term loan debt agreements and revolving credit facilities has been determined to be Level 3 as certain inputs used to determine the fair value of these agreements are unobservable. The Company utilizes a discounted cash flow method to estimate the fair value of the Level 3 long-term debt. The estimated fair value of the Company's publicly and non-publicly held EETC debt agreements and the Company's convertible notes has been determined to be Level 2 as the Company utilizes quoted market prices in markets with low trading volumes to estimate the fair value of its Level 2 long-term debt. The carrying amounts and estimated fair values of the Company's long-term debt at December 31, 2020 and December 31, 2019, were as follows: As of December 31, 2020 2019 Carrying Value Estimated Fair Value Carrying Value Estimated Fair Value Fair value level hierarchy (in millions) 8.00% senior secured notes $ 850.0 $ 886.0 $ — $ — Level 3 Fixed-rate term loans 1,301.9 1,362.9 1,074.3 1,120.0 Level 3 Unsecured term loans 73.3 83.1 — — Level 3 2015-1 EETC Class A 322.6 323.4 348.6 372.2 Level 2 2015-1 EETC Class B 64.0 62.5 72.0 74.5 Level 2 2015-1 EETC Class C 86.6 77.8 98.1 100.5 Level 2 2017-1 EETC Class AA 214.4 207.4 228.4 237.0 Level 2 2017-1 EETC Class A 71.5 68.8 76.1 78.8 Level 2 2017-1 EETC Class B 60.6 56.2 70.6 72.0 Level 2 2017-1 EETC Class C 85.5 76.3 85.5 88.0 Level 2 Convertible debt 175.0 380.3 — — Level 2 Revolving credit facilities 275.1 275.1 160.0 160.0 Level 3 Total long-term debt $ 3,580.5 $ 3,859.8 $ 2,213.6 $ 2,303.0 Cash and Cash Equivalents Cash and cash equivalents at December 31, 2020 and December 31, 2019 are comprised of liquid money market funds and cash and are categorized as Level 1 instruments. The Company maintains cash with various high-quality financial institutions. Restricted Cash Restricted cash is comprised of cash held in account subject to account control agreements or otherwise pledged as collateral against the Company's letters of credit and is categorized as a Level 1 instrument. As of December 31, 2020, the Company had a $30.0 million standby letter of credit secured by restricted cash, of which $23.6 million had been drawn upon for issued letters of credit. In addition, the Company had $41.4 million of restricted cash held in accounts subject to control agreements to be used for the payment of interest and fees on the Company's 8.00% senior secured notes. For additional information on the Company's 8.00% senior secured notes, refer to Note 14, Debt and Other Obligations. Short-term Investment Securities Short-term investment securities at December 31, 2020 and December 31, 2019 are classified as available-for-sale and generally consist of U.S. Treasury and U.S. government agency securities with contractual maturities of twelve months or less. The Company's short-term investment securities are categorized as Level 1 instruments, as the Company uses quoted market prices in active markets when determining the fair value of these securities. For additional information, refer to Note 9, Short-term Investment Securities. Assets Held for Sale The Company's assets held for sale consist of rotable aircraft parts. When long-lived assets are identified as held for sale and the required criteria are met, the Company reclassifies the assets from property and equipment to prepaid expenses and other current assets on the Company's consolidated balance sheets and discontinues depreciation. The assets are measured at the lower of the carrying amount or fair value less cost to sell and a loss is recognized for any initial adjustment of the asset’s carrying amount to fair value less cost to sell. Such valuations include estimations of fair values and incremental direct costs to transact a sale. The fair value measurements for our held-for-sale assets were based on Level 3 inputs, which include information obtained from third-party valuation sources. As of December 31, 2020 and 2019, the Company had $2.3 million in assets held for sale recorded within prepaid expenses and other current assets in the accompanying consolidated balance sheets. Assets and liabilities measured at gross fair value on a recurring basis are summarized below: Fair Value Measurements as of December 31, 2020 Total Level Level Level (in millions) Cash and cash equivalents $ 1,789.7 $ 1,789.7 $ — $ — Restricted cash 71.4 71.4 — — Short-term investment securities 106.3 106.3 — — Assets held for sale 2.3 — — 2.3 Total assets $ 1,969.7 $ 1,967.4 $ — $ 2.3 Total liabilities $ — $ — $ — $ — Fair Value Measurements as of December 31, 2019 Total Level Level Level (in millions) Cash and cash equivalents $ 979.0 $ 979.0 $ — $ — Restricted cash — — — — Short-term investment securities 105.3 105.3 — — Assets held for sale 2.3 — — 2.3 Total assets $ 1,086.6 $ 1,084.3 $ — $ 2.3 Total liabilities $ — $ — $ — $ — The Company had no transfers of assets or liabilities between any of the above levels during the years ended December 31, 2020 or 2019. Assets Held for Sale Activity for the Twelve Months Ended December 31, 2019 (in millions) Balance at December 31, 2018 $ — Purchases 5.4 Sales — Total realized or unrealized gains (losses) included in earnings, net (3.1) Balance at December 31, 2019 $ 2.3 The balance of the Company's held-for-sale assets remained the same during the twelve months ended December 31, 2020, as the Company had no purchases, sales nor realized and unrealized losses or gains related to these assets during this period.

Operating Segments and Related

Operating Segments and Related Disclosures	12 Months Ended
	Dec. 31, 2020
Segment Reporting [Abstract]
Operating Segments and Related Disclosures	Operating Segments and Related Disclosures The Company is managed as a single business unit that provides air transportation for passengers. Operating revenues by geographic region as defined by the Department of Transportation ("DOT") area are summarized below: 2020 2019 2018 (in millions) DOT—Domestic $ 1,660.7 $ 3,462.8 $ 2,990.7 DOT—Latin America and Caribbean 149.3 367.7 332.3 Total $ 1,810.0 $ 3,830.5 $ 3,323.0

Quarterly Financial Data (Unaud

Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2020
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Data (Unaudited)	Quarterly Financial Data (Unaudited) Quarterly results of operations for the years ended December 31, 2020 and 2019 are summarized below: Three Months Ended March 31 June 30 September 30 December 31 (in thousands, except per-share amounts) 2020 Operating revenue $ 771,081 $ 138,529 $ 401,922 $ 498,490 Operating income (loss) (57,992) (190,384) (99,471) (159,915) Net income (loss) (27,828) (144,428) (99,140) (157,304) Basic earnings (loss) per share (0.41) (1.81) (1.07) (1.61) Diluted earnings (loss) per share (0.41) (1.81) (1.07) (1.61) 2019 Operating revenue $ 855,796 $ 1,012,956 $ 991,968 $ 969,816 Operating income 87,804 163,938 124,681 124,624 Net income 56,076 114,501 83,464 81,214 Basic earnings per share 0.82 1.67 1.22 1.19 Diluted earnings per share 0.82 1.67 1.22 1.18 Interim results are not necessarily indicative of the results that may be expected for other interim periods or for the full year. The air transportation business is subject to significant seasonal fluctuations as demand is generally greater in the second

