Spirit Airlines (SAVE1) 10-K 5 Feb 20 2019 FY Annual report Financial data

Cover

Cover - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2019	Jan. 28, 2020	Jun. 30, 2019
Entity Information [Line Items]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2019
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
Entity Public Float			$ 3.2
Entity Common Stock, Shares Outstanding (in shares)		68,455,011
Documents Incorporated by Reference	Portions of the registrant's Proxy Statement for the registrant's 2020 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, 2019 .
Entity Central Index Key	0001498710
Amendment Flag	false
Document Fiscal Period Focus	FY
Document Fiscal Year Focus	2019
Current Fiscal Year End Date	--12-31
Voting Common Stock, $0.0001 par value
Entity Information [Line Items]
Title of 12(b) Security	Voting Common Stock, $0.0001 par value
Security Exchange Name	NYSE
Trading Symbol	SAVE
Non-Voting Common Stock, $0.0001 par value
Entity Information [Line Items]
Title of 12(b) Security	Non-Voting Common Stock, $0.0001 par value
Security Exchange Name	NYSE
Trading Symbol	SAVE

Statements Of Operations

Statements Of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating revenues:
Total operating revenues	$ 3,830,536	$ 3,323,034	$ 2,643,552
Operating expenses:
Aircraft fuel	993,478	939,324	615,581
Salaries, wages and benefits	865,019	719,635	527,959
Landing fees and other rents	256,275	214,677	180,655
Depreciation and amortization	225,264	176,727	140,152
Aircraft rent	182,609	177,641	205,852
Distribution	153,770	137,001	113,472
Maintenance, materials and repairs	143,575	129,078	110,439
Loss on disposal of assets	17,350	9,580	4,168
Special charges	717	88,921	12,629
Other operating	491,432	379,536	347,820
Total operating expenses	3,329,489	2,972,120	2,258,727
Operating income	501,047	350,914	384,825
Other (income) expense:
Interest expense	101,350	83,777	57,302
Capitalized interest	(12,471)	(9,841)	(13,793)
Interest income	(25,133)	(19,107)	(8,736)
Other expense	875	752	366
Special charges, non-operating	0	90,357	0
Total other (income) expense	64,621	145,938	35,139
Income before income taxes	436,426	204,976	349,686
Provision (benefit) for income taxes	101,171	49,227	(65,836)
Net income	$ 335,255	$ 155,749	$ 415,522
Basic earnings per share	$ 4.90	$ 2.28	$ 6
Diluted earnings per share	$ 4.89	$ 2.28	$ 5.99
Passenger
Operating revenues:
Total operating revenues	$ 3,757,605	$ 3,260,015	$ 2,572,887
Other
Operating revenues:
Total operating revenues	$ 72,931	$ 63,019	$ 70,665

Statements of Comprehensive Inc

Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 335,255	$ 155,749	$ 415,522
Unrealized gain (loss) on short-term investment securities and cash and cash equivalents, net of deferred taxes of $38, $44, and ($41)	167	30	(82)
Interest rate derivative loss reclassified into earnings, net of taxes of $76, $75, and $372	239	241	(37)
Other comprehensive income (loss)	406	271	(119)
Comprehensive income	$ 335,661	$ 156,020	$ 415,403

Statements of Comprehensive I_2

Statements of Comprehensive Income (Parentheticals) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Comprehensive Income [Abstract]
Deferred taxes on unrealized loss on investment securities	$ 38	$ 44	$ 41
Loss reclassified from AOCI into income, tax	$ 76	$ 75	$ 372

Balance Sheets

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 978,957	$ 1,004,733
Short-term investment securities	105,321	102,789
Accounts receivable, net	73,807	47,660
Aircraft maintenance deposits, net	102,906	106,901
Income tax receivable	21,013	0
Prepaid expenses and other current assets	103,439	83,383
Total current assets	1,385,443	1,345,466
Property and equipment:
Flight equipment	3,730,751	3,257,215
Ground property and equipment	291,998	191,661
Less accumulated depreciation	(492,447)	(332,864)
Total property and equipment	3,530,302	3,116,012
Operating lease right-of-use assets	1,369,555	0
Pre-delivery deposits on flight equipment	291,930	236,775
Long-term aircraft maintenance deposits	67,682	138,738
Deferred heavy maintenance, net	361,603	249,010
Other long-term assets	36,897	79,456
Total assets	7,043,412	5,165,457
Current liabilities:
Accounts payable	43,601	39,320
Air traffic liability	315,408	291,981
Current maturities of long-term debt and finance leases	258,852	163,557
Current maturities of operating leases	120,662	0
Other current liabilities	373,521	339,677
Total current liabilities	1,112,044	834,535
Long-term debt and finance leases, less current maturities	1,960,453	2,024,774
Operating leases, less current maturities	1,218,014	0
Deferred income taxes	469,292	355,141
Deferred gains and other long-term liabilities	22,277	22,503
Shareholders’ equity:
Common stock: Common stock, $0.0001 par value, 240,000,000 shares authorized at December 31, 2019 and 2018, respectively; 70,148,386 and 69,871,535 issued and 68,455,011 and 68,269,567 outstanding as of December 31, 2019 and 2018, respectively	7	7
Additional paid-in-capital	379,380	371,225
Treasury stock, at cost: 1,693,375 and 1,601,968 as of December 31, 2019 and 2018, respectively	(72,455)	(67,016)
Retained earnings	1,955,187	1,625,481
Accumulated other comprehensive income (loss)	(787)	(1,193)
Total shareholders’ equity	2,261,332	1,928,504
Total liabilities and shareholders’ equity	$ 7,043,412	$ 5,165,457

Balance Sheets (Parenthetical)

Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
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)	70,148,386	69,871,535
Common stock, shares outstanding (in shares)	68,455,011	68,269,567
Treasury stock (in shares)	1,693,375	1,601,968
Voting Common Stock, $0.0001 par value
Common stock, par value (in dollars per share)	$ 0.0001
Common stock, shares authorized (in shares)	240,000,000

Statements of Cash Flows

Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating activities:
Net income	$ 335,255	$ 155,749	$ 415,522
Adjustments to reconcile net income to net cash provided by operations:
Losses reclassified from other comprehensive income	315	315	335
Share-based compensation	8,154	11,021	8,522
Allowance for doubtful accounts (recoveries)	0	(11)	(53)
Amortization of deferred gains, losses and debt issuance costs	8,654	8,819	7,944
Depreciation and amortization	225,264	176,727	140,152
Deferred income tax expense (benefit)	115,689	46,303	(492)
Loss on disposal of assets	17,350	9,580	4,168
Lease termination costs	0	0	12,629
Special charges, non-operating	0	90,357	0
Changes in operating assets and liabilities:
Accounts receivable, net	(26,147)	1,674	(8,134)
Aircraft maintenance deposits, net	22,453	14,019	(37,930)
Other assets	14,999	(4,803)	(50,951)
Deferred heavy maintenance, net	(175,957)	(190,381)	(78,237)
Income tax receivable	(21,013)	69,844	(69,844)
Prepaid income taxes	1,431	0	0
Accounts payable	569	15,317	6,030
Air traffic liability	23,429	28,270	43,527
Other liabilities	(140,402)	74,038	31,672
Other	(822)	(375)	380
Net cash provided by operating activities	409,221	506,463	425,240
Investing activities:
Purchase of available-for-sale investment securities	(122,410)	(124,430)	(107,246)
Proceeds from the maturity and sale of available-for-sale investment securities	120,830	122,947	105,906
Proceeds from sale of property and equipment	0	11,400	0
Pre-delivery deposits on flight equipment, net of refunds	(102,102)	(177,424)	(149,477)
Capitalized interest	(10,774)	(8,729)	(12,305)
Assets under construction for others	(7,936)	(501)	0
Purchase of property and equipment	(192,437)	(606,971)	(628,881)
Net cash used in investing activities	(314,829)	(783,708)	(792,003)
Financing activities:
Proceeds from issuance of long-term debt	225,891	832,099	629,725
Proceeds from stock options exercised	1	51	45
Payments on debt obligations	(246,783)	(137,275)	(102,313)
Payments on finance lease obligations	(96,547)	(205,720)	(425)
Reimbursement for assets under construction for others	5,618	501	0
Repurchase of common stock	(5,439)	(1,162)	(46,580)
Debt issuance costs	(2,909)	(7,365)	(13,740)
Net cash (used in) provided by financing activities	(120,168)	481,129	466,712
Net increase (decrease) in cash and cash equivalents	(25,776)	203,884	99,949
Cash and cash equivalents at beginning of period	1,004,733	800,849	700,900
Cash and cash equivalents at end of period	978,957	1,004,733	800,849
Cash payments for:
Interest, net of capitalized interest	80,254	65,123	37,902
Income taxes paid (received), net	5,843	(73,489)	5,826
Operating cash flows for operating leases (1)	191,004
Financing cash flows for finance leases (1)	674
Non-cash transactions:
Capital expenditures funded by finance lease borrowings	45,608	$ 987	$ 1,370
Capital expenditures funded by operating lease borrowings (1)	$ 569,948

Statements of Shareholders_ Equ

Statements of Shareholders’ Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Treasury Stock	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Restatement AdjustmentRetained Earnings
Balance at Dec. 31, 2016	$ 1,385,184	$ 7	$ 551,004	$ (218,692)	$ 1,054,210	$ (1,345)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	8,522		8,522
Repurchase of common stock	(46,580)			(46,580)
Proceeds from options exercised	45		45
Retirement of treasury stock	0		(199,418)	199,418
Changes in comprehensive income	(119)					(119)
Net income	415,522				415,522
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]
Share-based compensation	11,021		11,021
Repurchase of common stock	(1,162)			(1,162)
Proceeds from options exercised	51		51
Retirement of treasury stock			0
Changes in comprehensive income	271					271
Net income	155,749				155,749
Balance at Dec. 31, 2018	1,928,504	7	371,225	(67,016)	1,625,481	(1,193)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	8,154		8,154
Repurchase of common stock	(5,439)			(5,439)
Proceeds from options exercised	1		1
Retirement of treasury stock			0
Changes in comprehensive income	406					406
Net income	335,255				335,255
Balance at Dec. 31, 2019	$ 2,261,332	$ 7	$ 379,380	$ (72,455)	$ 1,955,187	$ (787)
Balance (Accounting Standards Update 2016-02) at Dec. 31, 2019							$ (5,549)

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies Basis of Presentation Spirit Airlines, Inc. ("Spirit" or the "Company") headquartered in Miramar, Florida, is an ultra low-cost, low-fare airline that provides affordable travel opportunities principally throughout the domestic United States, the Caribbean and Latin America. The Company 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. 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 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. 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 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 statements of cash flows. Realized gains and losses on sales of investments, if any, are reflected in non-operating income (expense) in the 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 doubtful accounts for amounts not expected to be collected. The Company estimates the allowance based on historical write-offs as well as aging trends. The allowance for doubtful accounts was immaterial as of December 31, 2019 and 2018 . In addition, the provision for doubtful accounts and write-offs for 2019 , 2018 and 2017 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 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 years Spare rotables and flight assemblies 7 to 15 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, 2019 , the Company had 91 aircraft, 14 spare engines and 1 flight simulator capitalized within flight equipment with depreciable lives of 25 years . As of December 31, 2019 , the Company had 52 aircraft financed through operating leases and 2 aircraft financed through finance leases with lease terms from 8 to 18 years . In addition, the Company had 9 spare engines financed through operating leases with lease terms from 2 to 14 years . The following table illustrates the components of depreciation and amortization expense: Year Ended December 31, 2019 2018 2017 (in thousands) Depreciation $ 155,326 $ 129,412 $ 83,154 Amortization of heavy maintenance 63,364 41,286 53,855 Amortization of capitalized software 6,574 6,029 3,143 Total depreciation and amortization $ 225,264 $ 176,727 $ 140,152 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 balance sheets, net of amortization, was $13.0 million and $13.7 million at December 31, 2019 and 2018 , respectively. The Company records amortization of capitalized software on a straight-line basis within depreciation and amortization expense in the accompanying statements of operations. The Company placed in service internal-use software of $5.9 million , $12.0 million and $1.4 million , during the years ended 2019 , 2018 and 2017 , 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 has 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 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 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 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 balance sheets along with the related PDPs as they are included in the cost of the aircraft or engine. Capitalized interest for 2019, 2018 and 2017 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. Passenger Revenues Fare revenues. Tickets sold are initially deferred within air traffic liability on the Company's balance sheet. Passenger fare revenues are recognized at time of departure when transportation is provided. 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, 2019 and 2018 , the Company had air traffic liability ("ATL") balances of $315.4 million and $292.0 million , respectively. As of December 31, 2019 , all of the ATL balance as of December 31, 2018 has been recognized. The balance of the Company's ATL as of December 31, 2019 is expected to be recognized within 12 months. The Company adopted ASU 2014-09 on January 1, 2018 utilizing the full retrospective method of adoption. Passenger revenues reported prior to the adoption of ASU 2014-09 are now reported as fare revenues within passenger revenues in the Company's disaggregated revenue table within Note 3, Revenue Disaggregation. Non-fare revenues. The adoption of ASU 2014-09 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 3, 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 2019 2018 2017 (in thousands) Baggage Time of departure $ 734,243 $ 620,154 $ 488,434 Passenger usage fee Time of departure 669,177 531,459 411,742 Advance seat selection Time of departure 228,876 180,012 131,821 Other 238,454 224,283 177,495 Non-fare revenue $ 1,870,750 $ 1,555,908 $ 1,209,492 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 generally expires 60 days from the date the credit shell is created and which can be used towards the purchase of a new ticket and the Company’s other service offerings. Both the 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. However, given the relatively short period of time to expiration, this does not have a significant impact on the Company's financial statements. 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. The adoption of ASU 2014-09 eliminated the incremental cost method for frequent flyer program accounting, which required the Company to re-value and record a liability associated with customer flight miles earned with travel as part of the Company’s frequent flyer program with a relative fair value. Upon adoption of ASU 2014-09 on January 1, 2018, the Company recorded an increase to its air traffic liability of $12.4 million . 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-brand credit card agreement provides for joint marketing where cardholders earn mileage credits for making purchases using co-branded cards. During 2015, the Company extended its agreement with the administrator of the FREE SPIRIT affinity credit card program to extend through 2022. 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 Company defers the amount for award travel obligation as part of loyalty deferred revenue within air traffic liability on the 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 $29.8 million and $27.4 million at December 31, 2019 and 2018 , 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 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, 2019 $ 48,136 $ 37,151 December 31, 2018 39,194 30,353 December 31, 2017 49,453 37,960 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 $63.4 million , $41.3 million and $53.9 million for the years ended 2019 , 2018 and 2017 , respectively. During the years ended 2019 , 2018 and 2017 , the Company deferred $176.0 million , $190.5 million and $78.2 million , respectively, of costs for heavy maintenance. At December 31, 2019 and 2018 , the Company had deferred heavy maintenance balance of $504.2 million and $366.3 million , and accumulated heavy maintenance amortization of $142.6 million and $117.3 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, 2019 2018 2017 (in thousands) Flight hour-based maintenance expense $ 78,253 $ 68,039 $ 54,802 Non-flight hour-based maintenance expense 65,322 61,039 55,637 Total maintenance, materials and repairs $ 143,575 $ 129,078 $ 110,439 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, 2019 , 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 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 $6.3 million , $6.3 million and $5.1 million for the years ended 2019 , 2018 and 2017 , respectively, were recorded within distribution expense in the 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, 2019 and 2018 , the Company recorded a valuation allowance of $1.7 million and $0.3 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 11, Stock-Based Compensation. Cash Flow Presentation In the Company's Form 10-Q for the period ending September 30, 2019 and filed on October 23, 2019, the Company had a typographical error on the Company's Condensed Statements of Cash Flows for the nine months ended September 30, 2019 and the Liquidity Section of the Company's Management Discussion and Analysis as the amounts listed under the captions for "Pre-delivery deposits on flight equipment, net of refunds" and "Purchase of property and equipment" were interchanged. These amounts have been corrected in the Company's Statements of Cash Flows for the year ended December 31, 2019 filed herein. 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 30% , 32% and 27% of total operating expenses in 2019 , 2018 and 2017 , 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, 2019 , the Company had five union-represented employee groups that together represented approximately 81% of all employees. A strike or other significant labor dispute with the Company’s unionized employees is likely to adversely affect the Company’s ability to conduct business. Additional disclosures are included in Note 18, Commitments and Contingencies.

