Spirit Airlines (SAVEQ) 10-K 13 Feb 18 2017 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2017	Feb. 06, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Document Type	10-K
Document Period End Date	Dec. 31, 2017
Entity Registrant Name	Spirit Airlines, Inc.
Amendment Flag	false
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Entity Central Index Key	1,498,710
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 3.6
Entity Common Stock, Shares Outstanding		68,213,030

Statements Of Operations

Statements Of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating revenues:
Passenger	$ 1,366,034	$ 1,200,621	$ 1,169,338
Non-ticket	1,281,632	1,121,335	972,125
Total operating revenues	2,647,666	2,321,956	2,141,463
Operating expenses:
Aircraft fuel	615,581	447,553	461,447
Salaries, wages and benefits	527,959	472,471	377,508
Aircraft rent	205,852	201,675	211,531
Landing fees and other rents	180,655	151,679	131,077
Depreciation and amortization	140,152	101,136	73,908
Maintenance, materials and repairs	110,439	98,587	80,448
Distribution	113,620	96,627	86,576
Special charges	12,629	37,189	673
Loss on disposal of assets	4,168	4,187	1,604
Other operating	347,820	267,191	207,569
Total operating expenses	2,258,875	1,878,295	1,632,341
Operating income	388,791	443,661	509,122
Other (income) expense:
Interest expense	57,302	41,654	20,382
Capitalized interest	(13,793)	(12,705)	(11,553)
Interest income	(8,736)	(5,276)	(2,125)
Other expense	366	528	15
Total other (income) expense	35,139	24,201	6,719
Income before income taxes	353,652	419,460	502,403
Provision (benefit) for income taxes	(66,954)	154,581	185,183
Net income	$ 420,606	$ 264,879	$ 317,220
Basic earnings per share (in dollars per share)	$ 6.08	$ 3.77	$ 4.39
Diluted earnings per share (in dollars per share)	$ 6.06	$ 3.76	$ 4.38

Statements of Comprehensive Inc

Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 420,606	$ 264,879	$ 317,220
Unrealized gain (loss) on interest rate derivative instruments, net of deferred taxes of $0, $0, and ($550)	0	0	(910)
Unrealized gain (loss) on short-term investment securities, net of deferred taxes of ($41), ($13), and $0	(82)	(23)	0
Interest rate derivative (gain) loss reclassified into earnings, net of taxes of $372, $130, and $50	(37)	224	82
Other comprehensive income (loss)	(119)	201	(828)
Comprehensive income	$ 420,487	$ 265,080	$ 316,392

Statements of Comprehensive In4

Statements of Comprehensive Income (Parentheticals) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Unrealized gain (loss) on interest rate derivative instrument, tax	$ 0	$ 0	$ (550)
Deferred taxes on unrealized loss on investment securities	(41)	(13)	0
Loss reclassified from AOCI into income, tax	$ 372	$ 130	$ 50

Balance Sheets

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 800,849	$ 700,900
Short-term investment securities	100,937	100,155
Accounts receivable, net	49,323	41,136
Aircraft maintenance deposits, net	175,615	87,035
Income tax receivable	69,844	0
Prepaid expenses and other current assets	83,692	46,619
Total current assets	1,280,260	975,845
Property and equipment:
Flight equipment	2,291,110	1,461,525
Ground property and equipment	155,166	126,206
Less accumulated depreciation	(207,808)	(122,509)
Total property and equipment	2,238,468	1,465,222
Deposits on flight equipment purchase contracts	253,687	325,688
Long-term aircraft maintenance deposits	150,617	199,415
Deferred heavy maintenance, net	99,915	75,534
Other long-term assets	121,003	110,223
Total assets	4,143,950	3,151,927
Current liabilities:
Accounts payable	22,822	15,193
Air traffic liability	246,404	206,392
Current maturities of long-term debt	115,430	84,354
Other current liabilities	262,370	226,011
Total current liabilities	647,026	531,950
Long-term debt, less current maturities	1,387,498	897,359
Deferred income taxes	313,140	308,143
Deferred gains and other long-term liabilities	19,205	19,868
Shareholders’ equity:
Common stock: Common stock, $0.0001 par value, 240,000,000 shares authorized at December 31, 2017 and 2016, respectively; 69,770,795 and 73,549,872 issued and 68,196,964 and 69,326,202 outstanding as of December 31, 2017 and 2016, respectively	7	7
Additional paid-in-capital	360,153	551,004
Treasury stock, at cost: 1,573,831 and 4,223,670 shares as of December 31, 2017 and 2016, respectively	(65,854)	(218,692)
Retained earnings	1,484,239	1,063,633
Accumulated other comprehensive loss	(1,464)	(1,345)
Total shareholders’ equity	1,777,081	1,394,607
Total liabilities and shareholders’ equity	$ 4,143,950	$ 3,151,927

Balance Sheets (Parenthetical)

Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized	240,000,000	240,000,000
Common stock, shares issued	69,770,795	73,549,872
Common stock, shares outstanding	68,196,964	69,326,202
Treasury stock, shares	1,573,831	4,223,670
Voting Common Stock
Common stock, par value (in dollars per share)	$ 0.0001
Common stock, shares authorized	240,000,000

Statements of Cash Flows

Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities:
Net income	$ 420,606	$ 264,879	$ 317,220
Adjustments to reconcile net income to net cash provided by operations:
Unrealized losses on open derivative contracts, net	0	0	2,202
Losses reclassified from other comprehensive income	335	354	132
Stock-based compensation	8,522	7,105	9,222
Allowance for doubtful accounts (recoveries)	(53)	80	12
Amortization of deferred gains and losses and debt issuance costs	7,944	5,732	1,165
Depreciation and amortization	140,152	101,136	73,908
Deferred income tax expense (benefit)	(1,610)	86,146	155,614
Loss on disposal of assets	4,168	4,187	1,604
Lease termination costs	12,629	37,189	0
Changes in operating assets and liabilities:
Accounts receivable	(8,134)	(12,951)	(5,592)
Aircraft maintenance deposits, net	(37,930)	(45,869)	(32,101)
Long-term deposits and other assets	(50,803)	(45,558)	(22,208)
Deferred heavy maintenance	(78,237)	(30,222)	(9,127)
Income tax receivable	(69,844)	0	0
Prepaid income taxes	0	72,278	(72,278)
Accounts payable	6,030	(6,823)	2,706
Air traffic liability	40,011	(11,582)	36,387
Other liabilities	31,074	47,391	14,119
Other	380	206	0
Net cash provided by operating activities	425,240	473,678	472,985
Investing activities:
Purchase of available-for-sale investment securities	(107,246)	(103,258)	0
Proceeds from the maturity of available-for-sale investment securities	105,906	2,842	0
Proceeds from sale of property and equipment	0	50	0
Pre-delivery deposits for flight equipment, net of refunds	(149,477)	(173,947)	(142,323)
Capitalized interest	(12,305)	(10,834)	(10,159)
Purchase of property and equipment	(628,881)	(541,122)	(548,800)
Net cash used in investing activities	(792,003)	(826,269)	(701,282)
Financing activities:
Proceeds from issuance of long-term debt	629,725	417,275	536,780
Proceeds from stock options exercised	45	92	32
Payments on debt and capital lease obligations	(102,738)	(64,421)	(26,364)
Proceeds from sale leaseback transactions	0	0	7,300
Excess tax (deficiency) benefit from equity-based compensation	0	(470)	8,850
Repurchase of common stock	(46,580)	(102,510)	(112,261)
Debt issuance costs	(13,740)	(107)	(15,192)
Net cash provided by financing activities	466,712	249,859	399,145
Net (decrease) increase in cash and cash equivalents	99,949	(102,732)	170,848
Cash and cash equivalents at beginning of period	700,900	803,632	632,784
Cash and cash equivalents at end of period	800,849	700,900	803,632
Cash payments for:
Interest, net of capitalized interest	37,902	39,963	7,061
Income taxes paid, net of refunds	5,826	(5,579)	95,933
Non-cash transactions:
Capital expenditures funded by capital lease borrowings	$ (1,370)	$ (31)	$ 0

Statements of Shareholders_ Equ

Statements of Shareholders’ Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Treasury Stock	Retained Earnings (Accumulated Deficit)	Accumulated Other Comprehensive Income (Loss)
Balance at Dec. 31, 2014	$ 1,003,075	$ 7	$ 526,173	$ (3,921)	$ 481,534	$ (718)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	9,222		9,222
Repurchase of common stock	(112,261)			(112,261)
Proceeds from options exercised	32		32
Excess tax benefits from share-based compensation	8,850		8,850
Changes in comprehensive income	(828)					(828)
Net income	317,220				317,220
Balance at Dec. 31, 2015	1,225,310	7	544,277	(116,182)	798,754	(1,546)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	7,105		7,105
Repurchase of common stock	(102,510)			(102,510)
Proceeds from options exercised	92		92
Excess tax deficiency from share-based compensation	(470)		(470)
Changes in comprehensive income	201					201
Net income	264,879				264,879
Balance at Dec. 31, 2016	1,394,607	7	551,004	(218,692)	1,063,633	(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
Changes in comprehensive income	(119)					(119)
Retirement of treasury stock	0		(199,418)	199,418
Net income	420,606				420,606
Balance at Dec. 31, 2017	$ 1,777,081	$ 7	$ 360,153	$ (65,854)	$ 1,484,239	$ (1,464)