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited consolidated financial statements include the accounts of Spirit Airlines, Inc. ("Spirit") and its consolidated subsidiaries (the "Company"). Spirit is an ultra low-cost, low-fare airline that provides affordable travel opportunities principally throughout the domestic United States, the Caribbean and Latin America and is headquartered in Miramar, Florida. Spirit manages operations on a system-wide basis due to the interdependence of its route structure in the various markets served. As only one service is offered (i.e., air transportation), management has concluded there is only one reportable segment. In August 2020, Spirit formed several new subsidiaries; Spirit Finance Cayman 1 Ltd. (“HoldCo 1”), Spirit Finance Cayman 2 Ltd. (“HoldCo 2), Spirit IP Cayman Ltd. (“Spirit IP”) and Spirit Loyalty Cayman Ltd. (“Spirit Loyalty”). Each are Cayman Islands exempted companies incorporated with limited liability. Spirit IP and Spirit Loyalty are wholly-owned subsidiaries of HoldCo 2 (other than the special share issued to the special shareholder, who granted a proxy to vote such share to the collateral agent for the 8.00% senior secured notes (as defined herein)). HoldCo 1 and HoldCo 2 are special purpose holding companies. HoldCo 2 is a wholly-owned direct subsidiary of HoldCo 1 (other than the special share issued to the special shareholder, who granted a proxy to vote such share to the collateral agent for the 8.00% senior secured notes). HoldCo 1 is a wholly-owned subsidiary of Spirit (other than the special share issued to the special shareholder, who granted a proxy to vote such share to the collateral agent for the 8.00% senior secured notes). As a result, the Company's financial statements are presented on a consolidated basis.
Use of Estimates	Use of Estimates The preparation of financial statements in accordance with generally accepted accounting principles in the United States of America requires the Company's management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. The Company's estimates and assumptions are based on historical experience and changes in the business environment. However, actual results may differ from estimates under different conditions, sometimes materially. Critical accounting policies and estimates are defined as those that both (i) are most important to the portrayal of the Company's financial condition and results and (ii) require management's most subjective judgments. The Company's most critical accounting policies and estimates are described below.
Cash and Cash Equivalents, Restricted Cash	Cash and Cash Equivalents The Company considers all highly liquid investments with maturities of less than three months at the date of acquisition to be cash equivalents. Investments included in this category primarily consist of cash and money market funds. Cash and cash equivalents are stated at cost, which approximates fair value. Restricted Cash
Short-term Investment Securities	Short-term Investment Securities The Company's short-term investment securities are classified as available-for-sale and generally consist of U.S. Treasury and U.S. government agency securities with contractual maturities of twelve months or less. These securities are stated at fair value within current assets on the Company's consolidated balance sheet. For all short-term investments, at each reset period or upon reinvestment, the Company accounts for the transaction as proceeds from the maturity of short-term investment securities for the security relinquished, and purchase of short-term investment securities for the security purchased, in the Company's consolidated statements of cash flows. Realized gains and losses on sales of investments, if any, are reflected in non-operating income (expense) in the consolidated statements of operations. Unrealized gains and losses on investment securities are reflected as a component of accumulated other comprehensive income.
Accounts Receivable	Accounts Receivable Accounts receivable primarily consist of amounts due from credit card processors associated with the sales of tickets and amounts due from the Internal Revenue Service related to federal excise fuel tax. The Company records an allowance for amounts not expected to be collected. The Company estimates the allowance based on historical write-offs and aging trends as
Income Tax Receivable	Income Tax ReceivableIncome tax receivable consists of amounts due from tax authorities for recovery of income taxes paid in prior periods.Income TaxesThe Company accounts for income taxes using the asset and liability method. The Company records a valuation allowance to reduce the deferred tax assets reported if, based on the weight of the evidence, it is more likely than not that some portion or all of the deferred tax assets will be not realized.
Property and Equipment	Property and Equipment Property and equipment is stated at cost, less accumulated depreciation and amortization. Depreciation of operating property and equipment is computed using the straight-line method applied to each unit of property. Residual values for new aircraft, new engines, major spare rotable parts, avionics and assemblies are generally estimated to be 10%. Property under finance leases and related obligations are initially recorded at an amount equal to the present value of future minimum lease payments computed using the Company's incremental borrowing rate or, when known, the interest rate implicit in the lease. Amortization of property under finance leases is recorded on a straight-line basis over the lease term and is included in depreciation and amortization expense.
Operating Lease Right-of-Use Asset and Liabilities	Operating Lease Right-of-Use Asset and Liabilities The Company adopted Topic 842 utilizing the modified retrospective adoption method with an effective date of January 1, 2019. 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. Right-of-use assets represent the Company's right to use an underlying asset for the lease term and lease liabilities represent the Company's obligation to make lease payments arising from the lease. Right-of-use assets and liabilities are recognized at the lease commencement date based on the estimated present value of lease payments over the lease term. When available, the Company uses the rate implicit in the lease to discount lease payments to present value. However, the Company's leases generally do not provide a readily determinable implicit rate. Therefore, the Company estimates the incremental borrowing rate to discount lease payments based on information available at lease commencement. The Company uses publicly available data for instruments with similar characteristics when calculating its incremental borrowing rates. The Company has options to extend certain of its operating leases for an additional period of time and options to early terminate several of its operating leases. The lease term consists of the noncancellable period of the lease, periods covered by options to extend the lease if the Company is reasonably certain to exercise the option, periods covered by an option to terminate the lease if the Company is reasonably certain not to exercise the option and periods covered by an option to extend or not terminate the lease in which the exercise of the option is controlled by the lessor. The Company's lease agreements do not contain any residual value guarantees. The Company elected to not separate non-lease components from the associated lease component for all underlying classes of assets with lease and non-lease components. The Company elected not to apply the recognition requirements in Topic 842 to short-term leases (i.e., leases of 12 months or less) but instead recognize these lease payments in income on a straight-line basis over the lease term. The Company elected this accounting policy for all classes of underlying assets. In addition, in accordance with Topic 842, variable lease payments in the period in which the obligation for those payments is incurred are not included in the recognition of a lease liability or right-of-use asset.
Pre-Delivery Deposits On Flight Equipment	Pre-Delivery Deposits on Flight Equipment The Company is required to make pre-delivery deposit payments ("PDPs") towards the purchase price of each new aircraft and engine prior to the scheduled delivery date. These deposits are initially classified as pre-delivery deposits on flight equipment on the Company's consolidated balance sheets until the aircraft or engine is delivered, at which time the related PDPs are deducted from the final purchase price of the aircraft or engine and are reclassified to flight equipment on the Company's consolidated balance sheets. In addition, the Company capitalizes the interest that is attributable to the outstanding PDP balances as a percentage of the related debt on which interest is incurred. Capitalized interest represents interest cost incurred during the acquisition period of a long-term asset, and is the amount which theoretically could have been avoided had the Company not paid PDPs for the related aircraft or engines.
Measurement of Asset Impairments	Measurement of Asset Impairments The Company records impairment charges on long-lived assets used in operations when events and circumstances indicate that the assets may be impaired, the undiscounted cash flows estimated to be generated by those assets are less than the carrying amount of those assets, and the net book value of the assets exceeds their estimated fair value. In making these determinations, the Company uses certain assumptions, including, but not limited to: (i) estimated fair value of the assets; and (ii) estimated, undiscounted future cash flows expected to be generated by these assets, which are based on additional assumptions such as asset utilization, length of service the asset will be used in the Company’s operations, and estimated salvage values. The Company has assessed whether any impairment of its long-lived assets existed and has determined that no charges were deemed necessary under applicable accounting standards as of December 31, 2020. The Company's assumptions about future conditions
Revenue from Contract with Customer	Passenger Revenues Fare revenues. Tickets sold are initially deferred within air traffic liability on the Company's consolidated balance sheet. Passenger fare revenues are recognized at time of departure when transportation is provided. Generally, all tickets sold by the Company are nonrefundable. An unused ticket expires at the date of scheduled travel and is recognized as revenue at the date of scheduled travel. As of December 31, 2020 and 2019, the Company had air traffic liability ("ATL") balances of $402.0 million and $315.4 million, respectively. As of December 31, 2020, substantially all of the ATL balance as of December 31, 2019 has been recognized. Substantially all of the Company's ATL balance as of December 31, 2020 is expected to be recognized within 12 months. Non-fare revenues. The adoption of ASU 2014-09 on January 1, 2018 impacted the classification of certain ancillary items such as bags, seats and other travel-related fees, since they are deemed part of the single performance obligation of providing passenger transportation. These ancillary items are now recognized in non-fare revenues within passenger revenues, at the time of departure, in the Company's disaggregated revenue table within Note 4, Revenue Disaggregation. Changes and cancellations. Customers may elect to change or cancel their itinerary prior to the date of departure. For changes, a service charge is recognized at time of departure of newly scheduled travel and is deducted from the face value of the original purchase price of the ticket, and the original ticket becomes invalid. For cancellations, a service charge is assessed and the amount remaining after deducting the service charge is called a credit shell which prior to 2020 had an expiration of 60 days from the date the credit shell was created. During 2020, in response to the COVID-19 pandemic, the Company increased the expiration period on some of its credit shells from 60 days to up to 12 months and waived change and cancellation fees for the Guests who booked travel to occur by February 28, 2021. As the COVID-19 pandemic continues to evolve, the Company will evaluate any continued impact to travel plans and may decide to further extend credit shell expiration dates and/or waive change and cancellation fees in the future. Credit shells can be used towards the purchase of a new ticket and the Company’s other service offerings. Both service charge and credit shell amounts are recorded as deferred revenue and amounts expected to expire unused are estimated based on historical experience. Estimating the amount of credits that will go unused involves some level of subjectivity and judgment. Assumptions used to generate breakage estimates can be impacted by several factors including, but not limited to, changes to the Company's ticketing policies, changes to the Company’s refund, exchange, and credit shell policies, and economic factors. Given the unprecedented amount of cancellations in the current year and the related increase in credit shells provided, the Company expects additional variability in the amount of breakage revenue recorded in future periods, as the estimates of the portion of those funds that will expire unused may differ from historical experience. Other Revenues Other revenues primarily consist of the marketing component of the sale of frequent flyer miles to the Company's credit card partner and commissions revenue from the sale of various items such as hotels and rental cars.
Frequent Flier Program	Frequent Flyer Program The Company's frequent flyer program generates customer loyalty by rewarding customers with mileage credits to travel on Spirit. When traveling, customers earn redeemable mileage credits for each mile flown on Spirit. Customers can also earn mileage credits through participating companies such as the co-branded Spirit credit card. Mileage credits are redeemable by customers in future periods for air travel on Spirit. To reflect the mileage credits earned, the program includes two types of transactions that are considered revenue arrangements with multiple performance obligations: (1) mileage credits earned with travel and (2) mileage credits sold to co-branded credit card partner. Passenger ticket sales earning mileage credits. Passenger ticket sales earning mileage credits provide customers with (1) mileage credits earned and (2) air transportation. The Company values each performance obligation on a standalone basis. To value the mileage credits earned, the Company considers the quantitative value a passenger receives by redeeming miles for a ticket rather than paying cash, which is referred to as equivalent ticket value ("ETV"). The Company defers revenue for the mileage credits when earned and recognizes loyalty travel awards in passenger revenue as the miles are redeemed and services are provided. The Company records the air transportation portion of the passenger ticket sales in air traffic liability and recognizes passenger revenue when transportation is provided or if the ticket goes unused, at the date of scheduled travel. Sale of mileage credits. Customers may earn mileage credits based on their spending with the Company's co-branded credit card company with which the Company has an agreement to sell mileage credits. The contract to sell mileage credits under this agreement has multiple performance obligations, as discussed below. The Company's co-branded credit card agreement provides for joint marketing where cardholders earn mileage credits for making purchases using co-branded cards. During 2020, the Company extended its agreement with the administrator of the FREE SPIRIT affinity credit card program to extend through March 31, 2024. In connection with its extension of the agreement, in January 2021, the Company launched a new loyalty program with extended mileage expiration, additional benefits based on status tiers, and other changes. The Company accounts for this agreement consistently with the accounting method that allocates the consideration received to the individual products and services delivered. The value is allocated based on the relative selling prices of those products and services, which generally consists of (i) travel miles to be awarded, (ii) licensing of brand and access to member lists and (iii) advertising and marketing efforts. The Company determined the best estimate of the selling prices by considering discounted cash flow analysis using multiple inputs and assumptions, including: (1) the expected number of miles awarded and number of miles redeemed, (2) ETV for the award travel obligation, (3) licensing of brand and access to member lists and (4) advertising and marketing efforts. The new program terms will require updated estimates of the allocation of future revenues to the performance obligations described above. The Company defers the amount for award travel obligation as part of loyalty deferred revenue within air traffic liability on the consolidated balance sheet and recognizes loyalty travel awards in passenger revenue as the mileage credits are used for travel. Revenue allocated to the remaining performance obligations, primarily marketing components, is recorded in other revenue over time as miles are delivered. Total unrecognized revenue from future FREE SPIRIT award redemptions and the sale of mileage credits was $31.6 million and $29.8 million at December 31, 2020 and 2019, respectively. The current portion of this balance is recorded within air traffic liability and the long-term portion of this balance is recorded within deferred gains and other long-term liabilities in the accompanying consolidated balance sheets. Current activity of frequent flyer program. Mileage credits are combined in one homogeneous pool and are not separately identifiable. As such, revenue is comprised of miles that were part of the frequent flyer deferred revenue balance at the beginning of the period as well as miles that were issued during the period.
Airframe and Engine Maintenance	Airframe and Engine Maintenance The Company accounts for heavy maintenance and major overhaul under the deferral method whereby the cost of heavy maintenance and major overhaul is deferred and amortized until the earlier of the end of the useful life of the related asset, the end of the remaining lease term or the next scheduled heavy maintenance event.
Leased Aircraft Return Costs	Leased Aircraft Return Costs The Company's aircraft lease agreements often contain provisions that require the Company to return aircraft airframes, engines and other aircraft components to the lessor in a certain condition or pay an amount to the lessor based on the airframe and engine's actual return condition. Lease return costs include all costs that would be incurred at the return of the aircraft, including costs incurred to repair the airframe and engines to the required condition as stipulated by the lease. Lease return costs are recognized beginning when it is probable that such costs will be incurred and they can be estimated.
Aircraft Maintenance Deposits	Aircraft Maintenance Deposits Some of the Company's aircraft and engine master lease agreements provide that the Company pay maintenance reserves to aircraft lessors to be held as collateral in advance of the Company's required performance of major maintenance activities. A majority of these maintenance reserve payments are calculated based on a utilization measure, such as flight hours or cycles, while some maintenance reserve payments are fixed, time-based contractual amounts. These lease agreements
Aircraft Fuel	Aircraft Fuel Aircraft fuel expense includes jet fuel and associated into-plane costs, taxes, and oil, and realized and unrealized gains and losses associated with fuel derivative contracts, if any.
Advertising	AdvertisingThe Company expenses advertising and the production costs of advertising as incurred.
Income Taxes	Income Tax ReceivableIncome tax receivable consists of amounts due from tax authorities for recovery of income taxes paid in prior periods.Income TaxesThe Company accounts for income taxes using the asset and liability method. The Company records a valuation allowance to reduce the deferred tax assets reported if, based on the weight of the evidence, it is more likely than not that some portion or all of the deferred tax assets will be not realized.
Stock-Based Compensation	Stock-Based CompensationThe Company recognizes cost of employee services received in exchange for awards of equity instruments based on the fair value of each instrument at the date of grant. For the majority of awards, compensation expense is recognized on a straight-line basis over the period during which an employee is required to provide service in exchange for an award. Certain awards have performance conditions that must be achieved prior to vesting and are expensed based on the expected achievement at each reporting period. The Company has issued and outstanding restricted stock awards, stock option awards and performance share awards. Restricted stock awards are valued at the fair value of the shares on the date of grant. The fair value of share option awards is estimated on the date of grant using the Black-Scholes valuation model. The fair value of performance share awards based on a market condition is estimated through the use of a Monte Carlo simulation model. The fair value of performance share awards based on a performance condition is based on the fair value of the shares on the date of grant. The performance share awards based on a performance condition are evaluated at each report date and adjustments are made to stock-based compensation expense based on the number of shares deemed probable of issuance upon vesting.
Concentrations of Risk	Concentrations of Risk The Company’s business may be adversely affected by increases in the price of aircraft fuel, the volatility of the price of aircraft fuel, or both. Aircraft fuel, one of the Company’s largest expenditures, represented approximately 19%, 30% and 32% of total operating expenses in 2020, 2019 and 2018, respectively. The Company’s operations are largely concentrated in the southeast United States with Fort Lauderdale being the highest volume fueling point in the system. Gulf Coast Jet indexed fuel is the basis for a substantial majority of the Company’s fuel consumption. Any disruption to the oil production or refinery capacity in the Gulf Coast, as a result of weather or any other disaster, or disruptions in supply of jet fuel, dramatic escalations in the costs of jet fuel and/or the failure of fuel providers to perform under fuel arrangements for other reasons could have a material adverse effect on the Company’s financial condition and results of operations. The Company’s operations will continue to be vulnerable to weather conditions (including hurricane season or snow and severe winter weather), which could disrupt service or create air traffic control problems. These events may result in decreased revenue and/or increased costs.
Recent Accounting Developments	Recently Adopted Accounting Pronouncements 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. For accounts receivables, aircraft maintenance deposits and security deposits (recorded within other long-term assets on the Company's consolidated balance sheets) the Company is required to estimate lifetime expected credit losses. 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. As such, the Company is required to recognize an allowance for credit losses for its short-term available-for-sale investment securities, with the exception of U.S. Treasury securities which do not require an allowance for credit losses. The Company adopted this standard effective January 1, 2020. In connection with the adoption of this standard, the Company recognized a cumulative effect adjustment, net of tax, of $1.6 million to retained earnings on the Company's consolidated balance sheets with corresponding reserves against certain of our outstanding financial instruments. These amounts were not material to the Company's consolidated financial statements individually or in the aggregate. 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. The Company adopted this standard effective January 1, 2020 and is applying the standard prospectively to all implementation costs incurred after the date of adoption. This adoption has not had a material impact on the Company's consolidated financial statement presentation or results. Recently Issued Accounting Pronouncements Not Yet Adopted Convertible Instruments and Contracts In August 2020, the FASB issued ASU No. 2020-06, "Accounting for Convertible Instruments and Contracts in an Entity's Own Equity." This new standard simplifies and adds disclosure requirements for the accounting and measurement of convertible instruments. It eliminates the treasury stock method for convertible instruments and requires application of the “if-converted” method for certain agreements. This standard is effective for the Company for fiscal years, and interim periods within those years, beginning January 1, 2022. Early adoption is permitted, but no earlier than fiscal years beginning January 1,

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Depreciable Lives Used for the Principal Depreciable Asset Classifications and Components of Depreciation and Amortization	The depreciable lives used for the principal depreciable asset classifications are: Estimated Useful Life Aircraft, engines and flight simulators 25 Spare rotables and flight assemblies 7 to 25 years Other equipment and vehicles 5 to 7 years Internal use software 3 to 10 years Finance leases Lease term or estimated useful life of the asset Leasehold improvements Lesser of lease term or estimated useful life of the improvement Buildings Lesser of lease term or 30 years Year Ended December 31, 2020 2019 2018 (in thousands) Depreciation $ 179,470 $ 155,326 $ 129,412 Amortization of heavy maintenance 88,927 63,364 41,286 Amortization of capitalized software 10,191 6,574 6,029 Total depreciation and amortization $ 278,588 $ 225,264 $ 176,727
Primary Components of Non-fare Revenue and the Revenue Recognition Method Utilized for Each Service or Product	The following table summarizes the primary components of the Company's non-fare revenue and the revenue recognition method utilized for each service or product: Year Ended December 31, Non-fare revenue Recognition method 2020 2019 2018 (in thousands) Baggage Time of departure $ 404,896 $ 734,243 $ 620,154 Passenger usage fee Time of departure 358,561 669,177 531,459 Advance seat selection Time of departure 125,213 228,876 180,012 Other 120,638 238,454 224,283 Non-fare revenue $ 1,009,308 $ 1,870,750 $ 1,555,908
Total Cash Proceeds Received from the Sale of Mileage Credit	The following table illustrates total cash proceeds received from the sale of mileage credits and the portion of such proceeds recognized in non-ticket revenue immediately as marketing component: Consideration received from credit card mile programs Portion of proceeds recognized immediately as marketing component Year Ended (in thousands) December 31, 2020 $ 33,201 $ 25,918 December 31, 2019 48,136 37,151 December 31, 2018 39,194 30,353
Aircraft Maintenance Expense	The table below summarizes the components of the Company’s maintenance cost: Year Ended December 31, 2020 2019 2018 (in thousands) Flight hour-based maintenance expense $ 52,092 $ 78,253 $ 68,039 Non-flight hour-based maintenance expense 59,135 65,322 61,039 Total maintenance, materials and repairs $ 111,227 $ 143,575 $ 129,078