Recent Accounting Developments

Recent Accounting Developments	12 Months Ended
	Dec. 31, 2019
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Recent Accounting Developments	Recent Accounting Developments Recently Adopted Accounting Pronouncements Leases The Company adopted ASU No. 2016-02, "Leases (Topic 842)," effective January 1, 2019. The Company adopted Topic 842 utilizing the modified retrospective adoption method with an effective date of January 1, 2019 and elected the package of transition practical expedients for expired or existing contracts, which does not require reassessment of: (1) whether any of the Company's contracts are or contain leases, (2) lease classification and (3) initial direct costs. Therefore, the financial statements for 2019 are presented under the new standard, while the comparative periods presented are not adjusted and continue to be reported in accordance with the Company's historical accounting policy. This standard requires all lessees to recognize a right-of-use asset and a lease liability, initially measured at the present value of the lease payments, for all leases with a term greater than 12 months. The adoption of the new lease standard had a significant impact on the Company's balance sheets due to the recognition of $1.0 billion of right-of-use assets for operating leases, $128.1 million of current maturities of operating leases and $895.1 million of operating leases, less current maturities. In addition, the Company recognized a $5.5 million cumulative effect adjustment, net of tax, to retained earnings. This adjustment was driven by the recognition of unamortized deferred gains and losses related to aircraft sale-leaseback transactions entered into in prior periods. Prior to the adoption of Topic 842, gains and losses on sale-leaseback transactions were generally deferred and recognized in income over the lease term. The accounting for finance leases is substantially unchanged. The adoption of Topic 842 did not have a significant impact on the Company's lease classification or a material impact on its statements of operations and liquidity. Additionally, the adoption of Topic 842 did not have a material impact on the Company’s debt-covenant compliance under its current agreements. Refer to Note 14, Leases and Prepaid Maintenance Deposits for information regarding the Company's adoption of Topic 842 and the Company's undiscounted future lease payments and the timing of those payments. Recently Issued Accounting Pronouncements Not Yet Adopted Cloud Computing Arrangements In August 2018, the FASB issued ASU No. 2018-15, "Intangibles - Goodwill and Other - Internal-Use Software." This new standard requires a customer in a cloud computing arrangement that is a service contract to follow the internal-use software guidance in Accounting Standards Codification ("ASC") 350-40, "Accounting for Internal-Use Software," to determine which implementation costs to capitalize as assets and amortize over the term of the hosting arrangement or expense as incurred. This standard is effective for the Company for fiscal years, and interim periods within those years, beginning January 1, 2020. Early adoption is permitted, including during an interim period. The Company has elected not to early adopt. Entities have the option to apply this standard prospectively to all implementation costs incurred after the date of adoption or retrospectively. The Company is evaluating this new standard, but does not expect it to have a significant impact on its financial statement presentation or results. Accounting for Credit Losses In June 2016, the FASB issued ASU No. 2016-13, "Financial Instruments - Credit Losses." The standard requires the use of an "expected loss" model on certain types of financial instruments. For accounts receivables, aircraft maintenance deposits and security deposits, recorded within other long-term assets on the Company's balance sheets, the Company will be 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 will be 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. This standard is effective for the Company for fiscal years, and interim periods within those years, beginning January 1, 2020, with early adoption permitted. The Company has elected not to early adopt. The Company does not expect the standard to have a material impact on its financial statements.

Revenue Disaggregation

Revenue Disaggregation	12 Months Ended
	Dec. 31, 2019
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, 2019 , 2018 and 2017 . Twelve Months Ended December 31, 2019 2018 2017 (in thousands) Operating revenues: Fare $ 1,886,855 $ 1,704,107 $ 1,363,395 Non-fare 1,870,750 1,555,908 1,209,492 Total passenger revenues 3,757,605 3,260,015 2,572,887 Other 72,931 63,019 70,665 Total operating revenues $ 3,830,536 $ 3,323,034 $ 2,643,552

Special Charges

Special Charges	12 Months Ended
	Dec. 31, 2019
Special Charges and Credits [Abstract]
Special Charges	Special Charges Special Charges, Operating During the twelve months ended December 31, 2019 , the Company recorded $0.7 million in special charges within operating expenses in the 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 in special charges within operating expenses in the statement of operations for the twelve months ended December 31, 2018 . As of December 31, 2018 , the Company had paid the full ratification incentive related to the Company's new collective bargaining agreement with its pilots. During the twelve months ended December 31, 2017 , the Company purchased one engine and one aircraft which were previously financed under operating lease agreements. The purchase price of the one engine and one aircraft was $8.1 million and $20.0 million , respectively, comprised of a cash payment of $3.8 million and $12.6 million , respectively, and the non-cash application of maintenance and security deposits held by the previous lessor of $4.3 million and $7.4 million , respectively. The Company estimated the fair value of the engine and aircraft to be $3.1 million and $11.9 million , respectively, and has recorded the one purchased engine and one aircraft at fair value within flight equipment on the balance sheets. The Company determined the valuation of the engine and aircraft based on a third-party appraisal considering the condition of the engine and aircraft (a Level 3 measurement). The Company recognized $4.8 million and $7.9 million as a cost of terminating the lease within special charges on the statement of operations, respectively, made up of the excess of the purchase price paid over the fair value of the engine and the aircraft, less other non-cash items of $0.2 million and $0.2 million , respectively. Special Charges, Non-Operating During the twelve months ended December 31, 2019 , the Company had no special charges, non-operating within other (income) expense in the statement of operations. During the twelve months ended December 31, 2018 , the Company recorded $90.4 million , in special charges, non-operating within other (income) expense in the statement of operations. During the first quarter of 2018, the Company entered into an aircraft purchase agreement for the purchase of 14 A319 aircraft previously operated under operating leases by the Company. The aggregate gross purchase price for the 14 aircraft was $285.0 million , and the price for each aircraft at the time of the sale comprised a cash payment net of the amount of maintenance reserves and security deposits for such aircraft held by the applicable lessor pursuant to the lease for such aircraft. The contract was deemed a lease modification which resulted in a change of classification from operating leases to finance leases for the 14 aircraft. During the first quarter of 2018, the finance lease assets were recorded at the lower of cost or fair value of the aircraft within flight equipment on the Company's balance sheets. During the second quarter of 2018, the purchase of the 14 aircraft was completed and the obligation was accreted up to the net cash payment price with interest charges recognized in special charges, non-operating in the statement of operations. The Company determined the valuation of the aircraft based on third-party appraisals considering the condition of the aircraft (a Level 3 measurement). During the twelve months ended December 31, 2017 , the Company had no special charges, non-operating.

Loss on Disposal of Assets

Loss on Disposal of Assets	12 Months Ended
	Dec. 31, 2019
Discontinued Operations and Disposal Groups [Abstract]
Loss on Disposal of Assets	Loss on Disposal of Assets 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 consists 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. This project was suspended in the third quarter of 2019 and the Company has elected to re-evaluate and pursue an alternative solution. 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 14, 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 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. During the twelve months ended December 31, 2017 , the Company recorded $4.2 million

Letters of Credit

Letters of Credit	12 Months Ended
	Dec. 31, 2019
Financial Instruments Pledged as Collateral [Abstract]
Letters of Credit	Letters of Credit As of December 31, 2019 and 2018 , the Company had a $35.0 million and $35.0 million unsecured standby letter of credit facility, of which $23.3 million and $18.1 million

Credit Card Processing Arrangem

Credit Card Processing Arrangements	12 Months Ended
	Dec. 31, 2019
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, 2019 and 2018 , 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 memberships as of December 31, 2019 and 2018 , was $342.3 million and $321.0 million , respectively.

Short-term Investment Securitie

Short-term Investment Securities	12 Months Ended
	Dec. 31, 2019
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 balance sheet. Realized gains and losses on sales of investments, if any, are reflected in non-operating income (expense) in the 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, 2019 and December 31, 2018 , the Company had $105.3 million and $102.8 million in short-term available-for-sale investment securities, respectively. During the twelve months ended December 31, 2019 , 2018 and 2017 , these investments earned interest income at a weighted-average fixed rate of approximately 2.3% , 1.6% and 1.4% respectively. For the twelve months ended December 31, 2019 and December 31, 2018 , an unrealized gain of $104 thousand and an unrealized gain of $30 thousand , net of deferred taxes of $31 thousand and $44 thousand , respectively, was recorded within AOCI related to these investment securities. For the twelve months ended December 31, 2019 , a realized gain of $5 thousand was recorded within non-operating income (expense) in the statements of operations. For the twelve months ended December 31, 2018 and 2017 , 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, 2019 and December 31, 2018 , $104 thousand and $74 thousand , net of tax, respectively, remained in AOCI, related to these instruments.

Accrued Liabilities

Accrued Liabilities	12 Months Ended
	Dec. 31, 2019
Payables and Accruals [Abstract]
Accrued Liabilities	Accrued Liabilities Accrued liabilities included in other current liabilities as of December 31, 2019 and 2018 consist of the following: As of December 31, 2019 2018 (in thousands) Salaries and wages $ 89,163 $ 82,900 Airport obligations 80,134 52,029 Federal excise and other passenger taxes and fees payable 65,312 60,604 Aircraft maintenance 38,099 59,805 Fuel 28,510 25,368 Aircraft and facility lease obligations 20,656 15,149 Interest payable 16,941 18,086 Other 34,706 25,736 Other current liabilities $ 373,521 $ 339,677

Common Stock and Preferred Stoc

Common Stock and Preferred Stock	12 Months Ended
	Dec. 31, 2019
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 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 . 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. 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, 2019 and 2018 , 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, 2019 and 2018 , there were no

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2019
Share-based Payment Arrangement [Abstract]
Stock-Based Compensation	Stock-Based Compensation The 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, 2019 and December 31, 2018 , 1,897,809 and 2,110,598 shares of the Company’s common stock, respectively, remained available for future issuance under the 2015 Plan. Stock-based compensation cost amounted to $8.2 million , $11.0 million and $8.5 million for 2019 , 2018 and 2017 , respectively. During 2019 , 2018 and 2017 there was a $1.9 million , $2.6 million and $1.6 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 or four year graded vesting period. Each restricted stock unit represents the right to receive one share of common stock upon vesting of such restricted stock unit. Vesting of restricted stock units is based on time-based service conditions. In order to vest, the participant must still be employed by the Company, with certain contractual exclusions, at each vesting event. Generally, within 30 days after vesting, the shares underlying the award will be issued to the participant. In the event a successor corporation in a change in control situation fails to assume or substitute for the restricted stock units, the restricted stock units will automatically vest in full as of immediately prior to the consummation of such change in control. In the event of death or permanent disability of a participant, the restricted stock units will automatically vest in full. Compensation expense is recognized on a straight-line basis over the requisite service period. A summary of the status of the Company’s restricted stock shares (restricted stock awards and restricted stock unit awards) as of December 31, 2019 and changes during the year ended December 31, 2019 is presented below: Number of Shares Weighted-Average Outstanding at December 31, 2018 297,797 47.95 Granted 148,120 53.41 Vested (97,095 ) 48.97 Forfeited (15,856 ) 53.00 Outstanding at December 31, 2019 332,966 49.84 There were 148,120 and 185,054 restricted stock shares granted during the years ended December 31, 2019 and December 31, 2018 , respectively. As of December 31, 2019 and December 31, 2018 , there was $10.0 million and $9.1 million , respectively, of total unrecognized compensation cost related to nonvested restricted stock to be recognized over 2.7 years and 2.5 years , respectively. The weighted-average fair value of restricted stock granted during the years ended December 31, 2019 , 2018 and 2017 was $53.41 , $46.90 and $51.68 , respectively. The total fair value of restricted stock shares vested during the years ended December 31, 2019 , 2018 and 2017 was $5.4 million , $6.5 million and $4.3 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, 2019 Weighted-Average at Grant Date for Twelve Months Ended December 31, 2018 Expected volatility factor 0.38 0.39 Risk free interest rate 2.50 % 2.11 % Expected term (in years) 2.97 2.96 Expected dividend yield — % — % For grants awarded in 2019, 2018 and 2017, 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, 2019 : Number of Awards Weighted-Average Fair Value at Grant Date ($) Outstanding at December 31, 2018 82,907 53.92 Granted 50,293 70.77 Vested (29,293 ) 56.84 Forfeited (7,748 ) 57.15 Outstanding at December 31, 2019 96,159 61.58 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, 2019 : Number of Awards Weighted-Average Fair Value at Grant Date ($) Outstanding at December 31, 2018 41,454 49.81 Granted 24,861 60.11 Vested (14,648 ) 55.50 Forfeited (3,873 ) 52.10 Outstanding at December 31, 2019 47,794 53.24 As of December 31, 2019 and 2018 , there was $4.4 million and $3.4 million , respectively, of total unrecognized compensation cost related to performance share awards expected to be recognized over 1.73 years and 1.80 years , respectively. Stock Appreciation Rights During 2018, the Company issued stock appreciation awards to certain senior-level executives. These awards have 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. This is a market-condition performance award 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 will be recognized related to these awards. Treasury Stock During the year ended December 31, 2019 , 2018 and 2017 , the Company repurchased 91 thousand , 28 thousand and 1.2 million shares, respectively, for $5.4 million , $1.2 million and $46.6 million , respectively. Repurchases made during the twelve months ended December 31, 2019 and 2018 include repurchases made from employees who received restricted stock. Repurchases made during the twelve months ended December 31, 2017 include open market repurchases made under our stock repurchase program as well as repurchases made from employees who received restricted stock. During the year ended December 31, 2019 and 2018 , the Company did not retire any treasury shares. During the year ended December 31, 2017 , the Company retired 3.9 million treasury shares in a total aggregate amount of $199.4 million .