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2017
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. Certain prior period amounts have been reclassified to conform to the current year's presentation. 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 consist of available-for-sale asset-backed 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, 2017 and 2016 . In addition, the provision for doubtful accounts and write-offs for 2017 , 2016 and 2015 were each immaterial. Income Tax Receivable Income tax receivable consists of amounts due from tax authorities for recovery of income taxes paid in prior years. 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 capital 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 capital 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 Capital leases Lease term 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, 2017 , the Company had 54 aircraft, 4 spare engines and 1 flight simulator capitalized within flight equipment with depreciable lives of 25 years . As of December 31, 2017 , the Company had 58 aircraft financed through operating leases with lease terms from 8 to 18 years and 11 spare engines financed through operating leases with lease terms from 2 to 13 years . The following table illustrates the components of depreciation and amortization expense: Year Ended December 31, 2017 2016 2015 (in thousands) Depreciation $ 86,297 $ 57,325 $ 30,797 Amortization of heavy maintenance 53,855 43,811 43,111 Total depreciation and amortization $ 140,152 $ 101,136 $ 73,908 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 $7.7 million and $9.4 million at December 31, 2017 and 2016 , respectively. We record amortization of capitalized software on a straight-line basis within depreciation and amortization expense in the accompanying statements of operations. Amortization of capitalized software costs was $3.1 million , $3.2 million and $3.1 million for the years ended 2017 , 2016 and 2015 , respectively. The Company placed in service internal-use software of $1.4 million , $4.1 million and $4.3 million , during the years ended 2017 , 2016 and 2015 , respectively. 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 Revenue Recognition Tickets sold are initially deferred as “air traffic liability.” Passenger revenue is 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. Customers may elect to change their itinerary prior to the date of departure. A service charge is assessed and recognized on the date the change is initiated and is deducted from the face value of the original purchase price of the ticket, and the original ticket becomes invalid. 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 can be used towards the purchase of a new ticket and the Company’s other service offerings. The amount of credits expected to expire is recognized as revenue upon issuance of the credit and is 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. The Company is also required to collect certain taxes and fees from customers on behalf of government agencies and airports and remit to the applicable governmental entity or airport on a periodic basis. These taxes and fees include U.S. federal transportation taxes, federal security charges, airport passenger facility charges and international arrival and departure taxes. These items are collected from customers at the time they purchase their tickets, but are not included in passenger revenue. The Company records a liability upon collection from the customer and relieves the liability when payments are remitted to the applicable governmental agency or airport. Frequent Flier Program Flown Miles. The Company records unrecognized revenue for mileage credits earned by passengers under its FREE SPIRIT program, including mileage credits for members with an insufficient number of mileage credits to earn an award, based on the estimated incremental cost of providing free travel for credits that are expected to be redeemed. Incremental costs include fuel, insurance, security, ticketing and facility charges reduced by an estimate of fees required to be paid by the passenger when redeeming the award. Affinity Card Program. During 2015, the Company extended its agreement with the administrator of the FREE SPIRIT affinity credit card program, which was scheduled to expire in April 2016. The renegotiated program was extended through 2022. As part of the new agreement, the Company received a $10.7 million signing bonus that is being deferred over the contract term and has been reflected in the table below as consideration received from credit card mileage programs in 2015. This extension is considered a material change and thus at the inception of this new arrangement, the Company evaluated all deliverables in the arrangement to determine whether they represent separate units of accounting using the criteria as set forth in ASU No. 2009-13. Under the Company's affinity card program, funds received for the marketing of a co-branded Spirit credit card and delivery of award miles are accounted for as a multiple-deliverable arrangement. At the inception of the arrangement, the Company evaluated all deliverables in the arrangement to determine whether they represent separate units of accounting. The Company determined the arrangement had three separate units of accounting: (i) travel miles to be awarded, (ii) licensing of brand and access to member lists and (iii) advertising and marketing efforts. Arrangement consideration was allocated based on relative selling price. The Company established the estimated selling price for all deliverables that qualified for separation. The manner in which the selling price was established was based on a hierarchy of evidence the Company considered. Total arrangement consideration was then allocated to each deliverable on the basis of the deliverable’s relative selling price. In considering the hierarchy of evidence, the Company first determined whether vendor specific objective evidence of selling price or third-party evidence of selling price existed. It was determined by the Company that neither vendor specific objective evidence of selling price nor third-party evidence existed due to the uniqueness of the Company’s program. As such, the Company developed its best estimate of the selling price for all deliverables. For the award miles, the Company considered a number of entity-specific factors when developing the best estimate of the selling price, including the number of miles needed to redeem an award, average fare of comparable segments, breakage, restrictions and other charges. For licensing of brand and access to member lists, the Company considered both market-specific factors and entity-specific factors, including general profit margins realized in the marketplace/industry, brand power, market royalty rates and size of customer base. For the advertising element, the Company considered market-specific factors and entity-specific factors, including the Company’s internal costs (and fluctuations of costs) of providing services, volume of marketing efforts and overall advertising plan. Consideration allocated based on the relative selling price to both brand licensing and advertising elements is recognized as revenue when earned and recorded in non-ticket revenue. Consideration allocated to award miles is deferred and recognized ratably as passenger revenue over the estimated period the transportation is expected to be provided which is estimated at 17 months . 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, 2017 $ 49,453 $ 37,960 December 31, 2016 48,882 36,640 December 31, 2015 58,005 35,938 Total unrecognized revenue from future FREE SPIRIT award redemptions and the sale of mileage credits was $13.8 million and $15.3 million at December 31, 2017 and 2016 , 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. Non-ticket Revenue Recognition Non-ticket revenues are generated from air travel-related services for baggage, bookings through the Company’s call center or third-party vendors, advance seat selection, itinerary changes and loyalty programs. Non-ticket revenues also consist of services not directly related to providing transportation such as the FREE SPIRIT affinity credit card program, $9 Fare Club and the sale of advertising to third parties on Spirit’s website and on board aircraft. The following table summarizes the primary components of non-ticket revenue and the revenue recognition method utilized for each service or product: Year Ended December 31, Non-ticket revenue Recognition method 2017 2016 2015 (in thousands) Baggage Time of departure $ 488,434 $ 434,269 $ 381,386 Passenger usage fee Time of departure 411,742 358,920 298,092 Advance seat selection Time of departure 131,821 110,966 97,786 Other 249,635 217,180 194,861 Non-ticket revenue $ 1,281,632 $ 1,121,335 $ 972,125 Charges for services recognized at time of departure are initially recorded as a liability, within air traffic liability, until time of departure. The passenger usage fee is charged for tickets sold through the Company’s primary sales distribution channels. The primary sales distribution channels for which passenger usage fees are charged include sales through the Company’s website, sales through the third-party provided call center and sales through travel agents; the Company does not charge a passenger usage fee for sales made at its airport ticket counters. Other non-ticket revenues include revenues from other air related charges as well as non-air related charges. Other air related charges include optional services and products provided to passengers such as use of the Company’s call center or travel agents, commissions on travel insurance and commissions on sale of on-board products, among others. Non-air related charges primarily consist of revenues from advertising on the Company’s aircraft and website, the Company’s $9 Fare Club subscription-based membership program and the Company’s FREE SPIRIT affinity credit card program. 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 $53.9 million , $43.8 million and $43.1 million for the years ended 2017 , 2016 and 2015 , respectively. During the years ended 2017 , 2016 and 2015 , the Company deferred $78.2 million , $35.4 million and $9.1 million , respectively, of costs for heavy maintenance. At December 31, 2017 and 2016 , the Company had deferred heavy maintenance balance of $260.5 million and $238.3 million , and accumulated heavy maintenance amortization of $160.7 million and $162.8 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 extent to which the Company’s maintenance costs are rate capped due to flight hour maintenance contracts: Year Ended December 31, 2017 2016 2015 (in thousands) Flight hour-based maintenance expense $ 54,802 $ 48,471 $ 41,818 Non-flight hour-based maintenance expense 55,637 50,116 38,630 Total maintenance, materials and repairs $ 110,439 $ 98,587 $ 80,448 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 could include, but are not limited to redelivery cost, fuel, final inspections, reconfiguration of the cabin, repairs to the airframe, painting, overhaul of engines, replacement of components, and checks. Lease return costs are recognized beginning when it is probable that such costs will be incurred and they can be estimated. Incurrence of lease return costs becomes probable and the amount of those costs can typically be estimated near the end of the lease term. When determining probability and estimated cost, there are various other factors which need to be considered such as current condition of the aircraft, the age of the aircraft at lease expiration, utilization of engines and other components, the extent of repairs needed at return, return locations, current configuration of the aircraft and cost of repairs and materials at the time of return. In addition, typically near the lease return date, the lessors may allow reserves to be applied as return condition consideration or pass on certain return provisions if they do not align with their current plans to remarket the aircraft. When costs become both probable and estimable, they are accrued on a straight-line basis as contingent rent, a component of supplemental rent, through the remaining lease term. Management expects return costs to be estimable near the end of the lease term, as such, contingent rent for related aircraft will be higher near the end of the lease term. Maintenance Reserves 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, 2017 , 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. Derivative Instruments The Company accounts for derivative financial instruments at fair value and recognizes them in the balance sheet in prepaid expenses and other current assets or other current liabilities. For derivatives designated as cash flow hedges, changes in fair value of the derivative are generally reported in other comprehensive income and are subsequently reclassified into earnings when the hedged item affects earnings. During the third quarter of 2015, the Company settled six forward interest rate swaps having a total notional amount of $120 million . These interest rate swaps fixed the benchmark interest rate component of interest payments on the debt related to three Airbus A321 aircraft, which the Company took delivery of during the third quarter of 2015. These instruments limited the Company's exposure to changes in the benchmark interest rate in the period from the trade date through the date of maturity. The interest rate swaps were designated as cash flow hedges. The Company accounts for interest rate swaps at fair value and recognizes them in the balance sheet in prepaid expenses and other current assets or other current liabilities with changes in fair value recorded within accumulated other comprehensive income (AOCI). Realized gains and losses from cash flow hedges are recorded in the statement of cash flows as a component of cash flows from operating activities. Subsequent to the issuance of each debt instrument, amounts remaining in AOCI are amortized over the life of the fixed-rate debt instrument. For the years ended 2017 and 2016 , the Company did not hold any fuel derivative instruments. For the year ended 2015 , the Company did not hold fuel derivative instruments that were designated as cash flow hedges for accounting purposes. As a result, changes in the fair value of such fuel derivative contracts were recorded within aircraft fuel expense in the accompanying statements of operations. These amounts include both realized gains and losses and mark-to-market adjustments of the fair value of unsettled derivative instruments at the end of the period. For additional information, refer to Note 13, Financial Instruments and Risk Management. Advertising The Company expenses advertising and the production costs of advertising as incurred. Marketing and advertising expenses of $5.1 million , $3.2 million and $3.5 million for the years ended 2017 , 2016 and 2015 , respectively, were recorded within distribution expense in the statement of operations. Income Taxes The Company accounts for income taxes using the 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, 2017 , the Company recorded a valuation allowance of $0.5 million . For additional information, refer to Note 15, Income Taxes. As of December 31, 2016 , the Company had no valuation allowance recorded against any deferred tax assets. 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. 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. 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 is estimated through the use of a Monte Carlo simulation model. For additional information, refer to Note 9, Stock-Based Compensation. Concentrations of Risk The Company’s business may be adversely affected by increases in the price of aircraft fuel, the volatility of the price of aircraft fuel, or both. Aircraft fuel, one of the Company’s largest expenditures, represented approximately 27% , 24% and 28% of total operating expenses in 2017 , 2016 and 2015 , 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 fleet and high utilization rate, the unavailability of one or more 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, 2017 , the Company had four union-represented employee groups that together represented approximately 75% of all employees. As of December 31, 2016 , the Company had four union-represented employee groups that together represented approximately 73%