Revenue Disaggregation (Tables)

Revenue Disaggregation (Tables)	12 Months Ended
	Dec. 31, 2020
Revenue from Contract with Customer [Abstract]
Schedule of Revenue Disaggregation	The following table shows disaggregated operating revenues for the twelve months ended December 31, 2020, 2019 and 2018. Twelve Months Ended December 31, 2020 2019 2018 (in thousands) Operating revenues: Fare $ 756,225 $ 1,886,855 $ 1,704,107 Non-fare 1,009,308 1,870,750 1,555,908 Total passenger revenues 1,765,533 3,757,605 3,260,015 Other 44,489 72,931 63,019 Total operating revenues $ 1,810,022 $ 3,830,536 $ 3,323,034

Accrued Liabilities (Tables)

Accrued Liabilities (Tables)	12 Months Ended
	Dec. 31, 2020
Payables and Accruals [Abstract]
Accrued liabilities included in other current liabilities	Accrued liabilities included in other current liabilities as of December 31, 2020 and 2019 consist of the following: As of December 31, 2020 2019 (in thousands) Salaries, wages and benefits $ 112,838 $ 89,163 Airport obligations 68,677 80,134 Aircraft and facility lease obligations 67,374 20,656 Interest payable 37,202 16,941 Federal excise and other passenger taxes and fees payable 36,884 65,312 Aircraft maintenance 27,466 38,099 Fuel 11,704 28,510 Other 31,469 34,706 Other current liabilities $ 393,614 $ 373,521

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2020
Share-based Payment Arrangement [Abstract]
Summary of the Status of the Company's Restricted Stock Shares	A summary of the status of the Company’s restricted stock shares (restricted stock awards and restricted stock unit awards) as of December 31, 2020 and changes during the year ended December 31, 2020 is presented below: Number of Shares Weighted-Average Outstanding at December 31, 2019 332,966 49.84 Granted 222,401 35.48 Vested (146,096) 50.31 Forfeited (8,795) 47.86 Outstanding at December 31, 2020 400,476 41.74
Multiple Input Variables of Performance Share Awards	The inputs utilized for the performance share awards based on total shareholder return are as follows: Weighted-Average at Grant Date for Twelve Months Ended December 31, 2020 Weighted-Average at Grant Date for Twelve Months Ended December 31, 2019 Expected volatility factor 0.40 0.38 Risk free interest rate 1.58 % 2.50 % Expected term (in years) 2.96 2.97 Expected dividend yield — % — %
Performance Share Award Activity	The following table summarizes the Company’s market condition performance share awards for the year ended December 31, 2020: Number of Awards Weighted-Average Fair Value at Grant Date ($) Outstanding at December 31, 2019 96,159 61.58 Granted 36,328 47.43 Vested (47,240) 52.07 Forfeited (336) 47.43 Outstanding at December 31, 2020 84,911 60.88 The following table summarizes the Company’s performance condition performance share awards for the year ended December 31, 2020: Number of Awards Weighted-Average Fair Value at Grant Date ($) Outstanding at December 31, 2019 47,794 53.24 Granted 72,593 40.11 Vested (23,620) 46.21 Forfeited (669) 40.11 Outstanding at December 31, 2020 96,098 45.14

Earnings (Loss) per Share (Tabl

Earnings (Loss) per Share (Tables)	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
Computation of basic and diluted earnings per common share	The following table sets forth the computation of basic and diluted earnings per common share: Year Ended December 31, 2020 2019 2018 (in thousands, except per-share amounts) Numerator: Net income (loss) $ (428,700) $ 335,255 $ 155,749 Denominator: Weighted-average shares outstanding, basic 84,692 68,429 68,249 Effect of dilutive stock awards — 130 182 Adjusted weighted-average shares outstanding, diluted 84,692 68,559 68,431 Earnings (Loss) per Share: Basic earnings (loss) per common share $ (5.06) $ 4.90 $ 2.28 Diluted earnings (loss) per common share $ (5.06) $ 4.89 $ 2.28 Anti-dilutive weighted-average shares 441 143 145

Debt and Other Obligations (Tab

Debt and Other Obligations (Tables)	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
Schedule of Long-term Debt Instruments	Long-term debt is comprised of the following: As of December 31, 2020 December 31, 2019 December 31, 2020 December 31, 2019 (in millions) (weighted-average interest rates) 8.00% senior secured notes due in 2025 $ 850.0 $ — 8.00 % N/A Fixed-rate term loans due through 2032 1,301.9 1,074.3 3.36 % 3.79 % Unsecured term loans due in 2030 73.3 — 1.00 % N/A Fixed-rate class A 2015-1 EETC due through 2028 322.6 348.6 4.10 % 4.10 % Fixed-rate class B 2015-1 EETC due through 2024 64.0 72.0 4.45 % 4.45 % Fixed-rate class C 2015-1 EETC due through 2023 86.6 98.1 4.93 % 4.93 % Fixed-rate class AA 2017-1 EETC due through 2030 214.4 228.4 3.38 % 3.38 % Fixed-rate class A 2017-1 EETC due through 2030 71.5 76.1 3.65 % 3.65 % Fixed-rate class B 2017-1 EETC due through 2026 60.6 70.6 3.80 % 3.80 % Fixed-rate class C 2017-1 EETC due through 2023 85.5 85.5 5.11 % 5.11 % Convertible debt due in 2025 175.0 — 4.75 % N/A Revolving credit facility due in 2021 95.1 160.0 1.55 % 3.12 % Revolving credit facility due in 2022 180.0 — 2.15 % N/A Long-term debt $ 3,580.5 $ 2,213.6 Less current maturities 383.5 214.0 Less unamortized discount, net 131.4 40.4 Total $ 3,065.6 $ 1,959.2
Schedule of Maturities of Long-term Debt	At December 31, 2020, long-term debt principal payments for the next five years and thereafter are as follows: December 31, 2020 (in millions) 2021 $ 290.0 2022 372.0 2023 335.5 2024 221.0 2025 1,212.2 2026 and beyond 1,149.8 Total debt principal payments $ 3,580.5
Schedule of Interest Expense on Long-term Debt and Capital Leases	Interest expense related to long-term debt and finance leases consists of the following: Year Ended December 31, 2020 2019 (in thousands) 8.00% senior secured notes (1) $ 19,953 $ — Fixed-rate senior term loans 43,591 41,053 Fixed-rate junior term loans — 1,811 Unsecured term loans 409 — Class A 2015-1 EETC 13,730 14,894 Class B 2015-1 EETC 3,027 3,377 Class C 2015-1 EETC 4,565 5,117 Class AA 2017-1 EETC 7,412 7,887 Class A 2017-1 EETC 2,672 2,843 Class B 2017-1 EETC 2,445 2,870 Class C 2017-1 EETC 4,379 4,367 Convertible debt (2) 14,905 — Revolving credit facilities 5,380 5,792 Finance leases 195 408 Commitment and other fees 1,106 2,217 Amortization of deferred financing costs 10,751 8,714 Total $ 134,520 $ 101,350 (1) Includes $0.5 million of accretion and $19.5 million of interest expense for the twelve months ended December 31, 2020. (2) Includes $9.6 million of accretion and $5.3 million of interest expense for the twelve months ended December 31, 2020.

Leases and Aircraft Maintenan_2

Leases and Aircraft Maintenance Deposits (Tables)	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Schedule of Future Minimum Lease Payments for Finance Leases	The following table provides details of the Company's future minimum lease payments under finance lease liabilities and operating lease liabilities recorded on the Company's consolidated balance sheets as of December 31, 2020. The table does not include commitments that are contingent on events or other factors that are currently uncertain and unknown. Finance Leases Operating Leases Total Operating and Finance Lease Obligations Aircraft and Spare Engine Leases Property Facility Leases (in thousands) 2021 (1) $ 753 $ 236,101 $ 4,683 $ 241,537 2022 725 209,911 4,397 215,033 2023 349 197,267 3,383 200,999 2024 98 175,204 2,772 178,074 2025 — 153,146 1,060 154,206 2026 and thereafter — 975,682 143,093 1,118,775 Total minimum lease payments $ 1,925 $ 1,947,311 $ 159,388 $ 2,108,624 Less amount representing interest 137 562,493 134,628 697,258 Present value of minimum lease payments $ 1,788 $ 1,384,818 $ 24,760 $ 1,411,366 Less current portion 671 130,484 3,307 134,462 Long-term portion $ 1,117 $ 1,254,334 $ 21,453 $ 1,276,904
Schedule of Future Minimum Lease Payments for Operating Leases	The following table provides details of the Company's future minimum lease payments under finance lease liabilities and operating lease liabilities recorded on the Company's consolidated balance sheets as of December 31, 2020. The table does not include commitments that are contingent on events or other factors that are currently uncertain and unknown. Finance Leases Operating Leases Total Operating and Finance Lease Obligations Aircraft and Spare Engine Leases Property Facility Leases (in thousands) 2021 (1) $ 753 $ 236,101 $ 4,683 $ 241,537 2022 725 209,911 4,397 215,033 2023 349 197,267 3,383 200,999 2024 98 175,204 2,772 178,074 2025 — 153,146 1,060 154,206 2026 and thereafter — 975,682 143,093 1,118,775 Total minimum lease payments $ 1,925 $ 1,947,311 $ 159,388 $ 2,108,624 Less amount representing interest 137 562,493 134,628 697,258 Present value of minimum lease payments $ 1,788 $ 1,384,818 $ 24,760 $ 1,411,366 Less current portion 671 130,484 3,307 134,462 Long-term portion $ 1,117 $ 1,254,334 $ 21,453 $ 1,276,904
Schedule of Lease Costs, Lease Terms and Discount Rates	The table below presents information for lease costs related to the Company's finance and operating leases: Year Ended December 31, 2020 2019 (in thousands) Finance lease cost Amortization of leased assets $ 824 $ 998 Interest of lease liabilities 194 674 Operating lease cost Operating lease cost (1) 201,474 179,959 Short-term lease cost (1) 25,195 5,144 Variable lease cost (1) 125,534 140,417 Total lease cost $ 353,221 $ 327,192 (1) Expenses are classified within aircraft rent and landing fees and other rents on the Company's consolidated statements of operations. The table below presents lease-related terms and discount rates as of December 31, 2020: December 31, 2020 December 31, 2019 Weighted-average remaining lease term Operating leases 12.9 years 13.0 years Finance leases 2.6 years 0.1 years Weighted-average discount rate Operating leases 6.08 % 5.86 % Finance leases 5.54 % 2.46 %

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Significant Components of the Provision for Income Taxes from Continuing Operations	Significant components of the provision for income taxes from continuing operations are as follows: Year Ended December 31, 2020 2019 2018 (in thousands) Current: Federal $ (141,997) $ (22,429) $ (2,178) State and local (1,847) 1,218 410 Foreign (1,554) 6,693 4,692 Total current expense (benefit) (145,398) (14,518) 2,924 Deferred: Federal (33,494) 106,703 42,246 State and local (12,592) 8,986 4,057 Total deferred expense (benefit) (46,086) 115,689 46,303 Total income tax expense (benefit) $ (191,484) $ 101,171 $ 49,227
Reconciliation of Income Tax Expense	The income tax provision differs from that computed at the federal statutory corporate tax rate as follows: Year Ended December 31, 2020 2019 2018 Expected provision at federal statutory tax rate 21.0 % 21.0 % 21.0 % State tax expense, net of federal benefit 1.9 % 1.8 % 1.7 % Revaluation of deferred taxes 9.2 % (2.1) % — % Other (1.2) % 2.5 % 1.3 % Total income tax expense (benefit) 30.9 % 23.2 % 24.0 %
Schedule of Deferred Taxes	At December 31, 2020 and 2019, the significant components of the Company's deferred taxes consisted of the following: December 31, 2020 2019 (in thousands) Deferred tax assets: Income tax credits 4,298 9,632 Net operating losses 220,071 13,604 Deferred revenue 11,740 8,824 Nondeductible accruals 21,918 14,133 Deferred manufacturing credits 6,442 2,813 Accrued maintenance 568 1,668 Equity compensation 3,433 2,851 Operating lease liability 313,142 305,161 Other 465 482 Valuation allowance (2,949) (1,746) Deferred tax assets $ 579,128 $ 357,422 Deferred tax liabilities: Convertible debt 14,942 — Prepaid expenses 898 1,120 Property, plant and equipment 603,173 430,523 Deferred financing costs 124 154 Accrued aircraft and engine maintenance 80,916 84,479 Right-of-use asset 318,969 310,438 Deferred tax liabilities 1,019,022 826,714 Net deferred tax assets (liabilities) $ (439,894) $ (469,292)