Earnings per Share

Earnings per Share	12 Months Ended
	Dec. 31, 2019
Earnings Per Share [Abstract]
Earnings per Share	Earnings per Share The following table sets forth the computation of basic and diluted earnings per common share: Year Ended December 31, 2019 2018 2017 (in thousands, except per-share amounts) Numerator: Net income (1) $ 335,255 $ 155,749 $ 415,522 Denominator: Weighted-average shares outstanding, basic 68,429 68,249 69,221 Effect of dilutive stock awards 130 182 156 Adjusted weighted-average shares outstanding, diluted 68,559 68,431 69,377 Earnings per Share: Basic earnings per common share (1) $ 4.90 $ 2.28 $ 6.00 Diluted earnings per common share (1) $ 4.89 $ 2.28 $ 5.99 Anti-dilutive weighted-average shares 143 145 85 (1) During the twelve months ended December 31, 2017, the Company recorded a non-recurring income tax benefit of $196.7 million ( $2.84 and $2.84 per basic and diluted share, respectively) due to the enactment of the Tax Cuts and Jobs Act of 2017. For additional information, refer to Note 17, Income Taxes.

Debt and Other Obligations

Debt and Other Obligations	12 Months Ended
	Dec. 31, 2019
Debt Disclosure [Abstract]
Debt and Other Obligations	Debt and Other Obligations Long-term debt As of December 31, 2019 , the Company had outstanding non-public and public debt instruments. During 2019 , the Company acquired additional debt through fixed-rate term loans and a previously existing revolving credit facility described below. Fixed-rate term loans During 2019, the Company acquired additional debt under facility agreements, which as of December 31, 2019 provided $152.0 million of debt financing for 4 Airbus A320 aircraft delivered during 2019. 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 11 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 2.85% and 3.93% . As of December 31, 2019 , the Company has taken delivery of all 4 Airbus A320 aircraft financed through these facility agreements. Revolving credit facility During the fourth quarter of 2018, the Company entered into a revolving credit facility for up to $160 million secured by the collateral assignment of certain of the Company's rights under the purchase agreement with Airbus. As of December 31, 2019 , collateralized amounts were related to 34 Airbus A320neo aircraft scheduled to be delivered between January 2020 and December 2021. The final maturity of the facility is December 30, 2020 with final payment due in January 2021. As of December 31, 2019 , the Company had drawn $160.0 million on the facility of which $50.0 million is included in current maturities of long-term debt and finance leases and $110.0 million is included within long-term debt and finance leases, less current maturities on the Company's balance sheets. The revolving credit facility bears variable interest based on LIBOR. Long-term debt is comprised of the following: As of December 31, 2019 December 31, 2018 December 31, 2019 December 31, 2018 (in millions) (weighted-average interest rates) Fixed-rate senior term loans due through 2027 $ 296.1 $ 382.4 4.02 % 4.10 % Fixed-rate junior term loans due through 2022 — 31.1 N/A 6.90 % Fixed-rate loans due through 2030 778.2 625.1 3.70 % 3.88 % Fixed-rate class A 2015-1 EETC due through 2028 348.6 378.6 4.10 % 4.10 % Fixed-rate class B 2015-1 EETC due through 2024 72.0 80.0 4.45 % 4.45 % Fixed-rate class C 2015-1 EETC due through 2023 98.1 109.5 4.93 % 4.93 % Fixed-rate class AA 2017-1 EETC due through 2030 228.4 242.5 3.38 % 3.38 % Fixed-rate class A 2017-1 EETC due through 2030 76.1 80.8 3.65 % 3.65 % Fixed-rate class B 2017-1 EETC due through 2026 70.6 83.7 3.80 % 3.80 % Fixed-rate class C 2017-1 EETC due through 2023 85.5 85.5 5.11 % 5.11 % Revolving credit facility due in 2021 160.0 135.3 3.12 % 3.72 % Long-term debt $ 2,213.6 $ 2,234.5 Less current maturities 214.0 162.8 Less unamortized discount, net 40.4 47.7 Total $ 1,959.2 $ 2,024.0 The Company's debt financings are collateralized by first priority security interest in the individual aircraft being financed with the exception of the Company's revolving credit facility secured by the Company's rights under the purchase agreement with Airbus related to certain A320neo aircraft. During the year ended December 31, 2019 and 2018 , the Company made principal payments of $246.8 million and $137.3 million on its outstanding debt obligations, respectively. At December 31, 2019 , long-term debt principal payments for the next five years and thereafter are as follows: December 31, 2019 (in millions) 2020 $ 222.1 2021 273.8 2022 167.7 2023 310.1 2024 195.5 2025 and beyond 1,044.4 Total debt principal payments $ 2,213.6 Interest Expense Interest expense related to long-term debt and finance leases consists of the following: Year Ended December 31, 2019 2018 (in thousands) Fixed-rate senior term loans $ 15,225 $ 16,869 Fixed-rate junior term loans 1,811 2,475 Fixed-rate term loans 25,828 19,615 Class A 2015-1 EETC 14,894 16,138 Class B 2015-1 EETC 3,377 3,781 Class C 2015-1 EETC 5,117 3,575 Class AA 2017-1 EETC 7,887 6,026 Class A 2017-1 EETC 2,843 2,172 Class B 2017-1 EETC 2,870 2,420 Class C 2017-1 EETC 4,367 2,478 Revolving credit facility 5,792 793 Finance leases 408 104 Commitment fees and other 2,217 262 Amortization of deferred financing costs 8,714 7,069 Total $ 101,350 $ 83,777 As of December 31, 2019 and 2018 , the Company had a line of credit for $33.6 million related to corporate credit cards. Respectively, the Company had drawn $4.6 million and $3.5 million as of December 31, 2019 and 2018 , which is included in accounts payable. As of December 31, 2019 and 2018 , 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, 2019 and 2018 , the Company had drawn $25.3 million and $23.0 million on these lines of credit for physical fuel delivery, which is included within other current liabilities in the Company's balance sheets. The Company is required to post collateral for any excess above the lines of credit if the fuel derivatives are in a net liability position and make periodic payments in order to maintain an adequate undrawn portion for physical fuel delivery. As of December 31, 2019 and 2018

Leases and Aircraft Maintenance

Leases and Aircraft Maintenance Deposits	12 Months Ended
	Dec. 31, 2019
Leases [Abstract]
Leases and Prepaid 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 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, 2019 , the Company had a fleet consisting of 145 A320 family aircraft. As of December 31, 2019 , the Company had 52 aircraft financed under operating leases and 2 aircraft financed under finance leases with lease term expirations between 2020 and 2037 . In addition, the Company owned 91 aircraft of which 27 were purchased off lease and are currently unencumbered. As of December 31, 2019 , the Company also had 9 spare engines financed under operating leases with lease term expiration dates ranging from 2020 to 2027 and owned 14 spare engines of which 3 were purchased off lease and all 14 are currently unencumbered. The Company adopted Topic 842 utilizing the modified retrospective adoption method with an effective date of January 1, 2019. Prior to the adoption of Topic 842, the Company had entered into sale leaseback transactions with third-party aircraft lessors for the majority of its aircraft and engine leases. Upon adoption of Topic 842, the Company recognized a $5.5 million cumulative effect adjustment, net of tax, to retained earnings driven by the recognition of unamortized deferred gains and losses related to aircraft sale-leaseback transactions entered into in prior periods. Prior to the adoption of Topic 842, gains and losses on sale-leaseback transactions were generally deferred and recognized in income over the lease term. 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 statements of operations. Total rent expense for all leases charged to operations for the years ended 2019 , 2018 and 2017 was $345.0 million , $312.0 million and $309.8 million , respectively. Total rental expense charged to operations for aircraft and engine operating leases for the years ended December 31, 2019 , 2018 and 2017 was $182.6 million , $177.6 million and $205.9 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, 2019 , 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. Management assesses the need to accrue lease return costs throughout the lease as facts and circumstances warrant an assessment. 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. Supplemental rent 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 $4.8 million , $3.4 million and $8.4 million of supplemental rent recorded within aircraft rent during 2019 , 2018 and 2017 , respectively. These amounts include $0.5 million , $1.3 million and $0.4 million of paid maintenance reserves expensed as supplemental rent during 2019 , 2018 and 2017 , respectively. As of December 31, 2019 , the Company had $170.6 million of aircraft maintenance deposits ( $102.9 million in aircraft maintenance deposits and $67.7 million in long-term aircraft maintenance deposits) on the Company's balance sheets. During the twelve months ended December 31, 2019 , the Company took delivery of 4 aircraft under secured debt arrangements, 6 aircraft under sale-leaseback transactions and 7 aircraft under direct operating leases. In addition, the Company purchased 5 previously leased aircraft. The Company also purchased 4 new engines and purchased 2 previously leased engines. In June 2019, the Company entered into an aircraft sale agreement to acquire four A320 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 four aircraft. During the third quarter of 2019, the purchase of the four aircraft was completed for an aggregate purchase price of $141.3 million and the related interest in the amount of $0.3 million was recognized within interest expense on the Company's statement of operations for the year ended December 31, 2019. These aircraft were recorded within flight equipment on the Company's balance sheets. On December 31, 2019, the Company entered into an aircraft sale 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 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 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 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 3 to 5 years. Accounting for finance leases is substantially unchanged under Topic 842. 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 leases and long-term debt and finance leases, less current maturities on the Company's balance sheets. On December 18, 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 balance sheet as of December 31, 2019. The 99 -year lease was determined to be an operating lease. As of December 31, 2019, the Company recorded an operating lease right-of-use asset of $14.6 million and related operating lease liability of $9.3 million for this lease. Construction of the headquarters campus is expected to commence mid-year 2020 and take approximately two years to complete. As of December 31, 2019 , the Company estimates the total construction related outlay for the project will be approximately $200 million to $250 million . 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 balance sheets as of December 31, 2019 . 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 Other (in thousands) 2020 $ 44,980 $ 192,937 $ 3,544 $ 517 $ 241,978 2021 606 192,539 3,532 — 196,677 2022 578 193,616 3,221 — 197,415 2023 202 181,667 2,570 — 184,439 2024 — 159,668 2,046 — 161,714 2025 and thereafter — 921,721 138,696 — 1,060,417 Total minimum lease payments $ 46,366 $ 1,842,148 $ 153,609 $ 517 $ 2,042,640 Less amount representing interest 285 526,760 130,825 13 657,883 Present value of minimum lease payments $ 46,081 $ 1,315,388 $ 22,784 $ 504 $ 1,384,757 Less current portion 44,796 117,861 2,297 504 165,458 Long-term portion $ 1,285 $ 1,197,527 $ 20,487 $ — $ 1,219,299 Commitments related to the Company's noncancellable short-term operating leases not recorded on the Company's balance sheets are expected to be $0.6 million for 2020 and none for 2021 and beyond. The table below presents information for lease costs related to the Company's finance and operating leases: Year ended December 31. 2019 (in thousands) Finance lease cost Amortization of leased assets $ 998 Interest of lease liabilities 674 Operating lease cost Operating lease cost (1) 179,959 Short-term lease cost (1) 5,144 Variable lease cost (1) 140,417 Total lease cost $ 327,192 (1) Expenses are classified within aircraft rent and landing fees and other rents on the Company's statements of operations. The table below presents lease-related terms and discount rates as of December 31, 2019 : December 31, 2019 Weighted-average remaining lease term Operating leases 13.0 years Finance leases 0.1 years Weighted-average discount rate Operating leases 5.86 % Finance leases 2.46 %
Leases and Prepaid Mainenance 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 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, 2019 , the Company had a fleet consisting of 145 A320 family aircraft. As of December 31, 2019 , the Company had 52 aircraft financed under operating leases and 2 aircraft financed under finance leases with lease term expirations between 2020 and 2037 . In addition, the Company owned 91 aircraft of which 27 were purchased off lease and are currently unencumbered. As of December 31, 2019 , the Company also had 9 spare engines financed under operating leases with lease term expiration dates ranging from 2020 to 2027 and owned 14 spare engines of which 3 were purchased off lease and all 14 are currently unencumbered. The Company adopted Topic 842 utilizing the modified retrospective adoption method with an effective date of January 1, 2019. Prior to the adoption of Topic 842, the Company had entered into sale leaseback transactions with third-party aircraft lessors for the majority of its aircraft and engine leases. Upon adoption of Topic 842, the Company recognized a $5.5 million cumulative effect adjustment, net of tax, to retained earnings driven by the recognition of unamortized deferred gains and losses related to aircraft sale-leaseback transactions entered into in prior periods. Prior to the adoption of Topic 842, gains and losses on sale-leaseback transactions were generally deferred and recognized in income over the lease term. 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 statements of operations. Total rent expense for all leases charged to operations for the years ended 2019 , 2018 and 2017 was $345.0 million , $312.0 million and $309.8 million , respectively. Total rental expense charged to operations for aircraft and engine operating leases for the years ended December 31, 2019 , 2018 and 2017 was $182.6 million , $177.6 million and $205.9 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, 2019 , 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. Management assesses the need to accrue lease return costs throughout the lease as facts and circumstances warrant an assessment. 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. Supplemental rent 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 $4.8 million , $3.4 million and $8.4 million of supplemental rent recorded within aircraft rent during 2019 , 2018 and 2017 , respectively. These amounts include $0.5 million , $1.3 million and $0.4 million of paid maintenance reserves expensed as supplemental rent during 2019 , 2018 and 2017 , respectively. As of December 31, 2019 , the Company had $170.6 million of aircraft maintenance deposits ( $102.9 million in aircraft maintenance deposits and $67.7 million in long-term aircraft maintenance deposits) on the Company's balance sheets. During the twelve months ended December 31, 2019 , the Company took delivery of 4 aircraft under secured debt arrangements, 6 aircraft under sale-leaseback transactions and 7 aircraft under direct operating leases. In addition, the Company purchased 5 previously leased aircraft. The Company also purchased 4 new engines and purchased 2 previously leased engines. In June 2019, the Company entered into an aircraft sale agreement to acquire four A320 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 four aircraft. During the third quarter of 2019, the purchase of the four aircraft was completed for an aggregate purchase price of $141.3 million and the related interest in the amount of $0.3 million was recognized within interest expense on the Company's statement of operations for the year ended December 31, 2019. These aircraft were recorded within flight equipment on the Company's balance sheets. On December 31, 2019, the Company entered into an aircraft sale 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 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 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 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 3 to 5 years. Accounting for finance leases is substantially unchanged under Topic 842. 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 leases and long-term debt and finance leases, less current maturities on the Company's balance sheets. On December 18, 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 balance sheet as of December 31, 2019. The 99 -year lease was determined to be an operating lease. As of December 31, 2019, the Company recorded an operating lease right-of-use asset of $14.6 million and related operating lease liability of $9.3 million for this lease. Construction of the headquarters campus is expected to commence mid-year 2020 and take approximately two years to complete. As of December 31, 2019 , the Company estimates the total construction related outlay for the project will be approximately $200 million to $250 million . 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 balance sheets as of December 31, 2019 . 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 Other (in thousands) 2020 $ 44,980 $ 192,937 $ 3,544 $ 517 $ 241,978 2021 606 192,539 3,532 — 196,677 2022 578 193,616 3,221 — 197,415 2023 202 181,667 2,570 — 184,439 2024 — 159,668 2,046 — 161,714 2025 and thereafter — 921,721 138,696 — 1,060,417 Total minimum lease payments $ 46,366 $ 1,842,148 $ 153,609 $ 517 $ 2,042,640 Less amount representing interest 285 526,760 130,825 13 657,883 Present value of minimum lease payments $ 46,081 $ 1,315,388 $ 22,784 $ 504 $ 1,384,757 Less current portion 44,796 117,861 2,297 504 165,458 Long-term portion $ 1,285 $ 1,197,527 $ 20,487 $ — $ 1,219,299 Commitments related to the Company's noncancellable short-term operating leases not recorded on the Company's balance sheets are expected to be $0.6 million for 2020 and none for 2021 and beyond. The table below presents information for lease costs related to the Company's finance and operating leases: Year ended December 31. 2019 (in thousands) Finance lease cost Amortization of leased assets $ 998 Interest of lease liabilities 674 Operating lease cost Operating lease cost (1) 179,959 Short-term lease cost (1) 5,144 Variable lease cost (1) 140,417 Total lease cost $ 327,192 (1) Expenses are classified within aircraft rent and landing fees and other rents on the Company's statements of operations. The table below presents lease-related terms and discount rates as of December 31, 2019 : December 31, 2019 Weighted-average remaining lease term Operating leases 13.0 years Finance leases 0.1 years Weighted-average discount rate Operating leases 5.86 % Finance leases 2.46 %

Financial Instruments and Risk

Financial Instruments and Risk Management	12 Months Ended
	Dec. 31, 2019
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Financial Instruments and Risk Management	Financial Instruments and Risk Management As part of the Company’s risk management program, the Company from time to time uses a variety of financial instruments to reduce its exposure to fluctuations in the price of jet fuel and interest rates. The Company does not hold or issue derivative financial instruments for trading purposes.