Recent Accounting Developments

Recent Accounting Developments	12 Months Ended
	Dec. 31, 2017
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Recent Accounting Developments	Recent Accounting Developments Revenue from Contracts with Customers In May 2014, the Financial Accounting Standards Board (the FASB) issued Accounting Standards Update (ASU) No. 2014-09, (ASU 2014-09), "Revenue from Contracts with Customers." The objective of ASU 2014-09 is to establish a single comprehensive model for entities to use in accounting for revenue arising from contracts with customers. The new guidance is effective for the Company in the first quarter of 2018. Entities have the option to use either a full retrospective or a modified retrospective approach to adopt ASU 2014-09. The Company currently anticipates utilizing the full retrospective method of adoption allowed by the standard, in order to provide for comparative results in all periods presented, and will adopt the standard as of January 1, 2018. While the Company is still evaluating the impact, it currently believes the most significant impact of this ASU will be the elimination of the incremental cost method for frequent flier program accounting, which will require the Company to re-value and record a liability associated with customer flight miles earned as part of the Company’s frequent flier program with a relative fair value approach. While our evaluation is ongoing, the Company currently estimates that applying a relative fair value would increase its air traffic liability by approximately $10 million at the date of adoption. The Company also expects the classification and timing of recognition of certain ancillary fees to be impacted by the adoption of ASU 2014-09. While the Company believes the adoption will not have a significant impact on earnings, the classification of certain revenues, such as bags, seats and other travel-related fees may be deemed part of the single performance obligation of providing passenger transportation. The Company expects that these revenues currently classified as non-ticket revenue, approximately $1 billion annually, will be reclassified to passenger revenue after adoption. Financial Instruments In January 2016, the FASB issued ASU 2016-01, “Financial Instruments – Overall (Subtopic 825-10).” ASU 2016-01 makes several modifications to Subtopic 825-10, including the elimination of the available-for-sale classification of equity investments, and requires equity investments with readily determinable fair values to be measured at fair value with changes in fair value recognized in net income. ASU 2016-01 is effective for interim and annual periods beginning January 1, 2018 and is not expected to have a material impact on the Company’s financial statements. Leases In February 2016, the FASB issued ASU No. 2016-02, "Leases (Topic 842)." This standard will require all leases with durations greater than twelve months to be recognized on the balance sheet and is effective for the Company in the first quarter of 2019, with early adoption permitted. The Company is currently evaluating the new guidance and believes adoption of this standard will have a significant impact on its balance sheets although adoption is not expected to significantly change the recognition, measurement or presentation of lease expenses within the statements of operations and cash flows. Refer to Note 16, Commitments and Contingencies for information regarding the Company's undiscounted future lease payments and the timing of those payments. Share-Based Compensation In March 2016, the FASB issued ASU No. 2016-09, "Improvements to Employee Share-Based Payment Accounting," which simplifies several aspects of the accounting for employee share-based payment transactions, including the accounting for income taxes, forfeitures, and statutory tax withholding requirements, as well as classification on the statement of cash flows. The Company adopted this guidance on January 1, 2017. As a result, 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, 2017 , $0.5 million of income tax deficiency related to share-based compensation was included within income tax expense on the Company's statements of operations. Additionally, excess income tax benefits and deficiencies for share-based payments are now included in net operating cash flows rather than net financing cash flows. The changes have been applied prospectively in accordance with the guidance and prior periods have not been adjusted. Accounting for Credit Losses In June 2016, the FASB issued ASU No. 2016-13, "Financial Instruments – Credit Losses." The standard requires the use of an "expected loss" model on certain types of financial instruments. The standard also amends the impairment model for available-for-sale debt securities and requires estimated credit losses to be recorded as allowances instead of reductions to amortized cost of the securities. This standard is effective for fiscal years, and interim periods within those years, beginning January 1, 2020, with early adoption permitted. The Company is evaluating the new guidance, but does not expect it to have a material impact on its financial statements. Statement of Cash Flows

Special Charges

Special Charges	12 Months Ended
	Dec. 31, 2017
Special Charges and Credits [Abstract]
Special Charges	Special Charges 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 condensed 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 condensed 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. During the twelve months ended December 31, 2016 , the Company purchased seven A319 aircraft which were previously financed under operating lease agreements. The purchase price for the seven aircraft was $147.7 million , comprised of cash payments of $107.1 million and the application of maintenance and security deposits held by the previous lessors of $40.6 million . The Company estimated the fair value of the aircraft to be $95.7 million and has recorded the seven purchased aircraft within flight equipment on the balance sheets. The Company determined the valuation of the aircraft based on a third-party appraisal considering the condition of each aircraft (a Level 3 measurement). The Company recognized $37.2 million as a cost of terminating the leases within special charges on the statement of operations, made up of the excess of the purchase price paid over the fair value of the aircraft, less previously expensed supplemental rent and other non-cash items of $14.8 million . During the twelve months ended December 31, 2015 , the Company incurred $0.7 million

Letters of Credit

Letters of Credit	12 Months Ended
	Dec. 31, 2017
Financial Instruments Pledged as Collateral [Abstract]
Letters of Credit	Letters of Credit As of December 31, 2017 and 2016 , the Company had a $35.0 million and $25.2 million unsecured standby letter of credit facility, of which $17.5 million and $19.7 million

Credit Card Processing Arrangem

Credit Card Processing Arrangements	12 Months Ended
	Dec. 31, 2017
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, 2017 and 2016 , 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, 2017 and 2016 , was $286.3 million and $234.6 million

Short-term Investment Securitie

Short-term Investment Securities	12 Months Ended
	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Short-term Investment Securities	Short-term Investment Securities The Company's short-term investment securities consist of available-for-sale asset-backed 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, 2017 and December 31, 2016 , the Company had $100.9 million and $100.2 million in short-term available-for-sale investment securities, respectively. During the twelve months ended December 31, 2017 and December 31, 2016 , these investments earned interest income at a weighted-average fixed rate of approximately 1.4% and 1.1% , respectively. For the twelve months ended December 31, 2017 and December 31, 2016 , an unrealized loss of $82 thousand and $23 thousand , net of deferred taxes of $41 thousand and $13 thousand , respectively, was recorded within AOCI related to these investment securities. The Company has not recognized any realized gains or losses related to these securities as the Company has not transacted any sales of these securities. As of December 31, 2017 and December 31, 2016 , $105 thousand and $23 thousand

Accrued Liabilities

Accrued Liabilities	12 Months Ended
	Dec. 31, 2017
Payables and Accruals [Abstract]
Accrued Liabilities	Accrued Liabilities Accrued liabilities included in other current liabilities as of December 31, 2017 and 2016 consist of the following: As of December 31, 2017 2016 (in thousands) Salaries and wages $ 54,338 $ 54,578 Airport obligations 56,299 43,989 Federal excise and other passenger taxes and fees payable 42,036 42,064 Aircraft maintenance 33,033 30,233 Fuel 25,171 14,828 Aircraft and facility lease obligations 16,992 10,378 Interest payable 11,384 8,499 Other 23,117 21,442 Other current liabilities $ 262,370 $ 226,011