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Schedules of Concentration of Risk, by Risk Factor	The table below sets forth the Company's employee groups and status of the collective bargaining agreements as of December 31, 2020. Employee Groups Representative Amendable Date Percentage of Workforce Pilots Air Line Pilots Association, International (ALPA) February 2023 29% Flight Attendants Association of Flight Attendants (AFA-CWA) May 2021 46% Dispatchers Professional Airline Flight Control Association (PAFCA) October 2023 1% Ramp Service Agents International Association of Machinists and Aerospace Workers (IAMAW) June 2020 3% Passenger Service Agents Transport Workers Union of America (TWU) NA 3%

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Long-term debt measured at fair value	The carrying amounts and estimated fair values of the Company's long-term debt at December 31, 2020 and December 31, 2019, were as follows: As of December 31, 2020 2019 Carrying Value Estimated Fair Value Carrying Value Estimated Fair Value Fair value level hierarchy (in millions) 8.00% senior secured notes $ 850.0 $ 886.0 $ — $ — Level 3 Fixed-rate term loans 1,301.9 1,362.9 1,074.3 1,120.0 Level 3 Unsecured term loans 73.3 83.1 — — Level 3 2015-1 EETC Class A 322.6 323.4 348.6 372.2 Level 2 2015-1 EETC Class B 64.0 62.5 72.0 74.5 Level 2 2015-1 EETC Class C 86.6 77.8 98.1 100.5 Level 2 2017-1 EETC Class AA 214.4 207.4 228.4 237.0 Level 2 2017-1 EETC Class A 71.5 68.8 76.1 78.8 Level 2 2017-1 EETC Class B 60.6 56.2 70.6 72.0 Level 2 2017-1 EETC Class C 85.5 76.3 85.5 88.0 Level 2 Convertible debt 175.0 380.3 — — Level 2 Revolving credit facilities 275.1 275.1 160.0 160.0 Level 3 Total long-term debt $ 3,580.5 $ 3,859.8 $ 2,213.6 $ 2,303.0
Assets and liabilities measured at fair value on a recurring basis	Assets and liabilities measured at gross fair value on a recurring basis are summarized below: Fair Value Measurements as of December 31, 2020 Total Level Level Level (in millions) Cash and cash equivalents $ 1,789.7 $ 1,789.7 $ — $ — Restricted cash 71.4 71.4 — — Short-term investment securities 106.3 106.3 — — Assets held for sale 2.3 — — 2.3 Total assets $ 1,969.7 $ 1,967.4 $ — $ 2.3 Total liabilities $ — $ — $ — $ — Fair Value Measurements as of December 31, 2019 Total Level Level Level (in millions) Cash and cash equivalents $ 979.0 $ 979.0 $ — $ — Restricted cash — — — — Short-term investment securities 105.3 105.3 — — Assets held for sale 2.3 — — 2.3 Total assets $ 1,086.6 $ 1,084.3 $ — $ 2.3 Total liabilities $ — $ — $ — $ — Assets Held for Sale Activity for the Twelve Months Ended December 31, 2019 (in millions) Balance at December 31, 2018 $ — Purchases 5.4 Sales — Total realized or unrealized gains (losses) included in earnings, net (3.1) Balance at December 31, 2019 $ 2.3

Operating Segments and Relate_2

Operating Segments and Related Disclosures (Tables)	12 Months Ended
	Dec. 31, 2020
Segment Reporting [Abstract]
Operating revenues by geographic region	Operating revenues by geographic region as defined by the Department of Transportation ("DOT") area are summarized below: 2020 2019 2018 (in millions) DOT—Domestic $ 1,660.7 $ 3,462.8 $ 2,990.7 DOT—Latin America and Caribbean 149.3 367.7 332.3 Total $ 1,810.0 $ 3,830.5 $ 3,323.0

Quarterly Financial Data (Una_2

Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2020
Quarterly Financial Information Disclosure [Abstract]
Quarterly results of operations	Quarterly results of operations for the years ended December 31, 2020 and 2019 are summarized below: Three Months Ended March 31 June 30 September 30 December 31 (in thousands, except per-share amounts) 2020 Operating revenue $ 771,081 $ 138,529 $ 401,922 $ 498,490 Operating income (loss) (57,992) (190,384) (99,471) (159,915) Net income (loss) (27,828) (144,428) (99,140) (157,304) Basic earnings (loss) per share (0.41) (1.81) (1.07) (1.61) Diluted earnings (loss) per share (0.41) (1.81) (1.07) (1.61) 2019 Operating revenue $ 855,796 $ 1,012,956 $ 991,968 $ 969,816 Operating income 87,804 163,938 124,681 124,624 Net income 56,076 114,501 83,464 81,214 Basic earnings per share 0.82 1.67 1.22 1.19 Diluted earnings per share 0.82 1.67 1.22 1.18

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Basis of Presentation (Details) - segment	12 Months Ended
	Dec. 31, 2020	Sep. 17, 2020
Debt Instrument [Line Items]
Number of reportable segments	1
8.00% Senior Secured Notes Due 2025 | 8.00% senior secured notes due in 2025
Debt Instrument [Line Items]
Stated interest rate percentage		8.00%

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Property and Equipment (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)aircraft_EngineflightSimulatoraircraft	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Property, Plant and Equipment [Line Items]
Number of spare engines capitalized | aircraft_Engine	16
Number of flight simulators capitalized | flightSimulator	1
Depreciation, Depletion and Amortization, Nonproduction [Abstract]
Depreciation	$ 179,470	$ 155,326	$ 129,412
Amortization of capitalized software	10,191	6,574	6,029
Total depreciation and amortization	$ 278,588	225,264	176,727
Spare engines
Property, Plant and Equipment [Line Items]
Number of aircraft under operating lease | aircraft_Engine	8
Minimum | Aircraft
Property, Plant and Equipment [Line Items]
Operating leases, term	8 years
Maximum | Aircraft
Property, Plant and Equipment [Line Items]
Operating leases, term	18 years
Aircrafts, Major Spare Rotable Parts, Avionics, and Assemblies
Property, Plant and Equipment [Line Items]
Residual value, percentage	10.00%
Aircraft, engines and flight simulators
Property, Plant and Equipment [Line Items]
Depreciable lives used for the principal depreciable asset classifications	25 years
Spare rotables and flight assemblies | Minimum
Property, Plant and Equipment [Line Items]
Depreciable lives used for the principal depreciable asset classifications	7 years
Spare rotables and flight assemblies | Maximum
Property, Plant and Equipment [Line Items]
Depreciable lives used for the principal depreciable asset classifications	25 years
Other equipment and vehicles | Minimum
Property, Plant and Equipment [Line Items]
Depreciable lives used for the principal depreciable asset classifications	5 years
Other equipment and vehicles | Maximum
Property, Plant and Equipment [Line Items]
Depreciable lives used for the principal depreciable asset classifications	7 years
Internal use software
Depreciation, Depletion and Amortization, Nonproduction [Abstract]
Capitalized computer software, net	$ 24,300	13,000
Capitalized software costs during the year	$ 21,500	$ 5,900	$ 12,000
Internal use software | Minimum
Property, Plant and Equipment [Line Items]
Depreciable lives used for the principal depreciable asset classifications	3 years
Internal use software | Maximum
Property, Plant and Equipment [Line Items]
Depreciable lives used for the principal depreciable asset classifications	10 years
Spare engines | Minimum
Property, Plant and Equipment [Line Items]
Operating leases, term	12 years
Spare engines | Maximum
Property, Plant and Equipment [Line Items]
Operating leases, term	16 years
Buildings
Property, Plant and Equipment [Line Items]
Depreciable lives used for the principal depreciable asset classifications	30 years
Aircraft | Minimum
Property, Plant and Equipment [Line Items]
Finance lease contract term	8 years
Aircraft | Maximum
Property, Plant and Equipment [Line Items]
Finance lease contract term	18 years
A320 Family | Aircraft
Property, Plant and Equipment [Line Items]
Number of aircraft under operating lease | aircraft	56
A320 Family | Aircraft
Property, Plant and Equipment [Line Items]
Number of aircraft capitalized | aircraft	101

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Passenger Revenues (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Air traffic liability	$ 401,966				$ 315,408				$ 401,966	$ 315,408
Total operating revenues	$ 498,490	$ 401,922	$ 138,529	$ 771,081	$ 969,816	$ 991,968	$ 1,012,956	$ 855,796	$ 1,810,022	3,830,536	$ 3,323,034
Credit shell term of expiration									60 days
Baggage
Disaggregation of Revenue [Line Items]
Total operating revenues									$ 404,896	734,243	620,154
Passenger usage fee
Disaggregation of Revenue [Line Items]
Total operating revenues									358,561	669,177	531,459
Advance seat selection
Disaggregation of Revenue [Line Items]
Total operating revenues									125,213	228,876	180,012
Other
Disaggregation of Revenue [Line Items]
Total operating revenues									120,638	238,454	224,283
Non-fare revenue
Disaggregation of Revenue [Line Items]
Total operating revenues									$ 1,009,308	$ 1,870,750	$ 1,555,908

Summary of Significant Accoun_7

Summary of Significant Accounting Policies - Frequent Flier Program (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)revenue_arrangement	Dec. 31, 2019USD ($)
Accounting Policies [Abstract]
Frequent flyer program, number of types of revenue arrangements | revenue_arrangement	2
Air traffic liability	$ 401,966	$ 315,408
Unrecognized revenue from future FREE SPIRIT award redemptions and the sale of mileage credits	$ 31,600	$ 29,800

Summary of Significant Accoun_8

Summary of Significant Accounting Policies - Cash Proceeds Received from the Sale of Mileage Credits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Consideration received from credit card mile programs	$ 33,201	$ 48,136	$ 39,194
Portion of proceeds recognized immediately as marketing component	$ 25,918	$ 37,151	$ 30,353

Summary of Significant Accoun_9

Summary of Significant Accounting Policies - Maintenance Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Deferred costs for heavy maintenance	$ 75,200	$ 176,000	$ 190,500
Deferred heavy maintenance, gross	570,600	504,200
Accumulated heavy maintenance amortization	680,230	492,447
Airframe and Engine Maintenance Costs [Abstract]
Total maintenance, materials and repairs	111,227	143,575	129,078
Heavy maintenance and major overhaul
Property, Plant and Equipment [Line Items]
Amortization of heavy maintenance	88,927	63,364	41,286
Heavy Maintenance
Property, Plant and Equipment [Line Items]
Accumulated heavy maintenance amortization	222,700	142,600
Flight hour-based maintenance expense
Airframe and Engine Maintenance Costs [Abstract]
Total maintenance, materials and repairs	52,092	78,253	68,039
Non-flight hour-based maintenance expense
Airframe and Engine Maintenance Costs [Abstract]
Total maintenance, materials and repairs	$ 59,135	$ 65,322	$ 61,039

Summary of Significant Accou_10

Summary of Significant Accounting Policies - Advertising (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Marketing and advertising expenses	$ 5.5	$ 6.3	$ 6.3

Summary of Significant Accou_11

Summary of Significant Accounting Policies - Income Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Valuation allowance	$ 2,949	$ 1,746

Summary of Significant Accou_12

Summary of Significant Accounting Policies - Concentrations of Risk (Details) - employee_group	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Total operating expenses | Aircraft fuel expenditure concentration risk
Concentration Risk [Line Items]
Concentration of risk	19.00%	30.00%	32.00%
Number of employees, total | Unionized employees concentration risk
Concentration Risk [Line Items]
Concentration of risk	82.00%
Union-represented employee groups	5

Impact of COVID-19 - Capacity R

Impact of COVID-19 - Capacity Reductions Narrative (Details)	1 Months Ended
	Dec. 31, 2020	Nov. 30, 2020	May 31, 2020
COVID-19 Pandemic
Unusual or Infrequent Item, or Both [Line Items]
Capacity reduction in current year compared to prior year	20.10%	20.80%	94.00%