Defined Contribution 401(k) Pla

Defined Contribution 401(k) Plan	12 Months Ended
	Dec. 31, 2019
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. Employer contributions made to all plans were $51.1 million , $36.7 million and $19.6 million in 2019 , 2018 and 2017

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2019
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, 2019 2018 2017 (in thousands) Current: Federal $ (22,429 ) $ (2,178 ) $ (68,601 ) State and local 1,218 410 515 Foreign 6,693 4,692 2,742 Total current expense (benefit) (14,518 ) 2,924 (65,344 ) Deferred: Federal 106,703 42,246 (9,349 ) State and local 8,986 4,057 8,857 Total deferred expense (benefit) 115,689 46,303 (492 ) Total income tax expense (benefit) $ 101,171 $ 49,227 $ (65,836 ) The income tax provision differs from that computed at the federal statutory corporate tax rate as follows: Year Ended December 31, 2019 2018 2017 Expected provision at federal statutory tax rate 21.0 % 21.0 % 35.0 % State tax expense, net of federal benefit 1.8 % 1.7 % 1.7 % Revaluation of deferred taxes (2.1 )% — % (56.3 )% Other 2.5 % 1.3 % 0.7 % Total income tax expense (benefit) 23.2 % 24.0 % (18.9 )% The Company accounts for income taxes using the asset and liability method. Deferred taxes are recorded based on differences between the financial statement basis and tax basis of assets and liabilities and available tax loss and credit carryforwards. At December 31, 2019 and 2018 , the significant components of the Company's deferred taxes consisted of the following: December 31, 2019 2018 (in thousands) Deferred tax assets: Income tax credits $ 9,632 $ 10,004 Net operating losses 13,604 155,670 Deferred revenue 8,824 6,824 Nondeductible accruals 14,133 14,691 Deferred manufacturing credits 2,813 — Accrued maintenance 1,668 2,168 Equity compensation 2,851 2,592 Operating lease liability 305,161 — Other 482 5,262 Valuation allowance (1,746 ) (254 ) Deferred tax assets 357,422 196,957 Deferred tax liabilities: Deferred gain (loss) on leases, net — 1,672 Accrued rent — 6,068 Prepaid expenses 1,120 793 Property, plant and equipment 430,523 481,847 Deferred financing costs 154 189 Accrued aircraft and engine maintenance 84,479 61,529 Right-of-use asset 310,438 — Deferred tax liabilities 826,714 552,098 Net deferred tax assets (liabilities) $ (469,292 ) $ (355,141 ) On December 22, 2017 , the Tax Cuts and Jobs Act (“TCJA”) was enacted. The TCJA reduces the statutory federal tax rate from 35.0% to 21.0% effective for tax year 2018 in addition to various other tax law changes that impact the Company. Pursuant to ASC 740, the Company is required to recognize the effects of changes in tax laws and rates on deferred tax assets and liabilities upon enactment.The IRS issued new regulations for TCJA in 2019. The new regulations allowed the Company to generate additional net operating losses in 2017, of which a portion was utilized as a carryback to the prior years with the remainder as a carryforward into 2018. As a result, the Company changed its bonus depreciation policy in 2018 to elect out of bonus depreciation on its 2018 income tax return. The change to the bonus depreciation election for 2018 reduced the net operating loss carryforward deferred tax asset as of December 31, 2019 . 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, 2019 and 2018 , the Company had a valuation allowance of $1.7 million and $0.3 million , respectively, against certain deferred tax assets related to equity compensation for executives due to changes in tax law resulting from the TCJA. At December 31, 2019 , the Company had $8.2 million of foreign tax credits and $1.4 million of general business tax credits, $37.7 million of federal net operating loss and $109.4 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. In accordance with ASU No. 2016-09, excess income tax benefits and deficiencies related to share-based compensation are now included within income tax expense rather than additional paid in capital. For the twelve months ended December 31, 2019 and 2018, $1.4 million income tax benefit and $0.4 million of income tax deficiency related to share-based compensation was included within income tax expense, respectively. Prior to the adoption of ASU No. 2016-09, the excess tax (benefit)/deficiency was recorded as a (reduction)/increase to income tax payable and a corresponding entry to additional paid in capital. For tax years ended December 31, 2019 , 2018 and 2017 , the Company did not recognize any liabilities for uncertain tax positions nor any interest and penalties on unrecognized tax benefits. For tax years 2019 , 2018 and 2017 , 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 2016 through 2018 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 financial statements.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2019
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, 2019 , the Company's firm aircraft orders consisted of 135 A320 family aircraft with Airbus, including A319neos, A320neos and A321neos, with deliveries expected through 2027. In addition, the Company had 12 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 four spare engine orders for PurePower PW 1100G-JM engines with Pratt & Whitney. Spare engines are scheduled for delivery from 2020 through 2023 . As of December 31, 2019 , purchase commitments for these aircraft and engines, including estimated amounts for contractual price escalations and pre-delivery payments, are expected to be $988.0 million in 2020 , $744.8 million in 2021 , $123.7 million in 2022 , $491.6 million in 2023 , $1,002.5 million in 2024 , and $3,605.4 million in 2025 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, 2019 , the Company had secured debt financing commitments for 3 aircraft scheduled for delivery in 2020. In addition, as of December 31, 2019 , the Company had secured financing for 12 aircraft to be leased directly from third-party lessors, scheduled for delivery in 2020 through 2021. The Company did not have financing commitments in place for the remaining 132 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, 2019 , principal and interest commitments related to the Company's future secured debt financing of 3 undelivered aircraft are approximately $9.6 million in 2020 , $12.9 million in 2021 , $13.0 million in 2022 , $13.0 million in 2023 , $13.1 million in 2024 , and $83.3 million in 2025 and beyond . As of December 31, 2019 , aircraft rent commitments for future aircraft deliveries to be financed under direct leases from third-party lessors are expected to be approximately $6.0 million in 2020, $34.9 million in 2021, $44.1 million in 2022, $44.1 million in 2023, $44.1 million in 2024, and $356.1 million in 2025 and beyond. Interest commitments related to the secured debt financing of 64 delivered aircraft as of December 31, 2019 are $77.9 million in 2020 , $71.0 million in 2021 , $64.7 million in 2022 , $54.6 million in 2023 , $44.0 million in 2024 , and $118.1 million in 2025 and beyond . For principal commitments related to these financed aircraft, refer to Note 13, 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, 2019 : $21.9 million in 2020 , $17.1 million in 2021 , $17.2 million in 2022 , $13.7 million in 2023 , $13.9 million in 2024 , and $50.3 million in 2025 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 81% of all employees at December 31, 2019 . The table below sets forth the Company's employee groups and status of the collective bargaining agreements as of December 31, 2019 . Employee Groups Representative Amendable Date Percentage of Workforce Pilots Air Line Pilots Association, International (ALPA) February 2023 27% Flight Attendants Association of Flight Attendants (AFA-CWA) May 2021 47% 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 within operating expenses in the statement of operations for the year ended December 31, 2018. For additional information, refer to Note 4, Special Charges. The Company's passenger service agents are represented by the TWU, but the representation applies only 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, 2019, 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 $5.2 million and $4.4 million , for health care claims as of December 31, 2019 , and 2018 , respectively, recorded within other current liabilities on the Company's balance sheet.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2019
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 term loan debt agreements and revolving credit facility 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 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, 2019 and December 31, 2018 , were as follows: As of December 31, 2019 2018 Carrying Value Estimated Fair Value Carrying Value Estimated Fair Value Fair value level hierarchy (in millions) Fixed-rate senior term loans $ 296.1 $ 296.4 $ 382.4 $ 373.6 Level 3 Fixed-rate junior term loans — — 31.1 31.1 Level 3 Fixed-rate term loans 778.2 823.6 625.1 600.1 Level 3 2015-1 EETC Class A 348.6 372.2 378.6 374.8 Level 2 2015-1 EETC Class B 72.0 74.5 80.0 78.1 Level 2 2015-1 EETC Class C 98.1 100.5 109.5 107.9 Level 2 2017-1 EETC Class AA 228.4 237.0 242.5 228.8 Level 2 2017-1 EETC Class A 76.1 78.8 80.8 76.6 Level 2 2017-1 EETC Class B 70.6 72.0 83.7 79.1 Level 2 2017-1 EETC Class C 85.5 88.0 85.5 84.2 Level 2 Revolving credit facility 160.0 160.0 135.3 135.3 Level 3 Total long-term debt $ 2,213.6 $ 2,303.0 $ 2,234.5 $ 2,169.6 Cash and Cash Equivalents Cash and cash equivalents at December 31, 2019 and December 31, 2018 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. Short-term Investment Securities Short-term investment securities at December 31, 2019 and December 31, 2018 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 8, 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 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, 2019 , the Company had $2.3 million in assets held for sale recorded within prepaid expenses and other current assets in the accompanying balance sheets. As of December 31, 2018 , the Company did not hold any assets held for sale. Assets and liabilities measured at gross fair value on a recurring basis are summarized below: Fair Value Measurements as of December 31, 2019 Total Level Level Level (in millions) Cash and cash equivalents $ 979.0 $ 979.0 $ — $ — 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 $ — $ — $ — $ — Fair Value Measurements as of December 31, 2018 Total Level Level Level (in millions) Cash and cash equivalents $ 1,004.7 $ 1,004.7 $ — $ — Short-term investment securities 102.8 102.8 — — Total assets $ 1,107.5 $ 1,107.5 $ — $ — Total liabilities $ — $ — $ — $ — The Company had no transfers of assets or liabilities between any of the above levels during the years ended December 31, 2019 or 2018 . The following table presents the Company's activity for assets and liabilities measured at gross fair value on a recurring basis using significant unobservable inputs (Level 3): 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 Related

Operating Segments and Related Disclosures	12 Months Ended
	Dec. 31, 2019
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: 2019 2018 2017 (in millions) DOT—Domestic $ 3,462.8 $ 2,990.7 $ 2,432.1 DOT—Latin America and Caribbean 367.7 332.3 211.5 Total $ 3,830.5 $ 3,323.0 $ 2,643.6 During 2019 , 2018 and 2017 , no revenue from any one foreign country represented greater than 4% of the Company’s total passenger revenue. The Company attributes operating revenues by geographic region based upon the origin and destination of each passenger flight segment. The Company’s tangible assets consist primarily of flight equipment, which are mobile across geographic markets and, therefore, have not been allocated.