Common Stock and Preferred Stoc

Common Stock and Preferred Stock	12 Months Ended
	Dec. 31, 2017
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 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. As of December 31, 2017 and 2016 , 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, 2017 and 2016 , there were no

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [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 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. The number of shares reserved for issuance or transfer pursuant to awards under the 2015 Plan will be increased by the number of shares represented by awards outstanding under the Company's former equity plan, the 2011 Equity Incentive Award Plan (2011 Plan), that are forfeited or lapse unexercised and which, following the effective date of the 2015 Plan, are not issued under the Company's 2011 Plan. No further awards will be granted under the 2011 Stock Plan, and all outstanding awards will continue to be governed by their existing terms. As of December 31, 2017 and December 31, 2016 , 2,377,650 and 2,358,283 shares of the Company’s common stock, respectively, remained available for future issuance under the 2015 Plan. Stock-based compensation cost amounted to $8.5 million , $7.1 million and $9.2 million for 2017 , 2016 and 2015 , respectively. During 2017 , there was $1.6 million tax expense recognized in income related to stock-based compensation. During 2016 and 2015 , there was $2.6 million and $3.4 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, 2017 and changes during the year ended December 31, 2017 is presented below: Number of Shares Weighted-Average Outstanding at December 31, 2016 274,645 49.55 Granted 103,030 51.68 Vested (88,580 ) 47.64 Forfeited (22,573 ) 55.79 Outstanding at December 31, 2017 266,522 50.48 There were 103,030 and 237,001 restricted stock shares granted during the years ended December 31, 2017 and December 31, 2016 , respectively. As of December 31, 2017 and December 31, 2016 , there was $7.5 million and $9.2 million , respectively, of total unrecognized compensation cost related to nonvested restricted stock to be recognized over 2.0 years and 2.5 years , respectively. The weighted-average fair value of restricted stock granted during the years ended December 31, 2017 , 2016 and 2015 was $51.68 , $42.91 and $75.40 , respectively. The total fair value of restricted stock shares vested during the years ended December 31, 2017 , 2016 and 2015 was $4.3 million , $6.6 million and $9.2 million respectively. Stock Options Stock option awards are granted with an exercise price equal to the fair market value of the Company’s common stock at the date of grant, vest over four years of continuous service and have ten -year contractual terms. The fair value of each stock option award is estimated on the date of grant using the Black Scholes model. There were no options granted during 2017 , 2016 , or 2015 . The Company has not granted options since 2011, at which time the Company’s weighted average assumptions for expected volatility, dividends, term and risk-free interest rate were 46.25% , 0% , 6.25 years and 2.03% , respectively. Expected volatilities are based on the historical volatility of a group of peer entities within the same industry. The expected term of options is based upon the simplified method, which represents the average of the vesting term and the contractual term. The risk-free interest rate is based on U.S. Treasury yields for securities with terms approximating the expected term of the option. Prior to the Company's IPO, to the extent a market price was not available, the fair value of the Company’s common stock was estimated using a discounted cash flow analysis and market multiples, based on management’s estimates of revenue, driven by assumed market growth rates, and estimated costs as well as appropriate discount rates. These estimates were consistent with the plans and estimates management used to manage the Company’s business. A summary of share option activity as of December 31, 2017 and changes during the year ended December 31, 2017 is presented below: Number Weighted- Average Aggregate Outstanding at December 31, 2016 16,250 8.37 3.7 804 Exercised (5,750 ) 7.80 Forfeited or expired — — Outstanding at December 31, 2017 10,500 8.68 2.7 380 Exercisable at December 31, 2017 10,500 8.68 2.7 380 Vested at December 31, 2017 10,500 8.68 2.7 380 The total intrinsic value of share options exercised during the years ended December 31, 2017 , 2016 and 2015 was $0.2 million , $0.4 million and $0.2 million , respectively. There were no options that vested during the year ended December 31, 2017 and 2016 . The total fair value of options vested during the year ended December 31, 2015 was $4 thousand . As of December 31, 2017 , 2016 and 2015 , there was no unrecognized compensation cost related to options as all option awards were fully vested. 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. 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, 2017 Weighted-Average at Grant Date for Twelve Months Ended December 31, 2016 Expected volatility factor 0.40 0.39 Risk free interest rate 1.47 % 1.12 % Expected term (in years) 2.93 2.94 Expected dividend yield — % — % For grants awarded in 2017, 2016 and 2015, 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, 2017 : Number of Awards Weighted-Average Fair Value at Grant Date ($) Outstanding at December 31, 2016 133,907 50.54 Granted 89,338 48.86 Vested (17,592 ) 50.07 Forfeited (85,294 ) 45.64 Outstanding at December 31, 2017 120,359 52.84 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, 2017 : Number of Awards Weighted-Average Fair Value at Grant Date ($) Outstanding at December 31, 2016 — — Granted 37,649 46.28 Vested — — Forfeited (2,142 ) 55.50 Outstanding at December 31, 2017 35,507 45.72 As of December 31, 2017 and 2016 , there was $3.7 million and $4.3 million , respectively, of total unrecognized compensation cost related to performance share awards expected to be recognized over 1.52 years and 1.90 years , respectively. Treasury Stock During the year ended December 31, 2017 and 2016 , the Company repurchased 1.2 million and 2.4 million shares, respectively, for $46.6 million and $102.5 million , respectively. Repurchases of equity securities for these periods include open market repurchases made under our stock repurchase programs as well as repurchases made from employees who r eceived restricted stock or performance share awards. During the year ended December 31, 2017 , the Company retired 3.9 million treasury shares in a total aggregate amount of $199.4 million

Net Income per Share

Net Income per Share	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Net Income per Share	Net Income per Share The following table sets forth the computation of basic and diluted earnings per common share: Year Ended December 31, 2017 2016 2015 (in thousands, except per share amounts) Numerator: Net income (1) $ 420,606 $ 264,879 $ 317,220 Denominator: Weighted-average shares outstanding, basic 69,221 70,344 72,208 Effect of dilutive stock awards 156 164 218 Adjusted weighted-average shares outstanding, diluted 69,377 70,508 72,426 Net Income per Share: Basic earnings per common share (1) $ 6.08 $ 3.77 $ 4.39 Diluted earnings per common share (1) $ 6.06 $ 3.76 $ 4.38 Anti-dilutive weighted-average shares 85 66 52 (1) During the fourth quarter of 2017, the Company recorded a non-recurring income tax benefit of $199.3 million ( $2.88 and $2.87

Debt and Other Obligations

Debt and Other Obligations	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Debt and Other Obligations	Debt and Other Obligations Long-term debt As of December 31, 2017 , the Company held non-public and public debt instruments. During 2017 , the Company acquired additional debt through the 2017 fixed-rate loans and the 2017-1 EETCs described below. 2017 Fixed-rate Loans During 2017, the Company entered into fixed-rate debt, which as of December 31, 2017 , provided $527.3 million of debt financing for 4 Airbus A320 aircraft and 10 Airbus A321 aircraft. Each loan is secured by a first-priority security interest on the individual aircraft. Each loan amortizes on a mortgage-style basis and has a 10 to 12 year term. Loans bear interest payable quarterly on a fixed-rate basis. As of December 31, 2017 , the Company has taken delivery of all aircraft financed through the 2017 fixed-rate loans. 2017-1 EETCs In November 2017, the Company created three separate pass-through trusts, which issued $420.5 million aggregate face amount of Series 2017-1 Class AA, Class A and Class B EETCs in connection with the financing of seven new Airbus A320 aircraft and five new Airbus A321 aircraft. Each class of certificates represents a fractional undivided interest in the respective pass-through trusts and is not an obligation of the Company. The proceeds from the issuance of these certificates are initially held in escrow by a depositary and, upon satisfaction of certain terms and conditions, are released and used to purchase equipment notes which are issued by the Company and secured by the Company's aircraft. Interest on the issued and outstanding equipment notes are payable semiannually on February 15 and August 15 of each year, commencing on August 15, 2018, and principal on such equipment notes is scheduled for payment on February 15 and August 15 of certain years. Principal payments commence on August 15, 2018 in the case of five new Airbus A321 scheduled for delivery from February 2018 to March 2018 and three Airbus A320 scheduled for delivery from December 2017 to January 2018 and on February 15, 2019 for four Airbus A320 aircraft scheduled for delivery from April 2018 to October 2018. Issued and outstanding Series AA and Series A equipment notes mature in February 2030 and Series B equipment notes mature in February 2026. Issued and outstanding Series AA, Series A and Series B equipment notes accrue interest at a rate of 3.375% , 3.650% and 3.800% , respectively. As of December 31, 2017 , $63.9 million of the proceeds from the sale of the Series 2017-1 EETCs had been used to purchase equipment notes in connection with the financing of two Airbus A320 aircraft. The remaining $356.6 million of escrowed proceeds held by the pass-through trusts will be used to purchase equipment notes as the remaining 10 new aircraft are delivered. Equipment notes that are issued are reported as debt on the Company's condensed balance sheets. The Company evaluated whether the pass-through trusts formed are variable interest entities (VIEs) required to be consolidated by the Company under applicable accounting guidance. The Company determined that the pass-through trusts are VIEs and that it does not have a variable interest in the pass-through trusts. Based on this analysis, the Company determined that it is not required to consolidate these pass-through trusts. Long-term debt is comprised of the following: As of Twelve Months Ended December 31, 2017 2016 2017 2016 (in millions) (weighted-average interest rates) Fixed-rate senior term loans due through 2027 $ 417.9 $ 451.9 4.10 % 4.10 % Fixed-rate junior term loans due through 2022 39.3 47.1 6.90 % 6.90 % Fixed-rate loans due through 2029 518.0 — 3.83 % N/A Fixed-rate class A 2015-1 EETC due through 2028 408.6 409.8 4.10 % 4.10 % Fixed-rate class B 2015-1 EETC due through 2024 92.0 103.6 4.45 % 4.45 % Fixed-rate class AA 2017-1 EETC due through 2030 37.5 — 3.38 % N/A Fixed-rate class A 2017-1 EETC due through 2030 12.5 — 3.65 % N/A Fixed-rate class B 2017-1 EETC due through 2026 13.8 — 3.80 % N/A Long-term debt $ 1,539.6 $ 1,012.4 Less current maturities 115.4 84.4 Less unamortized discount, net 36.7 30.6 Total $ 1,387.5 $ 897.4 The Company's debt financings are collateralized by first priority security interest in the individual aircraft being financed. During the year ended December 31, 2017 and 2016 , the Company made scheduled principal payments of $102.3 million and $64.4 million on its outstanding debt obligations, respectively. At December 31, 2017 , long-term debt principal payments for the next five years and thereafter are as follows: December 31, 2017 (in millions) 2018 $ 121.0 2019 122.3 2020 121.8 2021 121.5 2022 118.8 2023 and beyond 934.2 Total debt principal payments $ 1,539.6 Interest Expense Interest expense related to long-term debt consists of the following: Twelve Months Ended December 31, 2017 2016 (in thousands) Senior term loans $ 18,328 $ 19,759 Junior term loans 3,035 3,568 Fixed-rate loans 8,610 — Class A 2015-1 EETC 17,230 11,509 Class B 2015-1 EETC 4,446 3,243 Class AA 2017-1 EETC 54 — Class A 2017-1 EETC 19 — Class B 2017-1 EETC 22 — Commitment fees 124 127 Amortization of deferred financing costs 5,280 3,435 Total $ 57,148 $ 41,641 As of December 31, 2017 and 2016 , the Company had a line of credit for $33.6 million and $23.6 million related to corporate credit cards. Respectively, the Company had drawn $1.7 million and $9.9 million as of December 31, 2017 and 2016 , which is included in accounts payable. As of December 31, 2017 and 2016 , the Company had lines of credit with counterparties for derivatives and physical fuel delivery in the amount of $51.5 million and $46.5 million , respectively. As of December 31, 2017 and 2016 , the Company had drawn $24.2 million and $8.0 million on these lines of credit for physical fuel delivery, which is included in other current liabilities. 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, 2017 and 2016