Impact of COVID-19 - COVID-19 L

Impact of COVID-19 - COVID-19 Legislation Narrative (Details) - USD ($)	Feb. 10, 2021	Dec. 31, 2020	Jan. 31, 2021	Dec. 31, 2020	Jul. 31, 2020	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Apr. 20, 2020
Unusual or Infrequent Item, or Both [Line Items]
Special charges (credits)							$ (302,761,000)	$ 717,000	$ 88,921,000
Tax loss carrybacks due to COVID-19							142,000,000
Federal fuel tax savings due to COVID-19							6,500,000
Deferred payment of employer portion of social security taxes from CARES Act		$ 23,200,000		$ 23,200,000			23,200,000
Unsecured term loans due in 2030
Unusual or Infrequent Item, or Both [Line Items]
Proceeds received from Payroll Support Program					$ 334,700,000	$ 344,400,000
Special charges (credits)							$ (266,800,000)
CARES Act
Unusual or Infrequent Item, or Both [Line Items]
Shares of common stock subject to warrants issued to the United States Treasury (in shares)										520,797
Strike price (in dollars per share)		$ 14.08		$ 14.08			$ 14.08			$ 14.08
Warrants, fair value										$ 3,900,000
Number of warrants issued as a percent of outstanding stock		1.00%		1.00%			1.00%
Payroll Support Program, Grant
Unusual or Infrequent Item, or Both [Line Items]
Proceeds received from Payroll Support Program						267,200,000
Payroll Support Program, Grant | Subsequent Event
Unusual or Infrequent Item, or Both [Line Items]
Proceeds received from Payroll Support Program			$ 92,200,000
Payroll Support Program 2 (PSP2)
Unusual or Infrequent Item, or Both [Line Items]
Proceeds expected from Payroll Support Program				$ 184,500,000
CARES Act Employee Retention
Unusual or Infrequent Item, or Both [Line Items]
Special charges (credits)							$ (38,500,000)
Payroll Support Program, Low-Interest Loan
Unusual or Infrequent Item, or Both [Line Items]
Number of warrants issued as a percent of outstanding stock		1.00%		1.00%			1.00%
Payroll Support Program | Payroll Support Program, Low-Interest Loan
Unusual or Infrequent Item, or Both [Line Items]
Debt face amount					$ 70,400,000	$ 2,900,000				$ 73,300,000
Debt term						10 years
Proceeds expected from Payroll Support Program		$ 25,000,000
Forecast | Payroll Support Program 2 (PSP2)
Unusual or Infrequent Item, or Both [Line Items]
Shares of common stock subject to warrants issued to the United States Treasury (in shares)	103,761
Strike price (in dollars per share)	$ 24.42
Forecast | Payroll Support Program | Payroll Support Program, Low-Interest Loan
Unusual or Infrequent Item, or Both [Line Items]
Debt term	10 years

Impact of COVID-19 - Income Tax

Impact of COVID-19 - Income Taxes Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Unusual or Infrequent Item, or Both [Line Items]
Effective income tax rate	30.90%	23.20%	24.00%
Effective income tax rate with discrete income tax benefit removed	21.80%
CARES Act
Unusual or Infrequent Item, or Both [Line Items]
Discrete income tax benefit due to CARES Act	$ 56.1

Impact of COVID-19 - Balance Sh

Impact of COVID-19 - Balance Sheet, Cash Flow and Liquidity Narrative (Details)	May 12, 2020USD ($)$ / sharesshares	Sep. 30, 2020shares	Dec. 31, 2020USD ($)aircraft_Engineaircraft	Mar. 31, 2020USD ($)	Dec. 31, 2020USD ($)aircraft_Engineaircraft	Dec. 31, 2020USD ($)aircraftaircraft_Engineshares	Mar. 31, 2021USD ($)	Sep. 17, 2020USD ($)	Jul. 22, 2020shares	Jun. 30, 2020USD ($)	Mar. 30, 2020USD ($)	Mar. 29, 2020$ / shares	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Unusual or Infrequent Item, or Both [Line Items]
Cash, cash equivalents and short-term investments			$ 1,896,100,000		$ 1,896,100,000	$ 1,896,100,000
Long-term debt			$ 3,580,500,000		$ 3,580,500,000	3,580,500,000
Equity component value of convertible debt issuance, net						$ 55,590,000
Purchase price (in dollars per share) | $ / shares												$ 60
Number of aircraft delivered | aircraft						12
Number of aircraft with pre-delivery deposit payments scheduled for delivery | aircraft			16		16	16
Debt discount			$ 131,400,000		$ 131,400,000	$ 131,400,000							$ 40,400,000
A320NEO
Unusual or Infrequent Item, or Both [Line Items]
Number of aircraft with secured debt financing commitments scheduled for delivery, pledged as collateral | aircraft_Engine			11		11	11
Over-Allotment Option | Common Stock
Unusual or Infrequent Item, or Both [Line Items]
Number of shares sold (in shares) | shares	2,625,000
At-The-Market Offering Program
Unusual or Infrequent Item, or Both [Line Items]
Number of shares sold (in shares) | shares		9,000,000				9,000,000
Consideration received on sale of shares					$ 156,700,000
Payments of stock issuance costs					5,000,000
Shares available for sale (in shares) | shares									9,000,000
Public Offering | Common Stock
Unusual or Infrequent Item, or Both [Line Items]
Number of shares sold (in shares) | shares	20,125,000
Consideration received on sale of shares	$ 192,400,000
Purchase price (in dollars per share) | $ / shares	$ 10
Payments of stock issuance costs	$ 8,900,000
2020 Revolving Credit Facility | Line of Credit | Revolving Credit Facility
Unusual or Infrequent Item, or Both [Line Items]
Maximum borrowing capacity										$ 180,000,000	$ 110,000,000
Proceeds from lines of credit						$ 180,000,000
4.75% Convertible Senior Notes | Convertible Debt
Unusual or Infrequent Item, or Both [Line Items]
Debt face amount	$ 175,000,000
Stated interest rate percentage	4.75%
Proceeds from convertible debt	$ 168,300,000
Payments of financing costs	$ 6,700,000
Long-term debt			$ 95,600,000		95,600,000	95,600,000
Debt issuance costs, net			3,800,000		3,800,000	3,800,000
Equity component value of convertible debt issuance, net						72,700,000
Payment of stock and warrants issuance costs						2,900,000
2020 Revolving Credit Facility | Line of Credit | Revolving Credit Facility
Unusual or Infrequent Item, or Both [Line Items]
Maximum borrowing capacity														$ 160,000,000
Proceeds from lines of credit				$ 160,000,000
2020 Amended Revolving Credit Facility, Due 2021 | Line of Credit | Revolving Credit Facility
Unusual or Infrequent Item, or Both [Line Items]
Maximum borrowing capacity			95,100,000		95,100,000	95,100,000				$ 111,200,000
Proceeds from lines of credit						95,100,000
8.00% Senior Secured Notes Due 2025 | 8.00% senior secured notes due in 2025
Unusual or Infrequent Item, or Both [Line Items]
Debt face amount								$ 850,000,000
Stated interest rate percentage								8.00%
Debt issuance costs, net			17,400,000		17,400,000	17,400,000
Proceeds from issuance of secured debt			823,900,000
Debt discount			$ 8,700,000		$ 8,700,000	$ 8,700,000
Subsequent Event | 2020 Revolving Credit Facility | Line of Credit | Revolving Credit Facility
Unusual or Infrequent Item, or Both [Line Items]
Maximum borrowing capacity							$ 54,000,000

Impact of COVID-19 - Workforce

Impact of COVID-19 - Workforce Reduction Narrative (Details) $ in Thousands	1 Months Ended	2 Months Ended	6 Months Ended		12 Months Ended
	Jul. 31, 2020employee	Oct. 02, 2020	Dec. 31, 2020USD ($)	Sep. 30, 2020employee	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Unusual or Infrequent Item, or Both [Line Items]
Special charges (credits) | $					$ (302,761)	$ 717	$ 88,921
COVID-19 Pandemic
Unusual or Infrequent Item, or Both [Line Items]
Involuntary terminated employees reduction due to voluntary leave program, percent (more than)		95.00%
Special charges (credits) | $			$ 2,500
COVID-19 Pandemic | U.S. Based Union Employees
Unusual or Infrequent Item, or Both [Line Items]
Expected number of positions to be eliminated | employee	2,500
Number of unionized employees involuntary furloughed | employee				0

Recent Accounting Developments

Recent Accounting Developments Recent Accounting Developments (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Jan. 01, 2020	Dec. 31, 2019
Adjustments for New Accounting Pronouncements [Line Items]
Retained earnings	$ 1,524,878		$ 1,955,187
Cumulative Effect, Period of Adoption, Adjustment | Accounting Standards Update 2016-13
Adjustments for New Accounting Pronouncements [Line Items]
Retained earnings		$ (1,600)

Revenue Disaggregation (Details

Revenue Disaggregation (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 498,490	$ 401,922	$ 138,529	$ 771,081	$ 969,816	$ 991,968	$ 1,012,956	$ 855,796	$ 1,810,022	$ 3,830,536	$ 3,323,034
Total value of refunds issued									$ 183,600
Maximum
Disaggregation of Revenue [Line Items]
Expiration period on credit shells									12 months
Minimum
Disaggregation of Revenue [Line Items]
Expiration period on credit shells									60 days
Passenger
Disaggregation of Revenue [Line Items]
Total operating revenues									$ 1,765,533	3,757,605	3,260,015
Fare
Disaggregation of Revenue [Line Items]
Total operating revenues									756,225	1,886,855	1,704,107
Non Fare
Disaggregation of Revenue [Line Items]
Total operating revenues									1,009,308	1,870,750	1,555,908
Other
Disaggregation of Revenue [Line Items]
Total operating revenues									$ 44,489	$ 72,931	$ 63,019

Special Charges and Credits - N

Special Charges and Credits - Narrative (Details)	1 Months Ended	6 Months Ended	12 Months Ended
	Jan. 31, 2020aircraft	Dec. 31, 2020USD ($)	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)aircraft
Property, Plant and Equipment [Line Items]
Special charges (credits)			$ (302,761,000)	$ 717,000	$ 88,921,000
Special charges, non-operating			0	$ 0	$ 90,357,000
Payroll Support Program
Property, Plant and Equipment [Line Items]
Special charges (credits)			(266,800,000)
CARES Act Employee Retention
Property, Plant and Equipment [Line Items]
Special charges (credits)			$ (38,500,000)
COVID-19 Pandemic
Property, Plant and Equipment [Line Items]
Special charges (credits)		$ 2,500,000
Airbus A319
Property, Plant and Equipment [Line Items]
Number of aircraft purchased | aircraft	2
Airbus A319 | Capital Lease Obligations
Property, Plant and Equipment [Line Items]
Number of aircraft purchased | aircraft					14
Consideration transferred					$ 285,000,000
Pilots
Property, Plant and Equipment [Line Items]
Special charges (credits)					88,700,000
Ratification Incentive					80,200,000
Collective bargaining agreement, contractual provision adjustments					$ 8,500,000

Loss on Disposal of Assets - Na

Loss on Disposal of Assets - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)aircraft	Dec. 31, 2019USD ($)aircraft	Dec. 31, 2018USD ($)aircraft_Engine
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Loss on disposal of assets	$ 2,264	$ 17,350	$ 9,580
Capitalized costs for enterprise accounting software write-off	1,500	2,400
Inventory write-down	$ 800	13,400	$ 4,400
Impairment of long-lived assets to be disposed of		3,100
Gain on sale-leaseback transaction		$ 1,500
Number of engines sold | aircraft_Engine			6
2020
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of sale and leaseback transactions | aircraft	1
2019
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of sale and leaseback transactions | aircraft		6
V2500 SelectOne Engine
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from the sale of used engines			$ 11,400
Loss on sale of used engines			$ 5,200

Letters of Credit (Details)

Letters of Credit (Details) - Standby letter of credit facility - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Unsecured Debt
Line of Credit Facility [Line Items]
Letters of credit, limit, amount	$ 30
Letter of credit facility, amount outstanding	$ 23.6
Secured Debt
Line of Credit Facility [Line Items]
Letters of credit, limit, amount		$ 35
Letter of credit facility, amount outstanding		$ 23.3

Credit Card Processing Arrang_2

Credit Card Processing Arrangements (Details) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Restricted Cash and Cash Equivalents Items [Line Items]
Maximum potential exposure to cash holdbacks by the credit card processors	$ 423,700,000	$ 342,300,000
Credit Card Processors
Restricted Cash and Cash Equivalents Items [Line Items]
Cash holdbacks	$ 0	$ 0

Short-term Investment Securit_2

Short-term Investment Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Short-term investment securities	$ 106,339	$ 105,321
Realized gain	3	5
Accumulated other comprehensive income (loss)	$ (618)	$ (787)
Available-for-sale Securities
Debt Securities, Available-for-sale [Line Items]
Weighted-average fixed rate	1.10%	2.30%	1.60%
Unrealized loss on investment securities, net of deferred taxes	$ (73)	$ 104
Deferred taxes on unrealized loss on investment securities	(21)	31
Accumulated other comprehensive income (loss)	$ 31	$ 104

Accrued Liabilities (Details)

Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Payables and Accruals [Abstract]
Salaries, wages and benefits	$ 112,838	$ 89,163
Airport obligations	68,677	80,134
Aircraft and facility lease obligations	67,374	20,656
Interest payable	37,202	16,941
Federal excise and other passenger taxes and fees payable	36,884	65,312
Aircraft maintenance	27,466	38,099
Fuel	11,704	28,510
Other	31,469	34,706
Other current liabilities	$ 393,614	$ 373,521

Common Stock and Preferred St_2

Common Stock and Preferred Stock - Narrative (Details) - $ / shares	Dec. 31, 2020	Mar. 29, 2020	Dec. 31, 2019
Class of Stock [Line Items]
Common stock, shares authorized (in shares)	240,000,000		240,000,000
Common stock, par value (in dollars per share)	$ 0.0001		$ 0.0001
Preferred stock, shares authorized (in shares)	10,000,000
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common Stock, $0.0001 par value
Class of Stock [Line Items]
Common stock, shares authorized (in shares)	240,000,000
Common stock, par value (in dollars per share)	$ 0.0001
Series A Preferred Stock Purchase Rights
Class of Stock [Line Items]
Common stock, shares authorized (in shares)	50,000,000
Common stock, par value (in dollars per share)	$ 0.0001

Common Stock and Preferred St_3

Common Stock and Preferred Stock - Non-Voting Common Stock Narrative (Details) - shares	Dec. 31, 2020	Dec. 31, 2019
Class of Stock [Line Items]
Common stock, shares outstanding (in shares)	97,689,583	68,455,011
Series A Preferred Stock Purchase Rights
Class of Stock [Line Items]
Common stock, shares outstanding (in shares)	0	0