Quarterly Financial Data (Unaud

Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2019
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Data (Unaudited)	Quarterly Financial Data (Unaudited) Quarterly results of operations for the years ended December 31, 2019 and 2018 are summarized below: Three Months Ended March 31 June 30 September 30 December 31 (in thousands, except per-share amounts) 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 2018 Operating revenue $ 704,138 $ 851,771 $ 904,330 $ 862,795 Operating income (loss) (38,797 ) 108,521 145,125 136,065 Net income (loss) (44,922 ) 11,254 97,480 91,937 Basic earnings (loss) per share (0.66 ) 0.16 1.43 1.35 Diluted earnings (loss) per share (0.66 ) 0.16 1.42 1.34 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 and third quarters of each year. The air transportation business is also volatile and highly affected by economic cycles and trends.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation Spirit Airlines, Inc. ("Spirit" or the "Company") headquartered in Miramar, Florida, is an ultra low-cost, low-fare airline that provides affordable travel opportunities principally throughout the domestic United States, the Caribbean and Latin America. The Company 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.
Use of Estimates	Use of Estimates
Cash and Cash Equivalents	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.
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 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 statements of cash flows. Realized gains and losses on sales of investments, if any, are reflected in non-operating income (expense) in the statements of operations. Unrealized gains and losses on investment securities are reflected as a component of accumulated other comprehensive income.
Accounts Receivable	Accounts Receivable
Property and Equipment	The Company records amortization of capitalized software on a straight-line basis within depreciation and amortization expense in the accompanying statements of operations. 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 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 years Spare rotables and flight assemblies 7 to 15 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, 2019 , the Company had 91 aircraft, 14 spare engines and 1 flight simulator capitalized within flight equipment with depreciable lives of 25 years . As of December 31, 2019 , the Company had 52 aircraft financed through operating leases and 2 aircraft financed through finance leases with lease terms from 8 to 18 years . In addition, the Company had 9 spare engines financed through operating leases with lease terms from 2 to 14 years .
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 has 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 statements of operations.
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 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 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 balance sheets along with the related PDPs as they are included in the cost of the aircraft or engine. Capitalized interest for 2019, 2018 and 2017 is primarily related to the interest incurred on long-term debt.
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.
Revenue from Contract with Customer	The Company's co-brand credit card agreement provides for joint marketing where cardholders earn mileage credits for making purchases using co-branded cards. During 2015, the Company extended its agreement with the administrator of the FREE SPIRIT affinity credit card program to extend through 2022. 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. 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 generally expires 60 days from the date the credit shell is created and which can be used towards the purchase of a new ticket and the Company’s other service offerings. Both the 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. However, given the relatively short period of time to expiration, this does not have a significant impact on the Company's financial statements. 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. Passenger Revenues Fare revenues. Tickets sold are initially deferred within air traffic liability on the Company's balance sheet. Passenger fare revenues are recognized at time of departure when transportation is provided. 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, 2019 and 2018 , the Company had air traffic liability ("ATL") balances of $315.4 million and $292.0 million , respectively. As of December 31, 2019 , all of the ATL balance as of December 31, 2018 has been recognized. The balance of the Company's ATL as of December 31, 2019 is expected to be recognized within 12 months. The Company adopted ASU 2014-09 on January 1, 2018 utilizing the full retrospective method of adoption. Passenger revenues reported prior to the adoption of ASU 2014-09 are now reported as fare revenues within passenger revenues in the Company's disaggregated revenue table within Note 3, Revenue Disaggregation. Non-fare revenues. The adoption of ASU 2014-09 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 3, Revenue Disaggregation.
Frequent Flier Program	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. 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. The adoption of ASU 2014-09 eliminated the incremental cost method for frequent flyer program accounting, which required the Company to re-value and record a liability associated with customer flight miles earned with travel as part of the Company’s frequent flyer program with a relative fair value. Upon adoption of ASU 2014-09 on January 1, 2018, the Company recorded an increase to its air traffic liability of $12.4 million . 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-brand credit card agreement provides for joint marketing where cardholders earn mileage credits for making purchases using co-branded cards. During 2015, the Company extended its agreement with the administrator of the FREE SPIRIT affinity credit card program to extend through 2022. 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 Company defers the amount for award travel obligation as part of loyalty deferred revenue within air traffic liability on the 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 $29.8 million and $27.4 million at December 31, 2019 and 2018 , 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 balance sheets.
Airframe and Engine Maintenance	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. 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 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, 2019 , 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 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 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	Advertising
Income Taxes	Income Taxes
Stock-Based Compensation	Stock-Based Compensation
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 30% , 32% and 27% of total operating expenses in 2019 , 2018 and 2017 , 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.
Recent Accounting Developments	Recent Accounting Developments Recently Adopted Accounting Pronouncements Leases The Company adopted ASU No. 2016-02, "Leases (Topic 842)," effective January 1, 2019. The Company adopted Topic 842 utilizing the modified retrospective adoption method with an effective date of January 1, 2019 and elected the package of transition practical expedients for expired or existing contracts, which does not require reassessment of: (1) whether any of the Company's contracts are or contain leases, (2) lease classification and (3) initial direct costs. Therefore, the financial statements for 2019 are presented under the new standard, while the comparative periods presented are not adjusted and continue to be reported in accordance with the Company's historical accounting policy. This standard requires all lessees to recognize a right-of-use asset and a lease liability, initially measured at the present value of the lease payments, for all leases with a term greater than 12 months. The adoption of the new lease standard had a significant impact on the Company's balance sheets due to the recognition of $1.0 billion of right-of-use assets for operating leases, $128.1 million of current maturities of operating leases and $895.1 million of operating leases, less current maturities. In addition, the Company recognized a $5.5 million cumulative effect adjustment, net of tax, to retained earnings. This adjustment was driven by the recognition of unamortized deferred gains and losses related to aircraft sale-leaseback transactions entered into in prior periods. Prior to the adoption of Topic 842, gains and losses on sale-leaseback transactions were generally deferred and recognized in income over the lease term. The accounting for finance leases is substantially unchanged. The adoption of Topic 842 did not have a significant impact on the Company's lease classification or a material impact on its statements of operations and liquidity. Additionally, the adoption of Topic 842 did not have a material impact on the Company’s debt-covenant compliance under its current agreements. Refer to Note 14, Leases and Prepaid Maintenance Deposits for information regarding the Company's adoption of Topic 842 and the Company's undiscounted future lease payments and the timing of those payments. Recently Issued Accounting Pronouncements Not Yet Adopted Cloud Computing Arrangements In August 2018, the FASB issued ASU No. 2018-15, "Intangibles - Goodwill and Other - Internal-Use Software." This new standard requires a customer in a cloud computing arrangement that is a service contract to follow the internal-use software guidance in Accounting Standards Codification ("ASC") 350-40, "Accounting for Internal-Use Software," to determine which implementation costs to capitalize as assets and amortize over the term of the hosting arrangement or expense as incurred. This standard is effective for the Company for fiscal years, and interim periods within those years, beginning January 1, 2020. Early adoption is permitted, including during an interim period. The Company has elected not to early adopt. Entities have the option to apply this standard prospectively to all implementation costs incurred after the date of adoption or retrospectively. The Company is evaluating this new standard, but does not expect it to have a significant impact on its financial statement presentation or results. Accounting for Credit Losses In June 2016, the FASB issued ASU No. 2016-13, "Financial Instruments - Credit Losses." The standard requires the use of an "expected loss" model on certain types of financial instruments. For accounts receivables, aircraft maintenance deposits and security deposits, recorded within other long-term assets on the Company's balance sheets, the Company will be 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 will be 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. This standard is effective for the Company for fiscal years, and interim periods within those years, beginning January 1, 2020, with early adoption permitted. The Company has elected not to early adopt. The Company does not expect the standard to have a material impact on its financial statements.
Sale Leaseback Transactions	Prior to the adoption of Topic 842, gains and losses on sale-leaseback transactions were generally deferred and recognized in income over the lease term. Under Topic 842, gains and losses on sale-leaseback transactions, subject to adjustment for off-market terms, are recognized immediately and recorded within loss on disposal of assets on the Company's statements of operations.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Depreciable lives used for the principal depreciable asset classifications and components of depreciation and amortization	The following table illustrates the components of depreciation and amortization expense: Year Ended December 31, 2019 2018 2017 (in thousands) Depreciation $ 155,326 $ 129,412 $ 83,154 Amortization of heavy maintenance 63,364 41,286 53,855 Amortization of capitalized software 6,574 6,029 3,143 Total depreciation and amortization $ 225,264 $ 176,727 $ 140,152 The depreciable lives used for the principal depreciable asset classifications are: Estimated Useful Life Aircraft, engines and flight simulators 25 years Spare rotables and flight assemblies 7 to 15 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
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 2019 2018 2017 (in thousands) Baggage Time of departure $ 734,243 $ 620,154 $ 488,434 Passenger usage fee Time of departure 669,177 531,459 411,742 Advance seat selection Time of departure 228,876 180,012 131,821 Other 238,454 224,283 177,495 Non-fare revenue $ 1,870,750 $ 1,555,908 $ 1,209,492
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, 2019 $ 48,136 $ 37,151 December 31, 2018 39,194 30,353 December 31, 2017 49,453 37,960
Aircraft maintenance expense	The table below summarizes the components of the Company’s maintenance cost: Year Ended December 31, 2019 2018 2017 (in thousands) Flight hour-based maintenance expense $ 78,253 $ 68,039 $ 54,802 Non-flight hour-based maintenance expense 65,322 61,039 55,637 Total maintenance, materials and repairs $ 143,575 $ 129,078 $ 110,439

Revenue Disaggregation (Tables)

Revenue Disaggregation (Tables)	12 Months Ended
	Dec. 31, 2019
Revenue from Contract with Customer [Abstract]
Schedule of Revenue Disaggregation	The following table shows disaggregated operating revenues for the twelve months ended December 31, 2019 , 2018 and 2017 . Twelve Months Ended December 31, 2019 2018 2017 (in thousands) Operating revenues: Fare $ 1,886,855 $ 1,704,107 $ 1,363,395 Non-fare 1,870,750 1,555,908 1,209,492 Total passenger revenues 3,757,605 3,260,015 2,572,887 Other 72,931 63,019 70,665 Total operating revenues $ 3,830,536 $ 3,323,034 $ 2,643,552

Accrued Liabilities (Tables)

Accrued Liabilities (Tables)	12 Months Ended
	Dec. 31, 2019
Payables and Accruals [Abstract]
Accrued liabilities included in other current liabilities	Accrued liabilities included in other current liabilities as of December 31, 2019 and 2018 consist of the following: As of December 31, 2019 2018 (in thousands) Salaries and wages $ 89,163 $ 82,900 Airport obligations 80,134 52,029 Federal excise and other passenger taxes and fees payable 65,312 60,604 Aircraft maintenance 38,099 59,805 Fuel 28,510 25,368 Aircraft and facility lease obligations 20,656 15,149 Interest payable 16,941 18,086 Other 34,706 25,736 Other current liabilities $ 373,521 $ 339,677

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 and changes during the year ended December 31, 2019 is presented below: Number of Shares Weighted-Average Outstanding at December 31, 2018 297,797 47.95 Granted 148,120 53.41 Vested (97,095 ) 48.97 Forfeited (15,856 ) 53.00 Outstanding at December 31, 2019 332,966 49.84
Market Condition Performance Share Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
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, 2019 Weighted-Average at Grant Date for Twelve Months Ended December 31, 2018 Expected volatility factor 0.38 0.39 Risk free interest rate 2.50 % 2.11 % Expected term (in years) 2.97 2.96 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, 2019 : Number of Awards Weighted-Average Fair Value at Grant Date ($) Outstanding at December 31, 2018 82,907 53.92 Granted 50,293 70.77 Vested (29,293 ) 56.84 Forfeited (7,748 ) 57.15 Outstanding at December 31, 2019 96,159 61.58
Performance Condition Performance Share Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance share award activity	The following table summarizes the Company’s performance condition performance share awards for the year ended December 31, 2019 : Number of Awards Weighted-Average Fair Value at Grant Date ($) Outstanding at December 31, 2018 41,454 49.81 Granted 24,861 60.11 Vested (14,648 ) 55.50 Forfeited (3,873 ) 52.10 Outstanding at December 31, 2019 47,794 53.24

Earnings per Share (Tables)

Earnings per Share (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 2018 2017 (in thousands, except per-share amounts) Numerator: Net income (1) $ 335,255 $ 155,749 $ 415,522 Denominator: Weighted-average shares outstanding, basic 68,429 68,249 69,221 Effect of dilutive stock awards 130 182 156 Adjusted weighted-average shares outstanding, diluted 68,559 68,431 69,377 Earnings per Share: Basic earnings per common share (1) $ 4.90 $ 2.28 $ 6.00 Diluted earnings per common share (1) $ 4.89 $ 2.28 $ 5.99 Anti-dilutive weighted-average shares 143 145 85

Debt and Other Obligations (Tab

Debt and Other Obligations (Tables)	12 Months Ended
	Dec. 31, 2019
Debt Disclosure [Abstract]
Schedule of long-term debt instruments	Long-term debt is comprised of the following: As of December 31, 2019 December 31, 2018 December 31, 2019 December 31, 2018 (in millions) (weighted-average interest rates) Fixed-rate senior term loans due through 2027 $ 296.1 $ 382.4 4.02 % 4.10 % Fixed-rate junior term loans due through 2022 — 31.1 N/A 6.90 % Fixed-rate loans due through 2030 778.2 625.1 3.70 % 3.88 % Fixed-rate class A 2015-1 EETC due through 2028 348.6 378.6 4.10 % 4.10 % Fixed-rate class B 2015-1 EETC due through 2024 72.0 80.0 4.45 % 4.45 % Fixed-rate class C 2015-1 EETC due through 2023 98.1 109.5 4.93 % 4.93 % Fixed-rate class AA 2017-1 EETC due through 2030 228.4 242.5 3.38 % 3.38 % Fixed-rate class A 2017-1 EETC due through 2030 76.1 80.8 3.65 % 3.65 % Fixed-rate class B 2017-1 EETC due through 2026 70.6 83.7 3.80 % 3.80 % Fixed-rate class C 2017-1 EETC due through 2023 85.5 85.5 5.11 % 5.11 % Revolving credit facility due in 2021 160.0 135.3 3.12 % 3.72 % Long-term debt $ 2,213.6 $ 2,234.5 Less current maturities 214.0 162.8 Less unamortized discount, net 40.4 47.7 Total $ 1,959.2 $ 2,024.0
Schedule of maturities of long-term debt	At December 31, 2019 , long-term debt principal payments for the next five years and thereafter are as follows: December 31, 2019 (in millions) 2020 $ 222.1 2021 273.8 2022 167.7 2023 310.1 2024 195.5 2025 and beyond 1,044.4 Total debt principal payments $ 2,213.6
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, 2019 2018 (in thousands) Fixed-rate senior term loans $ 15,225 $ 16,869 Fixed-rate junior term loans 1,811 2,475 Fixed-rate term loans 25,828 19,615 Class A 2015-1 EETC 14,894 16,138 Class B 2015-1 EETC 3,377 3,781 Class C 2015-1 EETC 5,117 3,575 Class AA 2017-1 EETC 7,887 6,026 Class A 2017-1 EETC 2,843 2,172 Class B 2017-1 EETC 2,870 2,420 Class C 2017-1 EETC 4,367 2,478 Revolving credit facility 5,792 793 Finance leases 408 104 Commitment fees and other 2,217 262 Amortization of deferred financing costs 8,714 7,069 Total $ 101,350 $ 83,777

Leases and Aircraft Maintenan_2

Leases and Aircraft Maintenance Deposits (Tables)	12 Months Ended
	Dec. 31, 2019
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 balance sheets as of December 31, 2019 . 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 Other (in thousands) 2020 $ 44,980 $ 192,937 $ 3,544 $ 517 $ 241,978 2021 606 192,539 3,532 — 196,677 2022 578 193,616 3,221 — 197,415 2023 202 181,667 2,570 — 184,439 2024 — 159,668 2,046 — 161,714 2025 and thereafter — 921,721 138,696 — 1,060,417 Total minimum lease payments $ 46,366 $ 1,842,148 $ 153,609 $ 517 $ 2,042,640 Less amount representing interest 285 526,760 130,825 13 657,883 Present value of minimum lease payments $ 46,081 $ 1,315,388 $ 22,784 $ 504 $ 1,384,757 Less current portion 44,796 117,861 2,297 504 165,458 Long-term portion $ 1,285 $ 1,197,527 $ 20,487 $ — $ 1,219,299
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 balance sheets as of December 31, 2019 . 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 Other (in thousands) 2020 $ 44,980 $ 192,937 $ 3,544 $ 517 $ 241,978 2021 606 192,539 3,532 — 196,677 2022 578 193,616 3,221 — 197,415 2023 202 181,667 2,570 — 184,439 2024 — 159,668 2,046 — 161,714 2025 and thereafter — 921,721 138,696 — 1,060,417 Total minimum lease payments $ 46,366 $ 1,842,148 $ 153,609 $ 517 $ 2,042,640 Less amount representing interest 285 526,760 130,825 13 657,883 Present value of minimum lease payments $ 46,081 $ 1,315,388 $ 22,784 $ 504 $ 1,384,757 Less current portion 44,796 117,861 2,297 504 165,458 Long-term portion $ 1,285 $ 1,197,527 $ 20,487 $ — $ 1,219,299
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. 2019 (in thousands) Finance lease cost Amortization of leased assets $ 998 Interest of lease liabilities 674 Operating lease cost Operating lease cost (1) 179,959 Short-term lease cost (1) 5,144 Variable lease cost (1) 140,417 Total lease cost $ 327,192 (1) Expenses are classified within aircraft rent and landing fees and other rents on the Company's statements of operations. The table below presents lease-related terms and discount rates as of December 31, 2019 : December 31, 2019 Weighted-average remaining lease term Operating leases 13.0 years Finance leases 0.1 years Weighted-average discount rate Operating leases 5.86 % Finance leases 2.46 %