Leases and Prepaid Maintenance

Leases and Prepaid Maintenance Deposits	12 Months Ended
	Dec. 31, 2017
Leases [Abstract]
Leases and Prepaid Maintenance Deposits	Leases and Aircraft Maintenance Deposits The Company leases various types of equipment and property, primarily aircraft, spare engines and airport facilities under leases, which expire in various years through 2047 . Lease terms are generally 8 to 18 years for aircraft and up to 30 years for other leased equipment and property. Total rental expense for all leases charged to operations for the years ended 2017 , 2016 and 2015 was $309.8 million , $283.9 million and $282.7 million , respectively. Total rental expense charged to operations for aircraft and engine operating leases for the years ended December 31, 2017 , 2016 and 2015 was $205.9 million , $201.7 million and $211.5 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. 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 for these aircraft and related flight equipment, including estimated amounts for contractual price escalations, are expected to be $8.4 million in 2018 , $5.9 million in 2019 , $5.6 million in 2020 , $5.7 million in 2021 , $4.9 million in 2022 , and $12.9 million in 2023 and beyond . 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, 2017 , 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. At lease inception and at each balance sheet date, the Company assesses whether the maintenance reserve payments required by the master lease agreements are substantively and contractually related to the maintenance of the leased asset. Maintenance reserve payments that are substantively and contractually related to the maintenance of the leased asset are accounted for as maintenance deposits to the extent they are expected to be recoverable and 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. 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 $8.4 million , $9.0 million and $7.7 million of supplemental rent recorded within aircraft rent during 2017 , 2016 and 2015 , respectively. These amounts include $0.4 million , $2.2 million and $2.3 million of paid maintenance reserves expensed as supplemental rent during 2017 , 2016 and 2015 , respectively. As of December 31, 2017 and 2016 , the Company had short-term and long-term aircraft maintenance deposits of $326.2 million and $286.4 million , respectively, on its balance sheets. As of December 31, 2017 , the Company had a fleet consisting of 112 A320 family aircraft. As of December 31, 2017 , the Company had 58 aircraft financed under operating leases with lease term expirations between 2020 and 2029 and owned 54 aircraft of which 8 were purchased off lease and are currently unencumbered. In addition, as of December 31, 2017 , the Company had 11 spare engines financed under operating leases with lease term expiration dates ranging from 2019 to 2027 and owned 4 spare engines of which 1 was purchased off lease and all 4 are currently unencumbered. One of the Company's leased aircraft have variable rent payments, which fluctuate based on changes in LIBOR (London Interbank Offered Rate). The Company entered into sale leaseback transactions with third-party aircraft lessors for the majority of these aircraft and engine leases. Deferred losses resulting from these sale leaseback transactions are included in other long-term assets on the accompanying balance sheet. Deferred losses are recognized as an increase to rent expense on a straight-line basis over the term of the respective operating leases. Deferred gains are included in deferred gains and other long-term liabilities on the accompanying balance sheet. Deferred gains are recognized as a decrease to rent expense on a straight-line basis over the term of the respective operating leases. During the twelve months ended December 31, 2017 , the Company took delivery of 17 aircraft under secured debt arrangements, purchased 1 previously leased aircraft, took delivery of 2 aircraft under operating leases and returned 2 aircraft to its lessors. In addition, the Company purchased two spare engines, purchased one previously leased engine and took delivery of one engine under an operating lease. All of the Company's aircraft and engine leases are accounted for as operating leases. Under the terms of the lease agreements, the Company will continue to operate and maintain the aircraft. Payments under 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. 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. The majority of the Company's capital lease obligations relate to the lease of computer equipment used by the Company's flight crew. Payments under this lease agreement are fixed for the 3 -year term of the lease which commenced in the second quarter of 2017. Future minimum lease payments under capital leases and noncancellable operating leases with initial or remaining terms in excess of one year at December 31, 2017 were as follows: Capital Leases Operating Leases Total Operating and Capital Lease Obligations Aircraft and Spare Engine Leases Property Facility Leases (in thousands) 2018 $ 537 $ 204,292 $ 45,737 $ 250,566 2019 504 189,106 36,852 226,462 2020 188 180,842 25,870 206,900 2021 28 170,643 12,740 183,411 2022 — 150,212 11,931 162,143 2023 and thereafter — 419,908 61,211 481,119 Total minimum lease payments $ 1,257 $ 1,315,003 $ 194,341 $ 1,510,601 Less amount representing interest 94 Present value of minimum lease payments $ 1,163 Less current portion 476 Long-term portion $ 687

Financial Instruments and Risk

Financial Instruments and Risk Management	12 Months Ended
	Dec. 31, 2017
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. The Company is exposed to credit losses in the event of nonperformance by counterparties to these financial instruments. The Company periodically reviews and seeks to mitigate exposure to the financial deterioration and nonperformance of any counterparty by monitoring the absolute exposure levels, each counterparty's credit ratings and the historical performance of the counterparties relating to hedge transactions. The credit exposure related to these financial instruments is limited to the fair value of contracts in a net receivable position at the reporting date. The Company also maintains security agreements that require the Company to post collateral if the value of selected instruments falls below specified mark-to-market thresholds. The Company records financial derivative instruments at fair value, which includes an evaluation of each counterparty's credit risk. As of December 31, 2017 and 2016 , the Company did not hold any derivatives. Fuel Derivative Instruments From time to time, the Company may enter into fuel derivative contracts in order to mitigate the risk of future volatility in fuel prices. The Company's fuel derivative contracts, if any, generally consist of United States Gulf Coast jet fuel swaps (jet fuel swaps) and United States Gulf Coast jet fuel options (jet fuel options). Both jet fuel swaps and jet fuel options are used at times to protect the refining price risk between the price of crude oil and the price of refined jet fuel, and to manage the risk of increasing fuel prices. Fair value of the instruments is determined using standard option valuation models. The Company accounts for any fuel derivative contracts at fair value and recognizes them in the balance sheet in prepaid expenses and other current assets or other current liabilities. The Company did not enter into any fuel derivative instruments during 2017 and 2016. The Company did not elect hedge accounting on any fuel derivative instruments entered into during 2015 and, as a result, changes in the fair value of these fuel derivative contracts are recorded in aircraft fuel expense. The following table summarizes the components of aircraft fuel expense for the years ended December 31, 2017 , 2016 and 2015 : Year Ended December 31, 2017 2016 2015 (in thousands) Into-plane fuel cost $ 615,581 $ 447,553 $ 454,747 Realized losses (gains) related to fuel derivative contracts, net — — 10,580 Unrealized losses (gains) related to fuel derivative contracts, net — — (3,880 ) Aircraft fuel $ 615,581 $ 447,553 $ 461,447 Premiums and settlements received or paid on fuel derivative contracts, if any, are reflected in the accompanying statements of cash flows in net cash provided by operating activities. As of December 31, 2017 and 2016 , the Company did not have any outstanding fuel derivatives and had no fuel hedging activity for the twelve months ended December 31, 2017 . Interest Rate Swaps During 2015, the Company settled six forward interest rate swaps that were designed to fix the benchmark interest rate component of the interest payments on the debt related to three Airbus A321 aircraft, which the company took delivery of during the third quarter of 2015. These instruments limited the Company's exposure to changes in the benchmark interest rate in the period from the trade date through the date of maturity. The interest rate swaps were designated as cash flow hedges. The Company accounts for these interest rate swaps at fair value and recognizes them in the balance sheet in prepaid expenses and other current assets or other current assets or other current liabilities with changes in fair value recorded within accumulated other comprehensive income (AOCI). As of December 31, 2017 and 2016 , the Company did not have any outstanding interest rate swaps. Realized gains and losses from cash flow hedges are recorded in the statement of cash flows as a component of cash flows from operating activities. Subsequent to the issuance of each debt instrument, amounts remaining in AOCI are amortized over the life of the fixed-rate debt instrument. During the twelve months ended December 31, 2017 and December 31, 2016 , there were no unrealized gains or losses recorded within AOCI related to these instruments as they settled in 2015. During the twelve months ended December 31, 2017 , the Company reclassified interest rate swap losses of $335 thousand , into earnings, resulting in a gain of $37 thousand , net of tax of $372 thousand . During the twelve months ended December 31, 2016 , the Company reclassified interest rate swap losses of $354 thousand , into earnings, resulting in a loss of $224 thousand , net of tax of $130 thousand . As of December 31, 2017 and December 31, 2016 , $1.4 million and $1.3 million