Common Stock and Preferred St_4

Common Stock and Preferred Stock - Preferred Stock Narrative (Details) - $ / shares	Mar. 29, 2020	Dec. 31, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
Preferred stock, shares authorized (in shares)		10,000,000
Preferred stock, shares outstanding (in shares)		0	0
Dividends, preferred stock purchase right per common stock (in shares)	1
Minimum threshold of outstanding common stock purchased without Board Of Director approval subject to penalty, percentage	10.00%
Purchase price (in dollars per share)	$ 60
Preferred stock, par value (in dollars per share)	0.0001	$ 0.0001
Preferred stock, redemption price per share (in dollars per share)	$ 0.001
Threshold of shares owned to qualify as Acquiring Person	10.00%
Quarterly dividend rate (in dollars per share)	$ 0.01
Amount per share received upon liquidation (in dollars per share)	$ 1
Threshold of shares to qualify as Beneficial Owner	50.00%
Percent of assets or earning power sold or transferred, allowed purchase for each right	50.00%

Common Stock and Preferred St_5

Common Stock and Preferred Stock - Warrants Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Dec. 31, 2020	Apr. 20, 2020
Payroll Support Program, Low-Interest Loan
Class of Stock [Line Items]
Number of warrants issued as a percent of outstanding stock	1.00%
CARES Act
Class of Stock [Line Items]
Warrants outstanding (in shares)	520,797
Strike price (in dollars per share)	$ 14.08	$ 14.08
Number of warrants issued as a percent of outstanding stock	1.00%
Adjustments to additional paid in capital, warrant issued	$ 3.9

Common Stock and Preferred St_6

Common Stock and Preferred Stock - Common Stock Offering Narrative (Details) - USD ($) $ / shares in Units, shares in Thousands	May 12, 2020	Dec. 31, 2020	Mar. 29, 2020	Dec. 31, 2019
Class of Stock [Line Items]
Purchase price (in dollars per share)			$ 60
Common stock, value, issued		$ 10,000		$ 7,000
Public Offering
Class of Stock [Line Items]
Common stock, value, issued	$ 2,000
Common Stock | Public Offering
Class of Stock [Line Items]
Number of shares sold (in shares)	20,125
Purchase price (in dollars per share)	$ 10
Consideration received on sale of shares	$ 192,400,000
Payments of stock issuance costs	$ 8,900,000
Common Stock | Over-Allotment Option
Class of Stock [Line Items]
Number of shares sold (in shares)	2,625

Common Stock and Preferred St_7

Common Stock and Preferred Stock - At-the-Market Offering Program Narrative (Details) - USD ($)	2 Months Ended	5 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2020	Jul. 22, 2020	Dec. 31, 2019
Class of Stock [Line Items]
Common stock, value, issued		$ 10,000	$ 10,000		$ 7,000
At-The-Market Offering Program
Class of Stock [Line Items]
Shares available for sale (in shares)				9,000,000
Number of shares sold (in shares)	9,000,000		9,000,000
Consideration received on sale of shares		156,700,000
Payments of stock issuance costs		5,000,000
Common stock, value, issued		$ 900	$ 900

Stock-Based Compensation - Narr

Stock-Based Compensation - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense		$ 11,600	$ 8,200	$ 11,000
Share-based payment arrangement, expense, tax benefit		$ 3,600	$ 1,900	$ 2,600
Shares repurchased (in shares)		44,000	91,000	28,000
Repurchase of common stock		$ 1,669	$ 5,439	$ 1,162
Number of treasury shares retired (in shares)		0	0	0
Equity Incentive Award Plan 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance (in shares)	1,897,809	1,618,417	1,897,809		1,897,809
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Period for share issuance after vesting		30 days
Granted (in shares)		222,401	148,120
Total compensation cost not yet recognized	$ 10,000	$ 10,400	$ 10,000		$ 10,000
Total compensation cost not yet recognized, period for recognition		2 years	2 years 8 months 12 days
Granted, weighted-average grant date fair value (in dollars per share)		$ 35.48	$ 53.41	$ 46.90
Total fair value of shares vested		$ 5,500	$ 5,400	$ 6,500
Restricted Stock | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period		3 years
Restricted Stock | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period		4 years
Performance Shares Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Period for share issuance after vesting		60 days
Total compensation cost not yet recognized	4,400	$ 4,800	$ 4,400		$ 4,400
Total compensation cost not yet recognized, period for recognition		1 year 7 months 28 days	1 year 8 months 23 days
Award requisite service period		3 years
Performance Shares Awards | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Potential vesting percentage		0.00%
Performance Shares Awards | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Potential vesting percentage		200.00%
Market Condition Performance Share Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)		36,328
Granted, weighted-average grant date fair value (in dollars per share)		$ 47.43
Expected dividend yield (percent)		0.00%	0.00%
Stock Appreciation Rights (SARs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense	$ (1,900)	$ 700	$ 1,200
Award requisite service period				4 years
Award performance period				2 years	2 years

Stock-Based Compensation - Rest

Stock-Based Compensation - Restricted Stock Activity (Details) - Restricted Stock - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Outstanding, beginning balance (in shares)	332,966
Granted (in shares)	222,401	148,120
Vested (in shares)	(146,096)
Forfeited (in shares)	(8,795)
Outstanding, ending balance (in shares)	400,476	332,966
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Outstanding, weighted-average grant date fair value at start of period (in dollars per share)	$ 49.84
Granted, weighted-average grant date fair value (in dollars per share)	35.48	$ 53.41	$ 46.90
Vested, weighted-average grant date fair value (in dollars per share)	50.31
Forfeited, weighted-average grant date fair value (in dollars per share)	47.86
Outstanding, weighted-average grant date fair value at end of period (in dollars per share)	$ 41.74	$ 49.84

Stock-Based Compensation - Simu

Stock-Based Compensation - Simulation Model Input Variables (Details) - Market Condition Performance Share Awards	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility factor	0.40%	0.38%
Risk free interest rate (percent)	1.58%	2.50%
Expected term (in years)	2 years 11 months 15 days	2 years 11 months 19 days
Expected dividend yield (percent)	0.00%	0.00%

Stock-Based Compensation - Perf

Stock-Based Compensation - Performance Share Awards (Details)	12 Months Ended
	Dec. 31, 2020$ / sharesshares
Market Condition Performance Share Awards
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Outstanding, beginning balance (in shares) | shares	96,159
Granted (in shares) | shares	36,328
Vested (in shares) | shares	(47,240)
Forfeited (in shares) | shares	(336)
Outstanding, ending balance (in shares) | shares	84,911
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Outstanding, weighted-average grant date fair value at start of period (in dollars per share) | $ / shares	$ 61.58
Granted, weighted-average grant date fair value (in dollars per share) | $ / shares	47.43
Vested, weighted-average grant date fair value (in dollars per share) | $ / shares	52.07
Forfeited, weighted-average grant date fair value (in dollars per share) | $ / shares	47.43
Outstanding, weighted-average grant date fair value at end of period (in dollars per share) | $ / shares	$ 60.88
Performance Condition Performance Share Awards
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Outstanding, beginning balance (in shares) | shares	47,794
Granted (in shares) | shares	72,593
Vested (in shares) | shares	(23,620)
Forfeited (in shares) | shares	(669)
Outstanding, ending balance (in shares) | shares	96,098
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Outstanding, weighted-average grant date fair value at start of period (in dollars per share) | $ / shares	$ 53.24
Granted, weighted-average grant date fair value (in dollars per share) | $ / shares	40.11
Vested, weighted-average grant date fair value (in dollars per share) | $ / shares	46.21
Forfeited, weighted-average grant date fair value (in dollars per share) | $ / shares	40.11
Outstanding, weighted-average grant date fair value at end of period (in dollars per share) | $ / shares	$ 45.14

Earnings (Loss) per Share (Deta

Earnings (Loss) per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Numerator:
Net income (loss)	$ (157,304)	$ (99,140)	$ (144,428)	$ (27,828)	$ 81,214	$ 83,464	$ 114,501	$ 56,076	$ (428,700)	$ 335,255	$ 155,749
Denominator:
Weighted-average shares outstanding, basic (in shares)									84,692	68,429	68,249
Effect of dilutive stock awards (in shares)									0	130	182
Adjusted weighted-average shares outstanding, diluted (in shares)									84,692	68,559	68,431
Earnings (Loss) per Share:
Basic earnings per common share (in dollars per share)	$ (1.61)	$ (1.07)	$ (1.81)	$ (0.41)	$ 1.19	$ 1.22	$ 1.67	$ 0.82	$ (5.06)	$ 4.90	$ 2.28
Diluted earnings per common share (in dollars per share)	$ (1.61)	$ (1.07)	$ (1.81)	$ (0.41)	$ 1.18	$ 1.22	$ 1.67	$ 0.82	$ (5.06)	$ 4.89	$ 2.28
Antidilutive awards excluded from computation of earnings per common share (in shares)									441	143	145

Debt and Other Obligations - Fi

Debt and Other Obligations - Fixed-Rate Term Loans Narrative (Details)	Dec. 31, 2020USD ($)aircraft
Fixed-rate term loans | Minimum
Debt Instrument [Line Items]
Debt term	10 years
Fixed-rate term loans | Maximum
Debt Instrument [Line Items]
Debt term	12 years
Fixed-rate term loans
Debt Instrument [Line Items]
Debt face amount | $	$ 333,000,000
Fixed-rate term loans | Minimum | Fixed-rate term loans
Debt Instrument [Line Items]
Stated interest rate percentage	1.90%
Fixed-rate term loans | Maximum | Fixed-rate term loans
Debt Instrument [Line Items]
Stated interest rate percentage	3.32%
Fixed-rate term loans | A320 | Airbus
Debt Instrument [Line Items]
Number of delivered aircraft with secured debt financing commitments | aircraft	8

Debt and Other Obligations - _2

Debt and Other Obligations - Fixed-Rate Unsecured Term Loans Narrative (Details) - USD ($)	5 Months Ended
	Sep. 30, 2020	Dec. 31, 2020	Jul. 31, 2020	Apr. 20, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Long-term debt, gross		$ 3,580,500,000			$ 2,213,600,000
Payroll Support Program, Low-Interest Loan | Payroll Support Program
Debt Instrument [Line Items]
Debt face amount	$ 2,900,000		$ 70,400,000	$ 73,300,000
Debt term	10 years
Unsecured term loans due in 2030 | Payroll Support Program
Debt Instrument [Line Items]
Debt face amount	$ 9,700,000
Stated interest rate percentage	1.00%
Unsecured term loans due in 2030 | Payroll Support Program | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Debt Instrument [Line Items]
Basis spread on variable rate	2.00%
Unsecured term loans due in 2030 | Unsecured term loans due in 2030
Debt Instrument [Line Items]
Long-term debt, gross		$ 73,300,000

Debt and Other Obligations - Re

Debt and Other Obligations - Revolving Credit Facility Due In 2022 Narrative (Details) - Revolving Credit Facility	Mar. 30, 2020USD ($)	Dec. 31, 2020USD ($)	Jun. 30, 2020USD ($)
Line of Credit
Debt Instrument [Line Items]
Debt instrument covenant, unrestricted cash and cash equivalents and unused commitments on all revolving credit facilities	$ 400,000,000
Debt instrument covenant, maximum Amount of unused commitments from 2020 Credit Facility	$ 200,000,000
Debt covenant, collateral coverage ratio	1
2020 Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 110,000,000		$ 180,000,000
Line of credit facility, optional maximum borrowing capacity subject to consent of lenders	$ 350,000,000
Proceeds from lines of credit		$ 180,000,000
Line of Credit | Maximum | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate	2.00%
Line of Credit | Maximum | Certain Market Interest Rates
Debt Instrument [Line Items]
Basis spread on variable rate	1.00%
Line of Credit | Minimum | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate	0.00%
Line of Credit | Minimum | Certain Market Interest Rates
Debt Instrument [Line Items]
Basis spread on variable rate	0.00%

Debt and Other Obligations - _3

Debt and Other Obligations - Revolving Credit Facility Due In 2021 Narrative (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020USD ($)	Dec. 31, 2020USD ($)aircraft_Engine	Dec. 31, 2021aircraft	Jun. 30, 2020USD ($)	Dec. 31, 2018USD ($)
A320NEO
Debt Instrument [Line Items]
Number of aircraft with secured debt financing commitments scheduled for delivery, pledged as collateral | aircraft_Engine		11
A320NEO | Forecast
Debt Instrument [Line Items]
Number of aircraft with secured debt financing commitments scheduled for delivery | aircraft			43
Revolving Credit Facility | 2020 Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity					$ 160
Proceeds from lines of credit	$ 160
Revolving Credit Facility | 2020 Amended Revolving Credit Facility, Due 2021 | Line of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity		$ 95.1		$ 111.2
Proceeds from lines of credit		$ 95.1

Debt and Other Obligations - Co

Debt and Other Obligations - Convertible Debt Narrative (Details) - USD ($)	May 12, 2020	Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Long-term debt		$ 3,580,500,000
Equity component value of convertible debt issuance, net		55,590,000
Current maturities of long-term debt and finance leases		384,197,000	$ 258,852,000
4.75% Convertible Senior Notes | Convertible Debt
Debt Instrument [Line Items]
Debt face amount	$ 175,000,000
Stated interest rate percentage	4.75%
Debt term	5 years
Proceeds from convertible debt	$ 168,300,000
Payments of financing costs	$ 6,700,000
Long-term debt		95,600,000
Debt issuance costs, net		3,800,000
Equity component value of convertible debt issuance, net		72,700,000
Payment of stock and warrants issuance costs		2,900,000
If-converted value in excess of principal		75,200,000
Conversion price threshold	130.00%
Number of trading days required for conversion price threshold	20 days
Number of consecutive days required for trading days for conversion price threshold	30 days
Number of consecutive business days for conversion price threshold	5 days
Number of consecutive trading days measurement period	5 days
Trading price threshold per principal amount portion	98.00%
Current maturities of long-term debt and finance leases		$ 109,000,000
Shares conversion rate per portion of principal amount (in shares)	78.4314
Principal amount portion for trading price threshold	$ 1,000
Conversion price	$ 12.75

Debt and Other Obligations - 8.