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 2018 2017 (in thousands) Current: Federal $ (22,429 ) $ (2,178 ) $ (68,601 ) State and local 1,218 410 515 Foreign 6,693 4,692 2,742 Total current expense (benefit) (14,518 ) 2,924 (65,344 ) Deferred: Federal 106,703 42,246 (9,349 ) State and local 8,986 4,057 8,857 Total deferred expense (benefit) 115,689 46,303 (492 ) Total income tax expense (benefit) $ 101,171 $ 49,227 $ (65,836 )
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, 2019 2018 2017 Expected provision at federal statutory tax rate 21.0 % 21.0 % 35.0 % State tax expense, net of federal benefit 1.8 % 1.7 % 1.7 % Revaluation of deferred taxes (2.1 )% — % (56.3 )% Other 2.5 % 1.3 % 0.7 % Total income tax expense (benefit) 23.2 % 24.0 % (18.9 )%
Deferred taxes	At December 31, 2019 and 2018 , the significant components of the Company's deferred taxes consisted of the following: December 31, 2019 2018 (in thousands) Deferred tax assets: Income tax credits $ 9,632 $ 10,004 Net operating losses 13,604 155,670 Deferred revenue 8,824 6,824 Nondeductible accruals 14,133 14,691 Deferred manufacturing credits 2,813 — Accrued maintenance 1,668 2,168 Equity compensation 2,851 2,592 Operating lease liability 305,161 — Other 482 5,262 Valuation allowance (1,746 ) (254 ) Deferred tax assets 357,422 196,957 Deferred tax liabilities: Deferred gain (loss) on leases, net — 1,672 Accrued rent — 6,068 Prepaid expenses 1,120 793 Property, plant and equipment 430,523 481,847 Deferred financing costs 154 189 Accrued aircraft and engine maintenance 84,479 61,529 Right-of-use asset 310,438 — Deferred tax liabilities 826,714 552,098 Net deferred tax assets (liabilities) $ (469,292 ) $ (355,141 )

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 . Employee Groups Representative Amendable Date Percentage of Workforce Pilots Air Line Pilots Association, International (ALPA) February 2023 27% Flight Attendants Association of Flight Attendants (AFA-CWA) May 2021 47% 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, 2019
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, 2019 and December 31, 2018 , were as follows: As of December 31, 2019 2018 Carrying Value Estimated Fair Value Carrying Value Estimated Fair Value Fair value level hierarchy (in millions) Fixed-rate senior term loans $ 296.1 $ 296.4 $ 382.4 $ 373.6 Level 3 Fixed-rate junior term loans — — 31.1 31.1 Level 3 Fixed-rate term loans 778.2 823.6 625.1 600.1 Level 3 2015-1 EETC Class A 348.6 372.2 378.6 374.8 Level 2 2015-1 EETC Class B 72.0 74.5 80.0 78.1 Level 2 2015-1 EETC Class C 98.1 100.5 109.5 107.9 Level 2 2017-1 EETC Class AA 228.4 237.0 242.5 228.8 Level 2 2017-1 EETC Class A 76.1 78.8 80.8 76.6 Level 2 2017-1 EETC Class B 70.6 72.0 83.7 79.1 Level 2 2017-1 EETC Class C 85.5 88.0 85.5 84.2 Level 2 Revolving credit facility 160.0 160.0 135.3 135.3 Level 3 Total long-term debt $ 2,213.6 $ 2,303.0 $ 2,234.5 $ 2,169.6
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, 2019 Total Level Level Level (in millions) Cash and cash equivalents $ 979.0 $ 979.0 $ — $ — 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 $ — $ — $ — $ — Fair Value Measurements as of December 31, 2018 Total Level Level Level (in millions) Cash and cash equivalents $ 1,004.7 $ 1,004.7 $ — $ — Short-term investment securities 102.8 102.8 — — Total assets $ 1,107.5 $ 1,107.5 $ — $ — Total liabilities $ — $ — $ — $ — The following table presents the Company's activity for assets and liabilities measured at gross fair value on a recurring basis using significant unobservable inputs (Level 3): 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, 2019
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: 2019 2018 2017 (in millions) DOT—Domestic $ 3,462.8 $ 2,990.7 $ 2,432.1 DOT—Latin America and Caribbean 367.7 332.3 211.5 Total $ 3,830.5 $ 3,323.0 $ 2,643.6

Quarterly Financial Data (Una_2

Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2019
Quarterly Financial Information Disclosure [Abstract]
Quarterly results of operations	Quarterly results of operations for the years ended December 31, 2019 and 2018 are summarized below: Three Months Ended March 31 June 30 September 30 December 31 (in thousands, except per-share amounts) 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 2018 Operating revenue $ 704,138 $ 851,771 $ 904,330 $ 862,795 Operating income (loss) (38,797 ) 108,521 145,125 136,065 Net income (loss) (44,922 ) 11,254 97,480 91,937 Basic earnings (loss) per share (0.66 ) 0.16 1.43 1.35 Diluted earnings (loss) per share (0.66 ) 0.16 1.42 1.34

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Reportable Segments (Details)	12 Months Ended
	Dec. 31, 2019segment
Accounting Policies [Abstract]
Number of reportable segments	1

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Property and Equipment (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)aircraft	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Property, Plant and Equipment [Line Items]
Number of spare engines capitalized | aircraft	14
Number of flight simulators capitalized | aircraft	1
Depreciation, Depletion and Amortization, Nonproduction [Abstract]
Depreciation	$ 155,326	$ 129,412	$ 83,154
Amortization of capitalized software	6,574	6,029	3,143
Total depreciation and amortization	$ 225,264	176,727	140,152
Aircrafts, Major Spare Rotable Parts, Avionics, and Assemblies
Property, Plant and Equipment [Line Items]
Residual value, percentage	10.00%
Aircraft
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	15 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	$ 13,000	13,700
Capitalized software costs during the year	$ 5,900	12,000	1,400
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
Heavy maintenance and major overhaul
Depreciation, Depletion and Amortization, Nonproduction [Abstract]
Amortization of heavy maintenance	$ 63,364	$ 41,286	$ 53,855
Spare engines | Minimum
Property, Plant and Equipment [Line Items]
Operating leases, term	2 years
Spare engines | Maximum
Property, Plant and Equipment [Line Items]
Operating leases, term	14 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]
Operating leases, term	8 years
Aircraft | Maximum
Property, Plant and Equipment [Line Items]
Operating leases, term	18 years
A320 Family | Aircraft
Property, Plant and Equipment [Line Items]
Number of aircraft under finance lease | aircraft	2
A320 Family | Aircraft
Property, Plant and Equipment [Line Items]
Number of aircraft capitalized | aircraft	91

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Passenger Revenues (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Air traffic liability	$ 315,408				$ 291,981				$ 315,408	$ 291,981
Credit shell term of expiration									60 days
Total operating revenues	$ 969,816	$ 991,968	$ 1,012,956	$ 855,796	$ 862,795	$ 904,330	$ 851,771	$ 704,138	$ 3,830,536	3,323,034	$ 2,643,552
Baggage
Disaggregation of Revenue [Line Items]
Total operating revenues									734,243	620,154	488,434
Passenger usage fee
Disaggregation of Revenue [Line Items]
Total operating revenues									669,177	531,459	411,742
Advance seat selection
Disaggregation of Revenue [Line Items]
Total operating revenues									228,876	180,012	131,821
Other
Disaggregation of Revenue [Line Items]
Total operating revenues									238,454	224,283	177,495
Non-fare revenue
Disaggregation of Revenue [Line Items]
Total operating revenues									$ 1,870,750	$ 1,555,908	$ 1,209,492

Summary of Significant Accoun_7

Summary of Significant Accounting Policies - Frequent Flier Program (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)revenue_arrangement	Dec. 31, 2018USD ($)	Jan. 01, 2018USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Frequent flyer program, number of types of revenue arrangements | revenue_arrangement	2
Air traffic liability	$ 315,408	$ 291,981
Unrecognized revenue from future FREE SPIRIT award redemptions and the sale of mileage credits	$ 29,800	$ 27,400
Difference between Revenue Guidance in Effect before and after Topic 606 | ASU 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Air traffic liability			$ 12,400

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, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Consideration received from credit card mile programs	$ 48,136	$ 39,194	$ 49,453
Portion of proceeds recognized immediately as marketing component	$ 37,151	$ 30,353	$ 37,960

Summary of Significant Accoun_9

Summary of Significant Accounting Policies - Maintenance Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Deferred costs for heavy maintenance	$ 176,000	$ 190,500	$ 78,200
Deferred heavy maintenance, gross	504,200	366,300
Accumulated heavy maintenance amortization	492,447	332,864
Airframe and Engine Maintenance Costs [Abstract]
Total maintenance, materials and repairs	143,575	129,078	110,439
Heavy maintenance and major overhaul
Property, Plant and Equipment [Line Items]
Amortization of heavy maintenance	63,364	41,286	53,855
Heavy Maintenance
Property, Plant and Equipment [Line Items]
Accumulated heavy maintenance amortization	142,600	117,300
Flight hour-based maintenance expense
Airframe and Engine Maintenance Costs [Abstract]
Total maintenance, materials and repairs	78,253	68,039	54,802
Non-flight hour-based maintenance expense
Airframe and Engine Maintenance Costs [Abstract]
Total maintenance, materials and repairs	$ 65,322	$ 61,039	$ 55,637

Summary of Significant Accou_10

Summary of Significant Accounting Policies - Advertising (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Marketing and advertising expenses	$ 6.3	$ 6.3	$ 5.1

Summary of Significant Accou_11

Summary of Significant Accounting Policies - Income Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Valuation allowance	$ 1,746	$ 254

Summary of Significant Accou_12

Summary of Significant Accounting Policies - Concentrations of Risk (Details) - employee_group	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Total operating expenses | Aircraft fuel expenditure concentration risk
Concentration Risk [Line Items]
Concentration of risk	30.00%	32.00%	27.00%
Number of employees, total | Unionized employees concentration risk
Concentration Risk [Line Items]
Concentration of risk	81.00%
Union-represented employee groups	5

Recent Accounting Developments

Recent Accounting Developments Recent Accounting Developments (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Adjustments for New Accounting Pronouncements [Line Items]
Operating lease, right-of-use asset	$ 1,369,555			$ 0
Current maturities of operating leases	120,662			0
Operating leases, less current maturities	$ 1,218,014			$ 0
Accounting Standards Update 2016-02
Adjustments for New Accounting Pronouncements [Line Items]
Operating lease, right-of-use asset		$ 1,000,000
Current maturities of operating leases		128,100
Operating leases, less current maturities		$ 895,100
Restatement Adjustment | Accounting Standards Update 2016-02
Adjustments for New Accounting Pronouncements [Line Items]
Retained earnings impact of Topic 842 adoption			$ (5,500)

Revenue Disaggregation (Details

Revenue Disaggregation (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Air traffic liability	$ 315,408				$ 291,981				$ 315,408	$ 291,981
Total operating revenues	$ 969,816	$ 991,968	$ 1,012,956	$ 855,796	$ 862,795	$ 904,330	$ 851,771	$ 704,138	3,830,536	3,323,034	$ 2,643,552
Fare
Disaggregation of Revenue [Line Items]
Total operating revenues									1,886,855	1,704,107	1,363,395
Non Fare
Disaggregation of Revenue [Line Items]
Total operating revenues									1,870,750	1,555,908	1,209,492
Passenger
Disaggregation of Revenue [Line Items]
Total operating revenues									3,757,605	3,260,015	2,572,887
Other
Disaggregation of Revenue [Line Items]
Total operating revenues									$ 72,931	$ 63,019	$ 70,665

Special Charges (Details)

Special Charges (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)aircraft	Dec. 31, 2018USD ($)aircraft	Dec. 31, 2017USD ($)aircraft_engineaircraft
Property, Plant and Equipment [Line Items]
Purchase of property and equipment	$ 192,437	$ 606,971	$ 628,881
Special charges	717	88,921	12,629
Special charges, non-operating	$ 0	$ 90,357	$ 0
Spare engines
Property, Plant and Equipment [Line Items]
Number of previously leased aircraft purchased off lease | aircraft	2
Aircraft
Property, Plant and Equipment [Line Items]
Number of previously leased aircraft purchased off lease | aircraft			1
Purchase of property and equipment			$ 12,600
Value of noncash consideration			7,400
Fair value of assets acquired			11,900
Special charges			7,900
Aircraft | Other Non-Cash Items
Property, Plant and Equipment [Line Items]
Value of noncash consideration			200
Airbus A319 | Aircraft
Property, Plant and Equipment [Line Items]
Purchase price			$ 20,000
Spare engines | Spare engines
Property, Plant and Equipment [Line Items]
Number of previously leased aircraft purchased off lease | aircraft_engine			1
Purchase price			$ 8,100
Purchase of property and equipment			3,800
Value of noncash consideration			4,300
Fair value of assets acquired			3,100
Special charges			4,800
Spare engines | Spare engines | Other Non-Cash Items
Property, Plant and Equipment [Line Items]
Value of noncash consideration			$ 200
Capital Lease Obligations | Airbus A319
Property, Plant and Equipment [Line Items]
Number of aircraft purchased | aircraft		14
Consideration transferred		$ 285,000
Pilots
Property, Plant and Equipment [Line Items]
Ratification Incentive		80,200
Collective bargaining agreement, contractual provision adjustments		8,500
Special charges		$ 88,700

Loss on Disposal of Assets - Na

Loss on Disposal of Assets - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)aircraft_engine	Dec. 31, 2017USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Loss on disposal of assets	$ 17,350	$ 9,580	$ 4,168
Inventory write-down	13,400	$ 4,400
Write down of assets held-for-sale	3,100
Capitalized costs for enterprise accounting software write-off	2,400
Gain on sale-leaseback transaction	$ 1,500
Number of engines sold | aircraft_engine		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) - Unsecured, standby letter of credit facility - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Line of Credit Facility [Line Items]
Letters of credit, limit, amount	$ 35	$ 35
Letter of credit facility, amount outstanding	$ 23.3	$ 18.1

Credit Card Processing Arrang_2

Credit Card Processing Arrangements (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Credit Card Processing Arrangements [Abstract]
Cash holdbacks	$ 0	$ 0
Maximum potential exposure to cash holdbacks by the credit card processors	$ 342,300,000	$ 321,000,000

Short-term Investment Securit_2

Short-term Investment Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Short-term investment securities	$ 105,321	$ 102,789
Unrealized loss on investment securities, net of deferred taxes	167	30	$ (82)
Deferred taxes on unrealized loss on investment securities	38	44	41
Debt Securities, Available-for-sale, Realized Gain (Loss)	5	0	$ 0
Accumulated other comprehensive income (loss)	$ (787)	$ (1,193)
Available-for-sale Securities
Debt Securities, Available-for-sale [Line Items]
Weighted-average fixed rate	2.30%	1.60%	1.40%
Unrealized loss on investment securities, net of deferred taxes	$ 104	$ 30
Deferred taxes on unrealized loss on investment securities	31	44
Accumulated other comprehensive income (loss)	$ 104	$ 74