Defined Contribution 401(k) Pla

Defined Contribution 401(k) Plan	12 Months Ended
	Dec. 31, 2017
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 Company may make a Qualified Discretionary Contribution, as defined in the plan, or provide matching contributions to this plan. For flight attendants and dispatchers participating in the first plan, the Company currently matches 100% of the employee's contribution, up to a maximum of 6% of the employee's annual compensation. Prior to May 1, 2016, the Company matched 50% of the flight attendant and dispatcher's contribution, up to a maximum of 6% of the employee's annual compensation. For all other employees participating in the first plan, the Company matches 50% of the employee’s contribution, up to a maximum of 6% of the employee’s annual compensation. The second plan is for the Company’s pilots, and contains the same service requirements as the first plan. Throughout 2017 , the Company matched 100% of the pilot's contribution, up to 9% of the individual pilot's annual compensation. Both the first and the second plans are subject to the annual IRS elective deferral limit, which was $18,000 for 2017 . 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. For pilots participating in the Puerto Rico plan, the Company matched 100% of their contribution, up to 9% of the individual pilot's annual compensation, but subject to the annual Puerto Rico pre-tax elective deferral limit, which was $15,000 for 2017 . For all other employees participating in the Puerto Rico plan, the Company matches 50% of the employee's contribution, up to 6% of the employee's annual compensation. Matching contributions made to all plans were $19.6 million , $16.2 million and $12.5 million in 2017 , 2016 and 2015

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
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, 2017 2016 2015 (in thousands) Current: Federal $ (68,601 ) $ 60,079 $ 21,632 State and local 515 6,322 6,702 Foreign 2,742 2,034 1,235 Total current expense (benefit) (65,344 ) 68,435 29,569 Deferred: Federal (10,370 ) 82,455 149,583 State and local 8,760 3,691 6,031 Total deferred expense (benefit) (1,610 ) 86,146 155,614 Total income tax expense (benefit) $ (66,954 ) $ 154,581 $ 185,183 The income tax provision differs from that computed at the federal statutory corporate tax rate as follows: Year Ended December 31, 2017 2016 2015 Expected provision at federal statutory tax rate 35.0 % 35.0 % 35.0 % State tax expense, net of federal benefit 1.7 % 1.6 % 1.7 % Income tax credits — % — % — % Revaluation of deferred taxes (56.3 )% — % — % Other 0.7 % 0.3 % 0.2 % Total income tax expense (benefit) (18.9 )% 36.9 % 36.9 % 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, 2017 and 2016 , the significant components of the Company's deferred taxes consisted of the following: December 31, 2017 2016 (in thousands) Deferred tax assets: Foreign tax credits $ 5,980 $ — Deferred revenue 1,631 3,057 Nondeductible accruals 10,107 20,547 Deferred manufacturing credits 258 910 Accrued maintenance 1,991 1,854 Equity compensation 2,392 3,882 Other 5,555 4,026 Valuation allowance (454 ) — Deferred tax assets 27,460 34,276 Deferred tax liabilities: Deferred gain (loss) on leases, net 1,605 2,435 Accrued rent 12,055 14,025 Prepaid expenses 754 1,217 Property, plant and equipment 298,703 278,872 Deferred financing costs 225 5,358 Accrued aircraft and engine maintenance 27,258 40,512 Deferred tax liabilities 340,600 342,419 Net deferred tax assets (liabilities) $ (313,140 ) $ (308,143 ) 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 Company reasonably estimated and recorded a provisional reduction in net deferred tax liabilities of $196.0 million upon enactment of the TCJA as written. In addition, the Company generated $6.3 million of alternative minimum tax credits, which are recorded within other long-term assets on the balance sheet. These provisional amounts may be affected as the Company gains a more thorough understanding of the tax law, including those related to the deductibility of acquired assets, state tax treatment and amounts related to employee compensation. 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. The Company recorded a valuation allowance of $0.5 million against certain deferred tax assets related to equity compensation for executives due to changes in tax law resulting from the TCJA. At December 31, 2017 , the Company has $6.0 million of foreign tax credits and $14.8 million of state net operating losses available, that may be applied against future tax liabilities. The foreign tax credits will start to expire in 2025 and the state net operating losses will start to expire in 2027 . 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, 2017, $0.5 million of income tax deficiency related to share-based compensation was included within income tax expense. 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. The Company recognized an excess tax deficiency of $0.5 million and an excess tax benefit of $8.9 million for tax years ended December 31, 2016 and 2015 , respectively. The Company's policy is to recognize interest and penalties accrued on any unrecognized tax benefits as a component of selling, general and administrative expenses. For tax years ended December 31, 2017 , 2016 and 2015 , the Company did not recognize any liabilities for uncertain tax positions nor any interest and penalties on unrecognized tax benefits. For tax years 2017 , 2016 and 2015 , the entire income before income taxes 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 2014 through 2016

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2017
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. During the first quarter of 2017, the Company negotiated revisions to its A320 aircraft order. The Company originally had four A320neo aircraft scheduled for delivery in 2018 of which two were converted to A320ceo aircraft and delivered in 2017, and the remaining two are deferred until 2019. As of December 31, 2017 , the Company's firm aircraft orders consisted of the following: Airbus A320ceo A320neo A321ceo Total 2018 5 5 10 2019 1 14 15 2020 16 16 2021 18 18 6 48 5 59 During the first quarter of 2018, the Company negotiated revisions to its A320 aircraft order. The Company originally had 14 A320neo aircraft scheduled for delivery in 2019. Pursuant to the revision, 5 of the 14 scheduled A320neo aircraft were converted to A320ceo aircraft but remain scheduled to be delivered in 2019. The Company also has four spare engine orders for V2500 SelectTwo engines with IAE and nine spare engine orders for PurePower PW 1100G-JM engines with Pratt & Whitney. Spare engines are scheduled for delivery from 2018 through 2023 . Purchase commitments for these aircraft and engines, including estimated amounts for contractual price escalations and pre-delivery payments, are expected to be $528.4 million in 2018 , $774.8 million in 2019 , $820.9 million in 2020 , $785.1 million in 2021 , $16.8 million in 2022 , and $7.9 million in 2023 and beyond . As of December 31, 2017 , the Company had secured financing commitments of $356.6 million for ten aircraft, scheduled for delivery in 2018, and did not have financing commitments in place for the remaining 49 Airbus aircraft currently on firm order, which are scheduled for delivery in 2019 through 2021 . Interest commitments related to the secured debt financing of 46 delivered aircraft as of December 31, 2017 are $60.8 million in 2018 , in $56.2 million 2019 , $51.0 million in 2020 , $45.8 million in 2021 , $40.7 million in 2022 , and $133.3 million in 2023 and beyond . For principal commitments related to these financed aircraft, refer to Note 11, Debt and Other Obligations. As of December 31, 2017 , principal and interest commitments related to the Company's future secured debt financing of ten undelivered aircraft under the Series 2017-1 EETC are approximately $19.6 million in 2018 , $39.4 million in 2019 , $35.8 million in 2020 , $30.3 million in 2021 , $29.0 million in 2022 , and $301.4 million in 2023 and beyond . In July 2015, the Company executed an upgrade service agreement with Airbus Americas Customer Services Inc. (Airbus) to reconfigure the seating and increase capacity in 40 of the Company’s A320ceos from 178 to 182 seats (reconfiguration). The reconfiguration of the aircraft commenced in the first quarter of 2016 and is expected to be completed in the second quarter of 2018 for a remaining committed cost of $0.6 million as of December 31, 2017 . These amounts will be capitalized within flight equipment on the balance sheet. In September 2015 , the Company executed a lease agreement with Wayne County Airport Authority (the Authority), which owns and operates Detroit Metropolitan Wayne County Airport (DTW). Under the lease agreement, the Company leases a 10 -acre site, adjacent to the airfield at DTW, in order to construct, operate and maintain an approximately 126,000 -square-foot hangar facility (the project). The project allows for the development of a maintenance hangar in order to fulfill the requirements of the Company's growing fleet and will reduce dependence on third-party facilities and contract line maintenance. The lease agreement has a 30 -year term with two 10 -year extension options. Upon termination of the lease, title of the project, which will be fully depreciated, will automatically pass to the Authority. The Company completed the project during the first quarter of 2017 and has no remaining capital commitments related to this project as of December 31, 2017 . The Company is contractually obligated to pay the following minimum guaranteed payments for its reservation system, new airport kiosks and other miscellaneous subscriptions and services as of December 31, 2017 : $5.9 million in 2018 , $1.8 million in 2019 , $1.3 million in 2020 , $0.7 million in 2021 , $0.4 million in 2022 , and $0.0 million in 2023 and beyond . The Company's current agreement with its reservation system provider expires in 2018. 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 four union-represented employee groups that together represent approximately 75% of all employees at December 31, 2017 . The Company had four union-represented employee groups that together represented approximately 73% of all employees at December 31, 2016 . The table below sets forth the Company's employee groups and status of the collective bargaining agreements as of December 31, 2017 . Employee Groups Representative Amendable Date Percentage of Workforce Pilots Air Line Pilots Association, International (ALPA) August 2015 26% Flight Attendants Association of Flight Attendants (AFA-CWA) May 2021 44% Dispatchers Transport Workers Union (TWU) August 2018 1% Ramp Service Agents International Association of Machinists and Aerospace Workers (IAMAW) June 2020 4% In August 2015 , the Company's collective bargaining agreement with its pilots, represented by ALPA, became amendable. In June 2016 , ALPA requested the services of the National Mediation Board (NMB) to facilitate negotiations for an amended agreement and the Company joined ALPA in the request. In January 2018 , under the guidance of the NMB assigned mediators, the parties reached a tentative amendable agreement which is subject to ratification by the pilot group. The ratification vote will take place throughout February 2018 . The current tentative agreement is for a five -year contract which includes a one-time $75 million ratification incentive and other negotiated contractual provisions. The Company can provide no assurance that the tentative agreement will be approved. Under the Railway Labor Act (RLA), the parties' current agreement remains in effect until an amended agreement is ratified. In March 2016 , under the supervision of the NMB, the Company and AFA-CWA reached a tentative agreement for a five -year contract with the Company's flight attendants. In May 2016 , the flight attendants voted to approve the new five -year contract with the Company. In connection with this agreement, the Company paid a $9.6 million ratification incentive payment to the flight attendants recorded within salaries, wages and benefits in the statement of operations. Of the total ratification incentive, $8.4 million was recorded during 2016 as the remaining $1.2 million was previously accrued in 2015. In December 2017 , the Professional Airline Flight Control Association (“PAFCA”) filed an application with the NMB seeking to represent our dispatchers, who are currently represented by the TWU. In January 2018 , the NMB determined that a representation election would be held. The voting period for the representation election will close on February 20, 2018 . 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 $3.9 million and $5.7 million , for health care claims as of December 31, 2017 , and 2016