Debt and Other Obligations - 8.00% Senior Secured Notes Due 2025 (Details) - USD ($)	Sep. 17, 2020	Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Less unamortized discount, net		$ 131,400,000	$ 40,400,000
8.00% Senior Secured Notes Due 2025 | 8.00% senior secured notes due in 2025
Debt Instrument [Line Items]
Debt face amount	$ 850,000,000
Stated interest rate percentage	8.00%
Proceeds from issuance of secured debt		823,900,000
Debt issuance costs, net		17,400,000
Less unamortized discount, net		$ 8,700,000
First-priority security interest, equity interests percentage	100.00%
Redemption price, percentage	100.00%
Debt covenant, percent of principal subject to prepayment	100.00%
Debt covenant, percent of principal subject to prepayment upon change of control	101.00%
Debt covenant, maximum pre-paid miles allowed during fiscal year, value	$ 25,000,000
Debt covenant, minimum liquidity requirement	$ 400,000,000

Debt and Other Obligations - Lo

Debt and Other Obligations - Long-term Debt (Details) - USD ($) $ in Millions	Dec. 31, 2020	Sep. 17, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Long-term debt, gross	$ 3,580.5		$ 2,213.6
Long-term debt, current maturities	383.5		214
Less unamortized discount, net	131.4		40.4
Total	$ 3,065.6		1,959.2
8.00% senior secured notes due in 2025
Debt Instrument [Line Items]
Weighted-average interest rates	8.00%
8.00% senior secured notes due in 2025 | 8.00% Senior Secured Notes Due 2025
Debt Instrument [Line Items]
Long-term debt, gross	$ 850
Less unamortized discount, net	8.7
Stated interest rate percentage		8.00%
Fixed-rate term loans
Debt Instrument [Line Items]
Long-term debt, gross	$ 1,301.9		$ 1,074.3
Weighted-average interest rates	3.36%		3.79%
Unsecured term loans due in 2030 | Unsecured term loans due in 2030
Debt Instrument [Line Items]
Long-term debt, gross	$ 73.3
Weighted-average interest rates	1.00%
2015-1 EETC Class A
Debt Instrument [Line Items]
Long-term debt, gross	$ 322.6		$ 348.6
Weighted-average interest rates	4.10%		4.10%
2015-1 EETC Class B
Debt Instrument [Line Items]
Long-term debt, gross	$ 64		$ 72
Weighted-average interest rates	4.45%		4.45%
2015-1 EETC Class C
Debt Instrument [Line Items]
Long-term debt, gross	$ 86.6		$ 98.1
Weighted-average interest rates	4.93%		4.93%
2017-1 EETC Class AA
Debt Instrument [Line Items]
Long-term debt, gross	$ 214.4		$ 228.4
Weighted-average interest rates	3.38%		3.38%
2017-1 EETC Class A
Debt Instrument [Line Items]
Long-term debt, gross	$ 71.5		$ 76.1
Weighted-average interest rates	3.65%		3.65%
2017-1 EETC Class B
Debt Instrument [Line Items]
Long-term debt, gross	$ 60.6		$ 70.6
Weighted-average interest rates	3.80%		3.80%
2017-1 EETC Class C
Debt Instrument [Line Items]
Long-term debt, gross	$ 85.5		$ 85.5
Weighted-average interest rates	5.11%		5.11%
Convertible Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 175
Weighted-average interest rates	4.75%
2020 Amended Revolving Credit Facility, Due 2021
Debt Instrument [Line Items]
Long-term debt, gross	$ 95.1
Weighted-average interest rates	1.55%
2020 Revolving Credit Facility
Debt Instrument [Line Items]
Long-term debt, gross			$ 160
Weighted-average interest rates			3.12%
Revolving Credit Facility Due 2020
Debt Instrument [Line Items]
Long-term debt, gross	$ 180
Weighted-average interest rates	2.15%

Debt and Other Obligations - Ad

Debt and Other Obligations - Additional Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Repayments of long-term debt	$ 254,300,000	$ 246,800,000
Corporate credit cards
Debt Instrument [Line Items]
Maximum borrowing capacity	3,100,000	33,600,000
Line of credit, current	600,000	4,600,000
Lines of Credit with Counterparties for Jet Fuel and Derivatives
Debt Instrument [Line Items]
Maximum borrowing capacity	41,500,000	41,500,000
Aircraft fuel
Debt Instrument [Line Items]
Line of credit, current	$ 3,700,000	$ 25,300,000

Debt and Other Obligations - Fu

Debt and Other Obligations - Future Maturities (Details) $ in Millions	Dec. 31, 2020USD ($)
Debt Disclosure [Abstract]
2021	$ 290
2022	372
2023	335.5
2024	221
2025	1,212.2
2026 and beyond	1,149.8
Long-term debt	$ 3,580.5

Debt and Other Obligations - In

Debt and Other Obligations - Interest Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Sep. 17, 2020
Debt Instrument [Line Items]
Commitment and other fees	$ 1,106	$ 2,217
Amortization of deferred financing costs	10,751	8,714
Total	134,520	101,350
Accretion expense	10,138	0	$ 0
8.00% senior secured notes due in 2025
Debt Instrument [Line Items]
Interest expense	19,953	0
Accretion expense	500
Interest expense	19,500
8.00% senior secured notes due in 2025 | 8.00% Senior Secured Notes Due 2025
Debt Instrument [Line Items]
Stated interest rate percentage				8.00%
Senior loans
Debt Instrument [Line Items]
Interest expense	43,591	41,053
Junior loans
Debt Instrument [Line Items]
Interest expense	0	1,811
Unsecured term loans due in 2030
Debt Instrument [Line Items]
Interest expense	409	0
2015-1 EETC Class A
Debt Instrument [Line Items]
Interest expense	13,730	14,894
2015-1 EETC Class B
Debt Instrument [Line Items]
Interest expense	3,027	3,377
2015-1 EETC Class C
Debt Instrument [Line Items]
Interest expense	4,565	5,117
2017-1 EETC Class AA
Debt Instrument [Line Items]
Interest expense	7,412	7,887
2017-1 EETC Class A
Debt Instrument [Line Items]
Interest expense	2,672	2,843
2017-1 EETC Class B
Debt Instrument [Line Items]
Interest expense	2,445	2,870
2017-1 EETC Class C
Debt Instrument [Line Items]
Interest expense	4,379	4,367
Convertible Debt
Debt Instrument [Line Items]
Interest expense	14,905	0
Accretion expense	9,600
Interest expense	5,300
Revolving Credit Facility
Debt Instrument [Line Items]
Interest expense	5,380	5,792
Finance leases
Debt Instrument [Line Items]
Interest expense	$ 195	$ 408

Leases and Aircraft Maintenan_3

Leases and Aircraft Maintenance Deposits - Narrative (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Jan. 31, 2020aircraft	Dec. 31, 2020USD ($)aaircraft_Engineaircraft	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Lessee, Lease, Description [Line Items]
Number of spare engines capitalized | aircraft_Engine		16
Total rental expense		$ 371,600	$ 345,000
Total rental expense				$ 312,000
Rental expense charged to operations for aircraft and engine operating leases		196,359	182,609	177,641
Supplemental rent expense		$ 3,300	$ 4,800
Supplemental rent expense				$ 3,400
Cost, Product and Service [Extensible List]		us-gaap:AircraftRental	us-gaap:AircraftRental	us-gaap:AircraftRental
Maintenance reserves, supplemental rent			$ 500	$ 1,300
Aircraft maintenance deposits		$ 126,300
Aircraft maintenance deposits, current		73,134	102,906
Long-term aircraft maintenance deposits		53,158	$ 67,682
Finance lease liability		$ 1,788
Area of land | a		8.5
Land		$ 41,000
A320 Family
Lessee, Lease, Description [Line Items]
Number of aircraft held | aircraft		157
Airbus A319
Lessee, Lease, Description [Line Items]
Number of aircraft to be purchased | aircraft		2
Number of aircraft purchased | aircraft	2
Aircraft
Lessee, Lease, Description [Line Items]
Number of previously leased aircraft purchased off lease | aircraft		2
Finance lease liability		$ 44,100
Finance lease, right-of-use asset		$ 48,400
Aircraft | A320 Family
Lessee, Lease, Description [Line Items]
Number of leased assets financed under operating leases | aircraft		56
Spare engines
Lessee, Lease, Description [Line Items]
Number of leased assets financed under operating leases | aircraft_Engine		8
Number of spare engines purchased | aircraft_Engine		2
Other leased equipment and property
Lessee, Lease, Description [Line Items]
Area of land | a		2.6
Noncancellable short-term operating leases commitments		$ 1,400
Minimum | Leased Computer And Office Equipment
Lessee, Lease, Description [Line Items]
Finance lease contract term		4 years
Minimum | Aircraft
Lessee, Lease, Description [Line Items]
Operating leases, term		8 years
Maximum | Leased Computer And Office Equipment
Lessee, Lease, Description [Line Items]
Finance lease contract term		5 years
Maximum | Aircraft
Lessee, Lease, Description [Line Items]
Operating leases, term		18 years
Maximum | Other leased equipment and property
Lessee, Lease, Description [Line Items]
Operating leases, term		99 years
2020
Lessee, Lease, Description [Line Items]
Number of sale and leaseback transactions | aircraft		1
2020 | Aircraft
Lessee, Lease, Description [Line Items]
Number of aircrafts delivered under secured debt arrangements | aircraft		8
2020 | Aircraft | A320 Family
Lessee, Lease, Description [Line Items]
Number of leased assets financed under operating leases | aircraft		3
Spare engines
Lessee, Lease, Description [Line Items]
Number of previously leased aircraft purchased off lease | aircraft		1
Spare engines | Minimum
Lessee, Lease, Description [Line Items]
Operating leases, term		12 years
Spare engines | Maximum
Lessee, Lease, Description [Line Items]
Operating leases, term		16 years
Aircraft
Lessee, Lease, Description [Line Items]
Number of capitalized aircraft or engine purchased off lease | aircraft		29
Aircraft | A320 Family
Lessee, Lease, Description [Line Items]
Number of aircraft capitalized | aircraft		101
Aircraft | Minimum
Lessee, Lease, Description [Line Items]
Finance lease contract term		8 years
Aircraft | Maximum
Lessee, Lease, Description [Line Items]
Finance lease contract term		18 years

Leases and Aircraft Maintenan_4

Leases and Aircraft Maintenance Deposits - Future Minimum Lease Payments Under Finance and Operating Leases (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Finance Leases
2021 (1)	$ 753
2022	725
2023	349
2024	98
2025	0
2026 and thereafter	0
Total minimum lease payments	1,925
Less amount representing interest	137
Present value of minimum lease payments	1,788
Less current portion	671
Long-term portion	1,117
Operating Leases
Less current portion	133,791	$ 120,662
Long-term portion	1,248,519	$ 1,218,014
Total Operating and Finance Lease Obligations
2021 (1)	241,537
2022	215,033
2023	200,999
2024	178,074
2025	154,206
2026 and thereafter	1,118,775
Total minimum lease payments	2,108,624
Less amount representing interest	697,258
Present value of minimum lease payments	1,411,366
Less current portion	134,462
Long-term portion	1,276,904
Aircraft and Spare Engine Leases
Operating Leases
2021 (1)	236,101
2022	209,911
2023	197,267
2024	175,204
2025	153,146
2026 and thereafter	975,682
Total minimum lease payments	1,947,311
Less amount representing interest	562,493
Present value of minimum lease payments	1,384,818
Less current portion	130,484
Long-term portion	1,254,334
Aircraft and Spare Engine Leases | COVID-19 Pandemic | Other Current Liabilities
Operating Leases
2021 (1)	27,300
Property Facility Leases
Operating Leases
2021 (1)	4,683
2022	4,397
2023	3,383
2024	2,772
2025	1,060
2026 and thereafter	143,093
Total minimum lease payments	159,388
Less amount representing interest	134,628
Present value of minimum lease payments	24,760
Less current portion	3,307
Long-term portion	$ 21,453

Leases and Aircraft Maintenan_5

Leases and Aircraft Maintenance Deposits - Lease Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Finance lease cost
Amortization of leased assets	$ 824	$ 998
Interest of lease liabilities	194	674
Operating lease cost
Operating lease cost	201,474	179,959
Short-term lease cost	25,195	5,144
Variable lease cost	125,534	140,417
Total lease cost	$ 353,221	$ 327,192
Weighted-Average Remaining Lease Term [Abstract]
Operating leases	12 years 10 months 24 days	13 years
Finance leases	2 years 7 months 6 days	1 month 6 days
Weighted-Average Lease Discount Rate [Abstract]
Operating leases	6.08%	5.86%
Finance leases	5.54%	2.46%