Accrued Liabilities (Details)

Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Salaries and wages	$ 89,163	$ 82,900
Airport obligations	80,134	52,029
Federal excise and other passenger taxes and fees payable	65,312	60,604
Aircraft maintenance	38,099	59,805
Fuel	28,510	25,368
Aircraft and facility lease obligations	20,656	15,149
Interest payable	16,941	18,086
Other	34,706	25,736
Other current liabilities	$ 373,521	$ 339,677

Common Stock and Preferred St_2

Common Stock and Preferred Stock (Details) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
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	10,000,000
Preferred stock, par value (in dollars per share)	$ 0.0001
Common stock, shares outstanding (in shares)	68,455,011	68,269,567
Preferred stock, shares outstanding	0	0
Voting 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
Non-Voting Common Stock, $0.0001 par value
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, shares outstanding (in shares)	0	0

Stock-Based Compensation - Narr

Stock-Based Compensation - Narrative (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense		$ 8,200	$ 11,000	$ 8,500
Share-based compensation		8,154	11,021	8,522
Share-based Payment Arrangement, Expense, Tax Benefit		$ 1,900	$ 2,600	$ 1,600
Shares repurchased (in shares)		91,000	28,000	1,200,000
Repurchase of common stock		$ 5,439	$ 1,162	$ 46,580
Number of treasury shares retired (in shares)		0	0	3,900,000
Retirement of treasury stock				$ 0
Additional Paid-In Capital
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Retirement of treasury stock		$ 0	$ 0	$ 199,418
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	1,897,809	2,110,598
Stock Appreciation Rights (SARs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense	$ (1,900)	$ 700	$ 1,200

Stock-Based Compensation - Rest

Stock-Based Compensation - Restricted Stock Activity (Details) - Restricted Stock - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Period for share issuance after vesting	30 days
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Outstanding (in shares)	297,797
Granted (in shares)	148,120	185,054
Vested (in shares)	(97,095)
Forfeited (in shares)	(15,856)
Outstanding (in shares)	332,966	297,797
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 (in dollars per share)	$ 47.95
Granted, weighted-average grant date fair value (in dollars per share)	53.41	$ 46.90	$ 51.68
Vested, weighted-average grant date fair value (in dollars per share)	48.97
Forfeited, weighted-average grant date fair value (in dollars per share)	53
Outstanding, weighted-average grant date fair value (in dollars per share)	$ 49.84	$ 47.95
Total compensation cost not yet recognized	$ 10	$ 9.1
Total compensation cost not yet recognized, period for recognition	2 years 8 months 12 days	2 years 6 months
Total fair value of shares vested	$ 5.4	$ 6.5	$ 4.3
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	4 years

Stock-Based Compensation - Simu

Stock-Based Compensation - Simulation Model Input Variables (Details) - Market Condition Performance Share Awards	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility factor	0.38%	0.39%
Risk free interest rate (percent)	2.50%	2.11%
Expected term (in years)	2 years 11 months 19 days	2 years 11 months 15 days
Expected dividend yield (percent)	0.00%	0.00%

Stock-Based Compensation - Perf

Stock-Based Compensation - Performance Share Awards (Details) - USD ($) $ / shares in Units, $ in Millions	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Allocated share-based compensation expense		$ 8.2	$ 11	$ 8.5
Performance Shares Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award requisite service period		3 years
Period for share issuance after vesting		60 days
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Total compensation cost not yet recognized	$ 4.4	$ 4.4	$ 3.4		$ 4.4
Total compensation cost not yet recognized, period for recognition		1 year 8 months 23 days	1 year 9 months 18 days
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]
Expected dividend yield (percent)		0.00%	0.00%
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Outstanding (in shares)		82,907
Granted (in shares)		50,293
Vested (in shares)		(29,293)
Forfeited (in shares)		(7,748)
Outstanding (in shares)	96,159	96,159	82,907		96,159
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 (in dollars per share)		$ 53.92
Granted, weighted-average grant date fair value (in dollars per share)		70.77
Vested, weighted-average grant date fair value (in dollars per share)		56.84
Forfeited, weighted-average grant date fair value (in dollars per share)		57.15
Outstanding, weighted-average grant date fair value (in dollars per share)	$ 61.58	$ 61.58	$ 53.92		$ 61.58
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 (in shares)		41,454
Granted (in shares)		24,861
Vested (in shares)		(14,648)
Forfeited (in shares)		(3,873)
Outstanding (in shares)	47,794	47,794	41,454		47,794
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 (in dollars per share)		$ 49.81
Granted, weighted-average grant date fair value (in dollars per share)		60.11
Vested, weighted-average grant date fair value (in dollars per share)		55.50
Forfeited, weighted-average grant date fair value (in dollars per share)		52.10
Outstanding, weighted-average grant date fair value (in dollars per share)	$ 53.24	$ 53.24	$ 49.81		$ 53.24
Stock Appreciation Rights (SARs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award requisite service period					4 years
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Award performance period					2 years
Allocated share-based compensation expense	$ (1.9)	$ 0.7	$ 1.2

Earnings per Share (Details)

Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net income	$ 81,214	$ 83,464	$ 114,501	$ 56,076	$ 91,937	$ 97,480	$ 11,254	$ (44,922)		$ 335,255	$ 155,749	$ 415,522
Denominator:
Weighted-average shares outstanding, basic (in shares)										68,429	68,249	69,221
Effect of dilutive stock awards (in shares)										130	182	156
Adjusted weighted-average shares outstanding, diluted (in shares)										68,559	68,431	69,377
Earnings per Share:
Basic earnings per common share (in dollars per share)	$ 1.19	$ 1.22	$ 1.67	$ 0.82	$ 1.35	$ 1.43	$ 0.16	$ (0.66)		$ 4.90	$ 2.28	$ 6
Diluted earnings per common share (in dollars per share)	$ 1.18	$ 1.22	$ 1.67	$ 0.82	$ 1.34	$ 1.42	$ 0.16	$ (0.66)		$ 4.89	$ 2.28	$ 5.99
Antidilutive awards excluded from computation of earnings per common share (in shares)										143	145	85
Non-recurring income tax benefit due to the Tax Cuts and Jobs Act of 2017									$ 196,700
Non-recurring income tax benefit due to the Tax Cuts and Jobs Act of 2017 per basic share (in dollars per share)												$ 2.84
Non-recurring income tax benefit due to the Tax Cuts and Jobs Act of 2017 per diluted share (in dollars per share)												$ 2.84

Debt and Other Obligations - Na

Debt and Other Obligations - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Line of Credit Facility [Line Items]
Long-term debt, gross	$ 2,213.6	$ 2,234.5
Long-term debt, current maturities	214	162.8
Long-term debt, excluding current maturities	1,959.2	2,024
Corporate credit cards
Line of Credit Facility [Line Items]
Maximum borrowing capacity	33.6	33.6
Line of credit, current	4.6	3.5
Lines of Credit with Counterparties for Jet Fuel and Derivatives [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	41.5	41.5
Aircraft fuel
Line of Credit Facility [Line Items]
Line of credit, current	$ 25.3	23
Fixed-rate term loans | Maximum
Line of Credit Facility [Line Items]
Long-term Debt, Term	12 years
Fixed-rate term loans | Minimum
Line of Credit Facility [Line Items]
Long-term Debt, Term	11 years
Revolving Credit Facility
Line of Credit Facility [Line Items]
Long-term debt, gross	$ 160	135.3
Long-term debt, current maturities	50
Long-term debt, excluding current maturities	110
Fixed-rate term loans
Line of Credit Facility [Line Items]
Long-term debt, gross	$ 778.2	$ 625.1
Fixed-rate term loans | Fixed-rate term loans | Maximum
Line of Credit Facility [Line Items]
Stated interest rate percentage	3.93%
Fixed-rate term loans | Fixed-rate term loans | Minimum
Line of Credit Facility [Line Items]
Stated interest rate percentage	2.85%

Debt and Other Obligations - Lo

Debt and Other Obligations - Long-term Debt (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019USD ($)aircraft	Dec. 31, 2018USD ($)
Debt Instrument [Line Items]
Long-term debt, gross	$ 2,213.6	$ 2,234.5
Repayments of Long-term Debt	246.8	137.3
Long-term debt, current maturities	214	162.8
Less unamortized discount, net	40.4	47.7
Total	1,959.2	2,024
Senior loans
Debt Instrument [Line Items]
Long-term debt, gross	$ 296.1	$ 382.4
Weighted-average interest rates	4.02%	4.10%
Junior loans
Debt Instrument [Line Items]
Long-term debt, gross	$ 0	$ 31.1
Weighted-average interest rates		6.90%
Fixed-rate term loans
Debt Instrument [Line Items]
Long-term debt, gross	778.2	$ 625.1
Debt instrument, face amount	$ 152
Weighted-average interest rates	3.70%	3.88%
2015-1 EETC Class A
Debt Instrument [Line Items]
Long-term debt, gross	$ 348.6	$ 378.6
Weighted-average interest rates	4.10%	4.10%
2015-1 EETC Class B
Debt Instrument [Line Items]
Long-term debt, gross	$ 72	$ 80
Weighted-average interest rates	4.45%	4.45%
2015-1 EETC Class C
Debt Instrument [Line Items]
Long-term debt, gross	$ 98.1	$ 109.5
Weighted-average interest rates	4.93%	4.93%
2017-1 EETC Class AA
Debt Instrument [Line Items]
Long-term debt, gross	$ 228.4	$ 242.5
Weighted-average interest rates	3.38%	3.38%
2017-1 EETC Class A
Debt Instrument [Line Items]
Long-term debt, gross	$ 76.1	$ 80.8
Weighted-average interest rates	3.65%	3.65%
2017-1 EETC Class B
Debt Instrument [Line Items]
Long-term debt, gross	$ 70.6	$ 83.7
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%
Revolving Credit Facility
Debt Instrument [Line Items]
Long-term debt, gross	$ 160	$ 135.3
Long-term debt, current maturities	50
Total	$ 110
Weighted-average interest rates	3.12%	3.72%
Minimum | Fixed-rate term loans
Debt Instrument [Line Items]
Long-term Debt, Term	11 years
Minimum | Fixed-rate term loans | Fixed-rate term loans
Debt Instrument [Line Items]
Stated interest rate percentage	2.85%
Maximum | Fixed-rate term loans
Debt Instrument [Line Items]
Long-term Debt, Term	12 years
Maximum | Fixed-rate term loans | Fixed-rate term loans
Debt Instrument [Line Items]
Stated interest rate percentage	3.93%
Airbus
Debt Instrument [Line Items]
Number of delivered aircraft with secured debt financing commitments | aircraft	64
Future aircraft to be received | aircraft	135
Airbus | A320 | Fixed-rate term loans
Debt Instrument [Line Items]
Number of delivered aircraft with secured debt financing commitments | aircraft	4
Airbus | A320NEO | Revolving Credit Facility
Debt Instrument [Line Items]
Future aircraft to be received | aircraft	34
Lines of Credit with Counterparties for Jet Fuel and Derivatives [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 41.5	$ 41.5
Revolving Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 160

Debt and Other Obligations - Fu

Debt and Other Obligations - Future Maturities (Details) $ in Millions	Dec. 31, 2019USD ($)
Debt Disclosure [Abstract]
2020	$ 222.1
2021	273.8
2022	167.7
2023	310.1
2024	195.5
2025 and beyond	1,044.4
Long-term debt	$ 2,213.6

Debt and Other Obligations - In

Debt and Other Obligations - Interest Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Commitment fees and other	$ 2,217	$ 262
Amortization of deferred financing costs	8,714	7,069
Total	101,350	83,777
Senior loans
Debt Instrument [Line Items]
Interest expense	15,225	16,869
Junior loans
Debt Instrument [Line Items]
Interest expense	1,811	2,475
Fixed-rate term loans
Debt Instrument [Line Items]
Interest expense	25,828	19,615
2015-1 EETC Class A
Debt Instrument [Line Items]
Interest expense	14,894	16,138
2015-1 EETC Class B
Debt Instrument [Line Items]
Interest expense	3,377	3,781
2015-1 EETC Class C
Debt Instrument [Line Items]
Interest expense	5,117	3,575
2017-1 EETC Class AA
Debt Instrument [Line Items]
Interest expense	7,887	6,026
2017-1 EETC Class A
Debt Instrument [Line Items]
Interest expense	2,843	2,172
2017-1 EETC Class B
Debt Instrument [Line Items]
Interest expense	2,870	2,420
2017-1 EETC Class C
Debt Instrument [Line Items]
Interest expense	4,367	2,478
Revolving Credit Facility
Debt Instrument [Line Items]
Interest expense	5,792	793
Capital Lease Obligations
Debt Instrument [Line Items]
Interest expense	$ 408	$ 104