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2017
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. Fuel Derivative Instruments From time to time, the Company may enter into fuel derivative contracts in order to mitigate the risk of future volatility in fuel prices. The Company’s fuel derivative contracts generally consist of jet fuel swaps and jet fuel options. These instruments are valued using energy and commodity market data, which is derived by combining raw inputs with quantitative models and processes to generate forward curves and volatilities. The Company utilizes the market approach to measure fair value for its fuel derivative instruments, if any. The market approach uses prices and other relevant information generated by market transactions involving identical or comparable assets or liabilities. The Company does not elect hedge accounting on its fuel derivative instruments, if any. As a result, the Company records the fair value adjustment of any fuel derivatives in the accompanying statement of operations within aircraft fuel and on the balance sheet within prepaid expenses and other current assets or other current liabilities, depending on whether the net fair value of the derivatives is in an asset or liability position as of the respective date. Fair values of any fuel derivative instruments are determined using standard option valuation models. The Company also considers counterparty risk and its own credit risk in its determination of all estimated fair values. The Company offsets fair value amounts recognized for any derivative instruments executed with the same counterparty under a master netting arrangement. The Company determines fair value of any jet fuel options utilizing an option pricing model based on inputs that are either readily available in public markets or can be derived from information available in publicly quoted markets. The Company has consistently applied these valuation techniques in all periods presented and believes it has obtained the most accurate information available for the types of derivative contracts it holds. The fair value of the Company's jet fuel swaps, if any, are determined based on inputs that are readily available in public markets or can be derived from information available in publicly quoted markets; therefore, the Company categorizes these instruments as Level 2. Due to the fact that certain inputs utilized to determine the fair value of jet fuel options are unobservable (principally implied volatility), the Company categorizes these derivatives as Level 3. Implied volatility of a jet fuel option is the volatility of the price of the underlying commodity that is implied by the market price of the option based on an option pricing model. Thus, it is the volatility that when used in a particular pricing model, yields a theoretical value for the option equal to the current market price of that option. Implied volatility, a forward-looking measure, differs from historical volatility because the latter is calculated from known past returns. At each balance sheet date, the Company substantiates and adjusts unobservable inputs. The Company routinely assesses the valuation model's sensitivity to changes in implied volatility. Based on the Company's assessment of the valuation model's sensitivity to changes in implied volatility, it concluded that holding other inputs constant, a significant increase (decrease) in implied volatility would result in significantly higher (lower) determination of fair value measurement for the Company's aircraft fuel derivatives. As of December 31, 2017 and 2016 , the Company had no outstanding jet fuel derivatives. Long-term Debt The estimated fair value of the Company's non publicly held debt agreements 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 held debt agreements has been determined to be Level 2 as the Company utilizes quoted market prices to estimate the fair value of it's public long-term debt. The carrying amounts and estimated fair values of the Company's long-term debt at December 31, 2017 and December 31, 2016 , were as follows: As of December 31, 2017 2016 Carrying Value Estimated Fair Value Carrying Value Estimated Fair Value Fair value level hierarchy (in millions) Senior term loans $ 417.9 $ 435.3 $ 451.9 463.9 Level 3 Junior term loans 39.3 40.4 47.1 48.1 Level 3 Fixed-rate loans 518.0 528.6 — N/A Level 3 2015-1 EETC Class A 408.6 420.9 409.8 416.0 Level 2 2015-1 EETC Class B 92.0 94.2 103.6 105.7 Level 2 2017-1 EETC Class AA 37.5 37.4 — N/A Level 2 2017-1 EETC Class A 12.5 12.6 — N/A Level 2 2017-1 EETC Class B 13.8 13.8 — N/A Level 2 Total long-term debt $ 1,539.6 $ 1,583.2 $ 1,012.4 $ 1,033.7 Cash and Cash Equivalents Cash and cash equivalents at December 31, 2017 and December 31, 2016 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, 2017 and December 31, 2016 are comprised of available-for-sale asset-backed securities with contractual maturities of twelve months or less and 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 6, Short-term Investment Securities. Assets and liabilities measured at gross fair value on a recurring basis are summarized below: Fair Value Measurements as of December 31, 2017 Total Level Level Level (in millions) Cash and cash equivalents $ 800.8 $ 800.8 $ — $ — Short-term investment securities 100.9 100.9 — — Total assets $ 901.7 $ 901.7 $ — $ — Total liabilities $ — $ — $ — $ — Fair Value Measurements as of December 31, 2016 Total Level Level Level (in millions) Cash and cash equivalents $ 700.9 $ 700.9 $ — $ — Short-term investment securities 100.2 100.2 — — Total assets $ 801.1 $ 801.1 $ — $ — Total liabilities $ — $ — $ — $ — The Company had no transfers of assets or liabilities between any of the above levels during the years ended December 31, 2017 or 2016 .

Operating Segments and Related

Operating Segments and Related Disclosures	12 Months Ended
	Dec. 31, 2017
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: 2017 2016 2015 (in millions) DOT—Domestic $ 2,435.9 $ 2,136.2 $ 1,940.2 DOT—Latin America 211.8 185.8 201.3 Total $ 2,647.7 $ 2,322.0 $ 2,141.5 During 2017 , 2016 and 2015 , no revenue from any one foreign country represented greater than 4%

Quarterly Financial Data (Unaud

Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Data (Unaudited)	Quarterly Financial Data (Unaudited) Quarterly results of operations for the years ended December 31, 2017 and 2016 are summarized below: Three Months Ended March 31 June 30 September 30 December 31 (in thousands, except per share amounts) 2017 Operating revenue $ 591,746 $ 701,682 $ 687,231 $ 667,007 Operating income 59,405 132,736 104,104 92,546 Net income (1) 31,935 78,143 60,190 250,338 Basic earnings per share (1) 0.46 1.13 0.87 3.64 Diluted earnings per share (1) 0.46 1.12 0.87 3.63 2016 Operating revenue $ 538,143 $ 584,133 $ 621,329 $ 578,351 Operating income 101,299 121,835 135,216 85,311 Net income 61,920 73,084 81,382 48,493 Basic earnings per share 0.87 1.03 1.17 0.70 Diluted earnings per share 0.86 1.03 1.17 0.70 (1) During the fourth quarter of 2017, the Company recorded a non-recurring income tax benefit of $199.3 million ( $2.90 and $2.89