Defined Contribution 401(k) P_2

Defined Contribution 401(k) Plan - Narrative (Details) $ in Millions	Mar. 01, 2022	Mar. 01, 2018	Feb. 28, 2018	Dec. 31, 2020USD ($)defined_contribution_plan	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Defined Contribution Plan Disclosure [Line Items]
Defined contribution 401(k) plan, number of plans | defined_contribution_plan				3
Defined contribution 401(k) plan, requisite service period				60 days
Defined contribution 401(k) plan, requisite age				21 years
Defined contribution 401(k) plan, matching contributions made in period | $				$ 58.6	$ 51.1	$ 36.7
Pilots’ Retirement Savings Plan
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, employer matching contribution, percent of match			100.00%
Defined contribution 401(k) plan, employer contribution, percent of employees' gross pay, initial percentage		11.00%
Defined contribution 401(k) plan, employer contribution, percent of employees' gross pay, annual percentage increase				1.00%
Pilots’ Retirement Savings Plan | Forecast
Defined Contribution Plan Disclosure [Line Items]
Defined contribution 401(k) plan, employer contribution, percent of employees' gross pay, final percentage	15.00%
Pilots’ Retirement Savings Plan | Maximum
Defined Contribution Plan Disclosure [Line Items]
Defined contribution 401(k) plan, maximum employer contribution percent of the employee’s annual compensation			9.00%

Income Taxes - Components of th

Income Taxes - Components of the Provision for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Current:
Federal	$ (141,997)	$ (22,429)	$ (2,178)
State and local	(1,847)	1,218	410
Foreign	(1,554)	6,693	4,692
Total current expense (benefit)	(145,398)	(14,518)	2,924
Deferred:
Federal	(33,494)	106,703	42,246
State and local	(12,592)	8,986	4,057
Total deferred expense (benefit)	(46,086)	115,689	46,303
Total income tax expense (benefit)	$ (191,484)	$ 101,171	$ 49,227

Income Taxes - Income Tax Recon

Income Taxes - Income Tax Reconciliation (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of Income Tax Expense
Expected provision at federal statutory tax rate	21.00%	21.00%	21.00%
State tax expense, net of federal benefit	1.90%	1.80%	1.70%
Revaluation of deferred taxes	9.20%	(2.10%)	0.00%
Other	(1.20%)	2.50%	1.30%
Total income tax expense (benefit)	30.90%	23.20%	24.00%

Income Taxes - Deferred Taxes (

Income Taxes - Deferred Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Income tax credits	$ 4,298	$ 9,632
Net operating losses	220,071	13,604
Deferred revenue	11,740	8,824
Nondeductible accruals	21,918	14,133
Deferred manufacturing credits	6,442	2,813
Accrued maintenance	568	1,668
Equity compensation	3,433	2,851
Operating lease liability	313,142	305,161
Other	465	482
Valuation allowance	(2,949)	(1,746)
Deferred tax assets	579,128	357,422
Deferred tax liabilities:
Convertible debt	14,942	0
Prepaid expenses	898	1,120
Property, plant and equipment	603,173	430,523
Deferred financing costs	124	154
Accrued aircraft and engine maintenance	80,916	84,479
Right-of-use asset	318,969	310,438
Deferred tax liabilities	1,019,022	826,714
Net deferred tax assets (liabilities)	$ (439,894)	$ (469,292)

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Summary Of Income Taxes [Line Items]
Valuation allowance	$ 2,949,000	$ 1,746,000
Foreign tax credits	2,800,000
State net operating losses	1,500,000
Income tax expense (benefit), share-based payment arrangement	(800,000)	1,400,000
Liability recorded for interest and penalties on unrecognized tax benefits	0	$ 0	$ 0
CARES Act
Summary Of Income Taxes [Line Items]
Federal NOL carryback	56,100,000
Income taxes receivable	142,000,000
Domestic Tax Authority
Summary Of Income Taxes [Line Items]
Operating loss carryforwards	956,900,000
State and Local Jurisdiction
Summary Of Income Taxes [Line Items]
Operating loss carryforwards	$ 360,000,000

Commitments and Contingencies -

Commitments and Contingencies - Aircraft-Related Commitments and Financing Arrangements (Details) $ in Millions	Dec. 31, 2020USD ($)aircraft_Engineaircraft	Dec. 31, 2019aircraft
Interest Commitments
Interest commitments, 2021	$ 162.5
Interest commitments, 2022	148.7
Interest commitments, 2023	138.1
Interest commitments, 2024	126.8
Interest commitments, 2025	110
Interest commitments, 2026 and beyond	103
Aircraft and Related Flight Equipment
Committed Expenditures
Committed expenditures, 2021	415.7
Committed expenditures, 2022	849.1
Committed expenditures, 2023	676
Committed expenditures, 2024	1,001.6
Committed expenditures, 2025	1,209.1
Committed expenditures, 2026 and beyond	2,367.8
Non-aircraft Related Commitments
Committed Expenditures
Committed expenditures, 2021	17.8
Committed expenditures, 2022	15.9
Committed expenditures, 2023	14.1
Committed expenditures, 2024	14.5
Committed expenditures, 2025	15
Committed expenditures, 2026 and beyond	$ 35.3
V2500 SelectTWO Engine | 2021-2023
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of spare aircraft engines ordered | aircraft_Engine	1
PurePower PW1100G-JM Engine | 2021-2023
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of spare aircraft engines ordered | aircraft_Engine	2
A320 Family
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of aircraft ordered | aircraft		100
Number of aircraft order conversions | aircraft		50
Airbus
Unrecorded Unconditional Purchase Obligation [Line Items]
Future aircraft to be received | aircraft	126
Airbus | 2021-2027
Committed Expenditures
Number of aircraft without secured financing commitments scheduled for delivery | aircraft	126
Third Party Lessor | A320NEO
Unrecorded Unconditional Purchase Obligation [Line Items]
Future aircraft to be received | aircraft	10
Aircraft Rent Commitments [Abstract]
Aircraft rent commitments, 2021	$ 18.1
Aircraft rent commitments, 2022	34.2
Aircraft rent commitments, 2023	34.2
Aircraft rent commitments, 2024	34.2
Aircraft rent commitments, 2025	34.2
Aircraft rent commitments, 2026 and beyond	$ 255.5

Commitments and Contingencies_2

Commitments and Contingencies - Employees (Details) $ in Millions	1 Months Ended	12 Months Ended
	Feb. 28, 2018	Dec. 31, 2020USD ($)employee_group	Dec. 31, 2018USD ($)	Dec. 31, 2019USD ($)
Health Insurance Product Line
Concentration Risk [Line Items]
Accrued health care claims		$ 7.3		$ 5.2
Air Line Pilots Association, International (ALPA)
Concentration Risk [Line Items]
Collective bargaining arrangement, term	5 years
Pilots
Concentration Risk [Line Items]
Ratification Incentive			$ 80.2
Collective bargaining agreement, contractual provision adjustments			$ 8.5
Unionized employees concentration risk | Number of employees (percent)
Concentration Risk [Line Items]
Union-represented employee groups | employee_group		5
Concentration of risk		82.00%
Unionized employees concentration risk | Number of employees (percent) | Air Line Pilots Association, International (ALPA)
Concentration Risk [Line Items]
Concentration of risk		29.00%
Unionized employees concentration risk | Number of employees (percent) | Association of Flight Attendants (AFA-CWA)
Concentration Risk [Line Items]
Concentration of risk		46.00%
Unionized employees concentration risk | Number of employees (percent) | Professional Airline Flight Control Association (PAFCA)
Concentration Risk [Line Items]
Concentration of risk		1.00%
Unionized employees concentration risk | Number of employees (percent) | International Association of Machinists and Aerospace Workers (IAMAW)
Concentration Risk [Line Items]
Concentration of risk		3.00%
Unionized employees concentration risk | Number of employees (percent) | Transport Workers Union of America (TWU)
Concentration Risk [Line Items]
Concentration of risk		3.00%

Fair Value Measurements - Long-

Fair Value Measurements - Long-term Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	$ 3,580,500	$ 2,213,600
Estimate of Fair Value Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	3,859,800	2,303,000
8.00% senior secured notes due in 2025 | 8.00% Senior Secured Notes Due 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	850,000
8.00% senior secured notes due in 2025 | Estimate of Fair Value Measurement | Level 3 | 8.00% Senior Secured Notes Due 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	886,000
Unsecured term loans due in 2030 | Unsecured term loans due in 2030
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	73,300
Unsecured term loans due in 2030 | Estimate of Fair Value Measurement | Level 3 | Unsecured term loans due in 2030
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	83,100
Fixed-rate term loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	1,301,900	1,074,300
Fixed-rate term loans | Estimate of Fair Value Measurement | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	1,362,900	1,120,000
2015-1 EETC Class A
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	322,600	348,600
2015-1 EETC Class A | Estimate of Fair Value Measurement | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	323,400	372,200
2015-1 EETC Class B
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	64,000	72,000
2015-1 EETC Class B | Estimate of Fair Value Measurement | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	62,500	74,500
2015-1 EETC Class C
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	86,600	98,100
2015-1 EETC Class C | Estimate of Fair Value Measurement | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	77,800	100,500
2017-1 EETC Class AA
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	214,400	228,400
2017-1 EETC Class AA | Estimate of Fair Value Measurement | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	207,400	237,000
2017-1 EETC Class A
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	71,500	76,100
2017-1 EETC Class A | Estimate of Fair Value Measurement | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	68,800	78,800
2017-1 EETC Class B
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	60,600	70,600
2017-1 EETC Class B | Estimate of Fair Value Measurement | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	56,200	72,000
2017-1 EETC Class C
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	85,500	85,500
2017-1 EETC Class C | Estimate of Fair Value Measurement | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	76,300	88,000
Convertible Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	175,000
Convertible Debt | Estimate of Fair Value Measurement | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	380,300
Revolving Credit Facility
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	275,100	160,000
Revolving Credit Facility | Estimate of Fair Value Measurement | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	$ 275,100	$ 160,000

Fair Value Measurements - Narra

Fair Value Measurements - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2020	Sep. 17, 2020	Dec. 31, 2019
Recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets held for sale	$ 2.3		$ 2.3
Control Agreements For Interest And Fee Payments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Restricted cash	$ 41.4
8.00% senior secured notes due in 2025 | 8.00% Senior Secured Notes Due 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Stated interest rate percentage		8.00%
Standby letter of credit facility | 8.00% senior secured notes due in 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Letters of credit, limit, amount			35
Letter of credit facility, amount outstanding			$ 23.3

Fair Value Measurements - Asset

Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - Recurring basis - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 1,789.7	$ 979
Restricted cash	71.4	0
Short-term investment securities	106.3	105.3
Assets held for sale	2.3	2.3
Total assets	1,969.7	1,086.6
Total liabilities	0	0
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	1,789.7	979
Restricted cash	71.4	0
Short-term investment securities	106.3	105.3
Assets held for sale	0	0
Total assets	1,967.4	1,084.3
Total liabilities	0	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0	0
Restricted cash	0	0
Short-term investment securities	0	0
Assets held for sale	0	0
Total assets	0	0
Total liabilities	0	0
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0	0
Restricted cash	0	0
Short-term investment securities	0	0
Assets held for sale	2.3	2.3
Total assets	2.3	2.3
Total liabilities	$ 0	$ 0

Fair Value Measurements - Ass_2

Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis using Significant Unobservable Inputs (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at December 31, 2018	$ 0
Purchases	5.4
Sales	0
Total realized or unrealized gains (losses) included in earnings, net	(3.1)
Balance at December 31, 2019	$ 2.3

Operating Segments and Relate_3

Operating Segments and Related Disclosures - Geographic Revenues (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues from External Customers and Long-Lived Assets [Line Items]
Total operating revenues	$ 498,490	$ 401,922	$ 138,529	$ 771,081	$ 969,816	$ 991,968	$ 1,012,956	$ 855,796	$ 1,810,022	$ 3,830,536	$ 3,323,034
DOT—Domestic
Revenues from External Customers and Long-Lived Assets [Line Items]
Total operating revenues									1,660,700	3,462,800	2,990,700
DOT—Latin America and Caribbean
Revenues from External Customers and Long-Lived Assets [Line Items]
Total operating revenues									$ 149,300	$ 367,700	$ 332,300

Operating Segments and Relate_4

Operating Segments and Related Disclosures - Foreign Revenues (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Total passenger revenue | Any individual foreign country
Concentration Risk [Line Items]
Concentration of risk (not greater than)	4.00%	4.00%	4.00%

Quarterly Financial Data (Una_3

Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Total operating revenues	$ 498,490	$ 401,922	$ 138,529	$ 771,081	$ 969,816	$ 991,968	$ 1,012,956	$ 855,796	$ 1,810,022	$ 3,830,536	$ 3,323,034
Operating income (loss)	(159,915)	(99,471)	(190,384)	(57,992)	124,624	124,681	163,938	87,804	(507,762)	501,047	350,914
Net income (loss)	$ (157,304)	$ (99,140)	$ (144,428)	$ (27,828)	$ 81,214	$ 83,464	$ 114,501	$ 56,076	$ (428,700)	$ 335,255	$ 155,749
Basic earnings (loss) per share ( in dollars per share)	$ (1.61)	$ (1.07)	$ (1.81)	$ (0.41)	$ 1.19	$ 1.22	$ 1.67	$ 0.82	$ (5.06)	$ 4.90	$ 2.28
Diluted earnings (loss) per share ( in dollars per share)	$ (1.61)	$ (1.07)	$ (1.81)	$ (0.41)	$ 1.18	$ 1.22	$ 1.67	$ 0.82	$ (5.06)	$ 4.89	$ 2.28