Leases and Aircraft Maintenan_3

Leases and Aircraft Maintenance Deposits Leases and Aircraft Maintenance Deposits - Narrative (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Jan. 31, 2020aircraft	Dec. 31, 2019USD ($)aaircraft_engineaircraft	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)aircraft	Mar. 31, 2019USD ($)	Jan. 01, 2019USD ($)
Lessee, Lease, Description [Line Items]
Total rental expense		$ 345,000	$ 312,000	$ 309,800
Number of spare engines capitalized | aircraft		14
Rental expense charged to operations for aircraft and engine operating leases		$ 182,609	177,641	205,852
Maintenance reserves, supplemental rent		500	1,300	400
Aircraft maintenance deposits		170,600
Aircraft maintenance deposits, current		102,906	106,901
Long-term aircraft maintenance deposits		67,682	138,738
Finance lease, interest expense		674
Finance lease liability		$ 46,081
Area of land | a		8.5
Land		$ 41,000
Operating lease, right-of-use asset		$ 1,369,555	0
Estimated time to complete construction		2 years
Aircraft Supplemental Rent
Lessee, Lease, Description [Line Items]
Supplemental rent expense		$ 4,800	$ 3,400	$ 8,400
Accounting Standards Update 2016-02
Lessee, Lease, Description [Line Items]
Operating lease, right-of-use asset					$ 1,000,000
Restatement Adjustment | Accounting Standards Update 2016-02
Lessee, Lease, Description [Line Items]
Retained earnings impact of Topic 842 adoption						$ 5,500
A320 Family
Lessee, Lease, Description [Line Items]
Number of aircraft held | aircraft		145
Number of aircraft purchased | aircraft		4
Consideration transferred		$ 141,300
Airbus A320
Lessee, Lease, Description [Line Items]
Finance lease, interest expense		$ 300
Airbus A319
Lessee, Lease, Description [Line Items]
Number of aircraft under finance lease | aircraft		2
Number of aircraft to be purchased | aircraft		2
Aircraft
Lessee, Lease, Description [Line Items]
Number of previously leased aircraft purchased off lease | aircraft		5
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		52
Spare engines
Lessee, Lease, Description [Line Items]
Number of leased assets financed under operating leases | aircraft_engine		9
Number of spare engines purchased | aircraft_engine		4
Other leased equipment and property
Lessee, Lease, Description [Line Items]
Area of land | a		2.6
Noncancellable short-term operating leases commitments		$ 600
Land
Lessee, Lease, Description [Line Items]
Operating lease, right-of-use asset		14,600
Operating lease rIght-of-use liability		9,300
Minimum
Lessee, Lease, Description [Line Items]
Development in process, estimated total costs		$ 200,000
Minimum | Leased Computer And Office Equipment
Lessee, Lease, Description [Line Items]
Finance lease contract term		3 years
Minimum | Aircraft
Lessee, Lease, Description [Line Items]
Operating leases, term		8 years
Maximum
Lessee, Lease, Description [Line Items]
Development in process, estimated total costs		$ 250,000
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
2019
Lessee, Lease, Description [Line Items]
Number of sale and leaseback transactions | aircraft		6
2019 | Aircraft
Lessee, Lease, Description [Line Items]
Number of aircrafts delivered under secured debt arrangements | aircraft		4
2019 | Aircraft | A320 Family
Lessee, Lease, Description [Line Items]
Number of leased assets financed under operating leases | aircraft		7
Subsequent Event | Airbus A319
Lessee, Lease, Description [Line Items]
Number of aircraft purchased | aircraft	2
Spare engines
Lessee, Lease, Description [Line Items]
Number of capitalized aircraft or engine purchased off lease | aircraft		3
Number of previously leased aircraft purchased off lease | aircraft		2
Spare engines | Minimum
Lessee, Lease, Description [Line Items]
Operating leases, term		2 years
Spare engines | Maximum
Lessee, Lease, Description [Line Items]
Operating leases, term		14 years
Aircraft
Lessee, Lease, Description [Line Items]
Number of capitalized aircraft or engine purchased off lease | aircraft		27
Number of previously leased aircraft purchased off lease | aircraft				1
Aircraft | A320 Family
Lessee, Lease, Description [Line Items]
Number of aircraft capitalized | aircraft		91
Aircraft | Minimum
Lessee, Lease, Description [Line Items]
Operating leases, term		8 years
Aircraft | Maximum
Lessee, Lease, Description [Line Items]
Operating leases, 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, 2019	Dec. 31, 2018
Finance Leases
2020	$ 44,980
2021	606
2022	578
2023	202
2024	0
2025 and thereafter	0
Total minimum lease payments	46,366
Less amount representing interest	285
Present value of minimum lease payments	46,081
Less current portion	44,796
Long-term portion	1,285
Operating Leases
Less current portion	120,662	$ 0
Long-term portion	1,218,014	$ 0
Total Operating and Finance Lease Obligations
2020	241,978
2021	196,677
2022	197,415
2023	184,439
2024	161,714
2025 and thereafter	1,060,417
Total minimum lease payments	2,042,640
Less amount representing interest	657,883
Present value of minimum lease payments	1,384,757
Less current portion	165,458
Long-term portion	1,219,299
Aircraft and Spare Engine Leases
Operating Leases
2020	192,937
2021	192,539
2022	193,616
2023	181,667
2024	159,668
2025 and thereafter	921,721
Total minimum lease payments	1,842,148
Less amount representing interest	526,760
Present value of minimum lease payments	1,315,388
Less current portion	117,861
Long-term portion	1,197,527
Property Facility Leases
Operating Leases
2020	3,544
2021	3,532
2022	3,221
2023	2,570
2024	2,046
2025 and thereafter	138,696
Total minimum lease payments	153,609
Less amount representing interest	130,825
Present value of minimum lease payments	22,784
Less current portion	2,297
Long-term portion	20,487
Other
Operating Leases
2020	517
2021	0
2022	0
2023	0
2024	0
2025 and thereafter	0
Total minimum lease payments	517
Less amount representing interest	13
Present value of minimum lease payments	504
Less current portion	504
Long-term portion	$ 0

Leases and Aircraft Maintenan_5

Leases and Aircraft Maintenance Deposits - Leases Lease Costs (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
Finance lease cost
Amortization of leased assets	$ 998
Interest of lease liabilities	674
Operating lease cost
Operating lease cost	179,959
Short-term lease cost	5,144
Variable lease cost	140,417
Total lease cost	$ 327,192
Weighted-Average Remaining Lease Term [Abstract]
Operating leases	13 years
Finance leases	1 month 6 days
Weighted-Average Lease Discount Rate [Abstract]
Operating leases	5.86%
Finance leases	2.46%

Defined Contribution 401(k) P_2

Defined Contribution 401(k) Plan (Details) $ in Millions	2 Months Ended	12 Months Ended
	Feb. 28, 2018	Dec. 31, 2019USD ($)defined_contribution_plan	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
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 | $		$ 51.1	$ 36.7	$ 19.6
Pilots’ Retirement Savings Plan
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, employer matching contribution, percent of match	100.00%
Defined Contribution Plan, Employer Contribution, Percent Of Employees' Gross Pay, Initial Percentage		11.00%
Defined Contribution Plan, Employer Contribution, Percent Of Employees' Gross Pay, Annual Percentage Increase		1.00%
Defined Contribution 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, 2019	Dec. 31, 2018	Dec. 31, 2017
Current:
Federal	$ (22,429)	$ (2,178)	$ (68,601)
State and local	1,218	410	515
Foreign	6,693	4,692	2,742
Total current expense (benefit)	(14,518)	2,924	(65,344)
Deferred:
Federal	106,703	42,246	(9,349)
State and local	8,986	4,057	8,857
Total deferred expense (benefit)	115,689	46,303	(492)
Total income tax expense (benefit)	$ 101,171	$ 49,227	$ (65,836)

Income Taxes - Income Tax Recon

Income Taxes - Income Tax Reconciliation (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of Income Tax Expense
Expected provision at federal statutory tax rate	21.00%	21.00%	35.00%
State tax expense, net of federal benefit	1.80%	1.70%	1.70%
Revaluation of deferred taxes	(2.10%)	0.00%	(56.30%)
Other	2.50%	1.30%	0.70%
Total income tax expense (benefit)	23.20%	24.00%	(18.90%)

Income Taxes - Deferred Taxes (

Income Taxes - Deferred Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Income tax credits	$ 9,632	$ 10,004
Net operating losses	13,604	155,670
Deferred revenue	8,824	6,824
Nondeductible accruals	14,133	14,691
Deferred manufacturing credits	2,813	0
Accrued maintenance	1,668	2,168
Equity compensation	2,851	2,592
Lease liability	305,161
Valuation allowance	(1,746)	(254)
Other	482	5,262
Deferred tax assets	357,422	196,957
Deferred tax liabilities:
Deferred gain (loss) on leases, net	0	1,672
Accrued rent	0	6,068
Prepaid expenses	1,120	793
Property, plant and equipment	430,523	481,847
Deferred financing costs	154	189
Accrued aircraft and engine maintenance	84,479	61,529
Right-of-use asset	310,438
Deferred tax liabilities	826,714	552,098
Net deferred tax assets (liabilities)	$ (469,292)	$ (355,141)

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Summary Of Income Taxes [Line Items]
Valuation allowance	$ 1,746,000	$ 254,000
Foreign tax credits	8,200,000
State net operating losses	1,400,000
Income tax deficiency	101,171,000	49,227,000	$ (65,836,000)
Liability recorded for interest and penalties on unrecognized tax benefits	0	0	$ 0
Domestic Tax Authority
Summary Of Income Taxes [Line Items]
Operating loss carryforwards	37,700,000
State and Local Jurisdiction
Summary Of Income Taxes [Line Items]
Operating loss carryforwards	109,400,000
Accounting Standards Update 2016-09
Summary Of Income Taxes [Line Items]
Income tax deficiency	$ (1,400,000)	$ 400,000

Commitments and Contingencies -

Commitments and Contingencies - Aircraft-Related Commitments and Financing Arrangements (Details) $ in Millions	Dec. 31, 2019USD ($)aircraft_engineaircraft
Secured Debt
Principal and Interest Commitments
Principal and interest commitments, 2020	$ 9.6
Principal and interest commitments, 2021	12.9
Principal and interest commitments, 2022	13
Principal and interest commitments, 2023	13
Principal and interest commitments, 2024	13.1
Principal and interest commitments, 2025 and beyond	83.3
Interest Commitments
Interest commitments, 2020	77.9
Interest commitments, 2021	71
Interest commitments, 2022	64.7
Interest commitments, 2023	54.6
Interest commitments, 2024	44
Interest commitments, 2025 and beyond	118.1
Aircraft and Related Flight Equipment
Committed Expenditures
Committed expenditures, 2020	988
Committed expenditures, 2021	744.8
Committed expenditures, 2022	123.7
Committed expenditures, 2023	491.6
Committed expenditures, 2024	1,002.5
Committed expenditures, 2025 and beyond	3,605.4
Non-aircraft Related Commitments
Committed Expenditures
Committed expenditures, 2020	21.9
Committed expenditures, 2021	17.1
Committed expenditures, 2022	17.2
Committed expenditures, 2023	13.7
Committed expenditures, 2024	13.9
Committed expenditures, 2025 and beyond	$ 50.3
V2500 SelectTWO Engine | 2020-2023
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of spare aircraft engines ordered | aircraft_engine	1
PurePower PW1100G-JM Engine | 2020-2023
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of spare aircraft engines ordered | aircraft_engine	4
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	135
Number of delivered aircraft with secured debt financing commitments | aircraft	64
Airbus | 2020
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of aircraft with secured debt financing commitments scheduled for delivery | aircraft	3
Airbus | 2020-2027
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of aircraft without secured financing commitments scheduled for delivery | aircraft	132
Third Party Lessor | A320NEO
Unrecorded Unconditional Purchase Obligation [Line Items]
Future aircraft to be received | aircraft	12
Aircraft Rent Commitments [Abstract]
Aircraft rent commitments, 2020	$ 6
Aircraft rent commitments, 2021	34.9
Aircraft rent commitments, 2022	44.1
Aircraft rent commitments, 2023	44.1
Aircraft rent commitments, 2024	44.1
Aircraft rent commitments, 2025 and thereafter	$ 356.1

Commitments and Contingencies_2

Commitments and Contingencies - Employees (Details) $ in Millions	1 Months Ended	12 Months Ended
	Feb. 28, 2018	Dec. 31, 2019USD ($)employee_group	Dec. 31, 2018USD ($)
Health Insurance Product Line
Concentration Risk [Line Items]
Accrued health care claims		$ 5.2	$ 4.4
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		81.00%
Unionized employees concentration risk | Number of employees (percent) | Air Line Pilots Association, International (ALPA)
Concentration Risk [Line Items]
Concentration of risk		27.00%
Unionized employees concentration risk | Number of employees (percent) | Association of Flight Attendants (AFA-CWA)
Concentration Risk [Line Items]
Concentration of risk		47.00%
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 Millions	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	$ 2,213.6	$ 2,234.5
Estimate of Fair Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	2,303	2,169.6
Senior loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	296.1	382.4
Senior loans | Estimate of Fair Value Measurement [Member] | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	296.4	373.6
Junior loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	0	31.1
Junior loans | Estimate of Fair Value Measurement [Member] | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0	31.1
Fixed-rate term loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	778.2	625.1
Fixed-rate term loans | Estimate of Fair Value Measurement [Member] | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	823.6	600.1
2015-1 EETC Class A
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	348.6	378.6
2015-1 EETC Class A | Estimate of Fair Value Measurement [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	372.2	374.8
2015-1 EETC Class B
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	72	80
2015-1 EETC Class B | Estimate of Fair Value Measurement [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	74.5	78.1
2015-1 EETC Class C
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	98.1	109.5
2015-1 EETC Class C | Estimate of Fair Value Measurement [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	100.5	107.9
2017-1 EETC Class AA
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	228.4	242.5
2017-1 EETC Class AA | Estimate of Fair Value Measurement [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	237	228.8
2017-1 EETC Class A
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	76.1	80.8
2017-1 EETC Class A | Estimate of Fair Value Measurement [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	78.8	76.6
2017-1 EETC Class B
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	70.6	83.7
2017-1 EETC Class B | Estimate of Fair Value Measurement [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	72	79.1
2017-1 EETC Class C
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	85.5	85.5
2017-1 EETC Class C | Estimate of Fair Value Measurement [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	88	84.2
Revolving Credit Facility
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	160	135.3
Revolving Credit Facility | Estimate of Fair Value Measurement [Member] | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	$ 160	$ 135.3

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, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 979	$ 1,004.7
Short-term investment securities	105.3	102.8
Assets Held-for-sale, Long Lived, Fair Value Disclosure	2.3
Total assets	1,086.6	1,107.5
Total liabilities	0	0
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	979	1,004.7
Short-term investment securities	105.3	102.8
Assets Held-for-sale, Long Lived, Fair Value Disclosure	0
Total assets	1,084.3	1,107.5
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
Short-term investment securities	0	0
Assets Held-for-sale, Long Lived, Fair Value Disclosure	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
Short-term investment securities	0	0
Assets Held-for-sale, Long Lived, Fair Value Disclosure	2.3
Total assets	2.3	0
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, 2019USD ($)
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 0
Purchases	5.4
Sales	0
Total realized or unrealized gains (losses) included in earnings, net	(3.1)
Ending balance	$ 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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues from External Customers and Long-Lived Assets [Line Items]
Total operating revenues	$ 969,816	$ 991,968	$ 1,012,956	$ 855,796	$ 862,795	$ 904,330	$ 851,771	$ 704,138	$ 3,830,536	$ 3,323,034	$ 2,643,552
DOT—Domestic
Revenues from External Customers and Long-Lived Assets [Line Items]
Total operating revenues									3,462,800	2,990,700	2,432,100
DOT—Latin America and Caribbean
Revenues from External Customers and Long-Lived Assets [Line Items]
Total operating revenues									$ 367,700	$ 332,300	$ 211,500

Operating Segments and Relate_4

Operating Segments and Related Disclosures - Foreign Revenues (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Total operating revenues	$ 969,816	$ 991,968	$ 1,012,956	$ 855,796	$ 862,795	$ 904,330	$ 851,771	$ 704,138		$ 3,830,536	$ 3,323,034	$ 2,643,552
Operating income	124,624	124,681	163,938	87,804	136,065	145,125	108,521	(38,797)		501,047	350,914	384,825
Net income	$ 81,214	$ 83,464	$ 114,501	$ 56,076	$ 91,937	$ 97,480	$ 11,254	$ (44,922)		$ 335,255	$ 155,749	$ 415,522
Basic earnings per share	$ 1.19	$ 1.22	$ 1.67	$ 0.82	$ 1.35	$ 1.43	$ 0.16	$ (0.66)		$ 4.90	$ 2.28	$ 6
Diluted earnings per share	$ 1.18	$ 1.22	$ 1.67	$ 0.82	$ 1.34	$ 1.42	$ 0.16	$ (0.66)		$ 4.89	$ 2.28	5.99
Non-recurring income tax benefit due to the Tax Cuts and Jobs Act of 2017									$ 196,700
Non-recurring income tax benefit due to the Tax Cuts and Jobs Act of 2017 per basic share (in dollars per share)												2.84
Non-recurring income tax benefit due to the Tax Cuts and Jobs Act of 2017 per diluted share (in dollars per share)												$ 2.84