Summary of Significant Accoun28

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
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 EstimatesThe 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.
Cash and Cash Equivalents	Cash and Cash EquivalentsThe 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 SecuritiesThe Company's short-term investment securities consist of available-for-sale asset-backed 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 ReceivableAccounts 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.
Property and Equipment	Property and Equipment Property and equipment is stated at cost, less accumulated depreciation and amortization. Depreciation of operating property and equipment is computed using the straight-line method applied to each unit of property. Residual values for new aircraft, new engines, major spare rotable parts, avionics and assemblies are generally estimated to be 10% . Property under capital 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 capital 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 Capital leases Lease term 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, 2017 , the Company had 54 aircraft, 4 spare engines and 1 flight simulator capitalized within flight equipment with depreciable lives of 25 years . As of December 31, 2017 , the Company had 58 aircraft financed through operating leases with lease terms from 8 to 18 years and 11 spare engines financed through operating leases with lease terms from 2 to 13 years
Measurement of Asset Impairments	Measurement of Asset ImpairmentsThe 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 Revenue Recognition	Passenger Revenue Recognition Tickets sold are initially deferred as “air traffic liability.” Passenger revenue is 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. Customers may elect to change their itinerary prior to the date of departure. A service charge is assessed and recognized on the date the change is initiated and is deducted from the face value of the original purchase price of the ticket, and the original ticket becomes invalid. 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 can be used towards the purchase of a new ticket and the Company’s other service offerings. The amount of credits expected to expire is recognized as revenue upon issuance of the credit and is 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.
Frequent Flier Program	Frequent Flier Program Flown Miles. The Company records unrecognized revenue for mileage credits earned by passengers under its FREE SPIRIT program, including mileage credits for members with an insufficient number of mileage credits to earn an award, based on the estimated incremental cost of providing free travel for credits that are expected to be redeemed. Incremental costs include fuel, insurance, security, ticketing and facility charges reduced by an estimate of fees required to be paid by the passenger when redeeming the award. Affinity Card Program. During 2015, the Company extended its agreement with the administrator of the FREE SPIRIT affinity credit card program, which was scheduled to expire in April 2016. The renegotiated program was extended through 2022. As part of the new agreement, the Company received a $10.7 million signing bonus that is being deferred over the contract term and has been reflected in the table below as consideration received from credit card mileage programs in 2015. This extension is considered a material change and thus at the inception of this new arrangement, the Company evaluated all deliverables in the arrangement to determine whether they represent separate units of accounting using the criteria as set forth in ASU No. 2009-13. Under the Company's affinity card program, funds received for the marketing of a co-branded Spirit credit card and delivery of award miles are accounted for as a multiple-deliverable arrangement. At the inception of the arrangement, the Company evaluated all deliverables in the arrangement to determine whether they represent separate units of accounting. The Company determined the arrangement had three separate units of accounting: (i) travel miles to be awarded, (ii) licensing of brand and access to member lists and (iii) advertising and marketing efforts. Arrangement consideration was allocated based on relative selling price. The Company established the estimated selling price for all deliverables that qualified for separation. The manner in which the selling price was established was based on a hierarchy of evidence the Company considered. Total arrangement consideration was then allocated to each deliverable on the basis of the deliverable’s relative selling price. In considering the hierarchy of evidence, the Company first determined whether vendor specific objective evidence of selling price or third-party evidence of selling price existed. It was determined by the Company that neither vendor specific objective evidence of selling price nor third-party evidence existed due to the uniqueness of the Company’s program. As such, the Company developed its best estimate of the selling price for all deliverables. For the award miles, the Company considered a number of entity-specific factors when developing the best estimate of the selling price, including the number of miles needed to redeem an award, average fare of comparable segments, breakage, restrictions and other charges. For licensing of brand and access to member lists, the Company considered both market-specific factors and entity-specific factors, including general profit margins realized in the marketplace/industry, brand power, market royalty rates and size of customer base. For the advertising element, the Company considered market-specific factors and entity-specific factors, including the Company’s internal costs (and fluctuations of costs) of providing services, volume of marketing efforts and overall advertising plan. Consideration allocated based on the relative selling price to both brand licensing and advertising elements is recognized as revenue when earned and recorded in non-ticket revenue. Consideration allocated to award miles is deferred and recognized ratably as passenger revenue over the estimated period the transportation is expected to be provided which is estimated at 17 months
Affinity Card Program, Multiple-deliverable arrangement	Affinity Card Program. During 2015, the Company extended its agreement with the administrator of the FREE SPIRIT affinity credit card program, which was scheduled to expire in April 2016. The renegotiated program was extended through 2022. As part of the new agreement, the Company received a $10.7 million signing bonus that is being deferred over the contract term and has been reflected in the table below as consideration received from credit card mileage programs in 2015. This extension is considered a material change and thus at the inception of this new arrangement, the Company evaluated all deliverables in the arrangement to determine whether they represent separate units of accounting using the criteria as set forth in ASU No. 2009-13. Under the Company's affinity card program, funds received for the marketing of a co-branded Spirit credit card and delivery of award miles are accounted for as a multiple-deliverable arrangement. At the inception of the arrangement, the Company evaluated all deliverables in the arrangement to determine whether they represent separate units of accounting. The Company determined the arrangement had three separate units of accounting: (i) travel miles to be awarded, (ii) licensing of brand and access to member lists and (iii) advertising and marketing efforts. Arrangement consideration was allocated based on relative selling price. The Company established the estimated selling price for all deliverables that qualified for separation. The manner in which the selling price was established was based on a hierarchy of evidence the Company considered. Total arrangement consideration was then allocated to each deliverable on the basis of the deliverable’s relative selling price. In considering the hierarchy of evidence, the Company first determined whether vendor specific objective evidence of selling price or third-party evidence of selling price existed. It was determined by the Company that neither vendor specific objective evidence of selling price nor third-party evidence existed due to the uniqueness of the Company’s program. As such, the Company developed its best estimate of the selling price for all deliverables. For the award miles, the Company considered a number of entity-specific factors when developing the best estimate of the selling price, including the number of miles needed to redeem an award, average fare of comparable segments, breakage, restrictions and other charges. For licensing of brand and access to member lists, the Company considered both market-specific factors and entity-specific factors, including general profit margins realized in the marketplace/industry, brand power, market royalty rates and size of customer base. For the advertising element, the Company considered market-specific factors and entity-specific factors, including the Company’s internal costs (and fluctuations of costs) of providing services, volume of marketing efforts and overall advertising plan. Consideration allocated based on the relative selling price to both brand licensing and advertising elements is recognized as revenue when earned and recorded in non-ticket revenue. Consideration allocated to award miles is deferred and recognized ratably as passenger revenue over the estimated period the transportation is expected to be provided which is estimated at 17 months
Non-ticket Revenue Recognition	Non-ticket Revenue RecognitionNon-ticket revenues are generated from air travel-related services for baggage, bookings through the Company’s call center or third-party vendors, advance seat selection, itinerary changes and loyalty programs. Non-ticket revenues also consist of services not directly related to providing transportation such as the FREE SPIRIT affinity credit card program, $9 Fare Club and the sale of advertising to third parties on Spirit’s website and on board aircraft. Charges for services recognized at time of departure are initially recorded as a liability, within air traffic liability, until time of departure. The passenger usage fee is charged for tickets sold through the Company’s primary sales distribution channels. The primary sales distribution channels for which passenger usage fees are charged include sales through the Company’s website, sales through the third-party provided call center and sales through travel agents; the Company does not charge a passenger usage fee for sales made at its airport ticket counters. Other non-ticket revenues include revenues from other air related charges as well as non-air related charges. Other air related charges include optional services and products provided to passengers such as use of the Company’s call center or travel agents, commissions on travel insurance and commissions on sale of on-board products, among others. Non-air related charges primarily consist of revenues from advertising on the Company’s aircraft and website, the Company’s $9 Fare Club subscription-based membership program and the Company’s FREE SPIRIT affinity credit card program.
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 MaintenanceThe 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 Fuel	Aircraft FuelAircraft 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.
Derivative Instruments	Derivative Instruments The Company accounts for derivative financial instruments at fair value and recognizes them in the balance sheet in prepaid expenses and other current assets or other current liabilities. For derivatives designated as cash flow hedges, changes in fair value of the derivative are generally reported in other comprehensive income and are subsequently reclassified into earnings when the hedged item affects earnings. During the third quarter of 2015, the Company settled six forward interest rate swaps having a total notional amount of $120 million . These interest rate swaps fixed the benchmark interest rate component of interest payments on the debt related to three Airbus A321 aircraft, which the Company took delivery of during the third quarter of 2015. These instruments limited the Company's exposure to changes in the benchmark interest rate in the period from the trade date through the date of maturity. The interest rate swaps were designated as cash flow hedges. The Company accounts for interest rate swaps at fair value and recognizes them in the balance sheet in prepaid expenses and other current assets or other current liabilities with changes in fair value recorded within accumulated other comprehensive income (AOCI). Realized gains and losses from cash flow hedges are recorded in the statement of cash flows as a component of cash flows from operating activities. Subsequent to the issuance of each debt instrument, amounts remaining in AOCI are amortized over the life of the fixed-rate debt instrument. For the years ended 2017 and 2016 , the Company did not hold any fuel derivative instruments. For the year ended 2015
Advertising	AdvertisingThe Company expenses advertising and the production costs of advertising as incurred.
Income Taxes	Income TaxesThe Company accounts for income taxes using the liability method. The Company records a valuation allowance to reduce the deferred tax assets reported if, based on the weight of the evidence, it is more likely than not that some portion or all of the deferred tax assets will be not realized.
Stock-Based Compensation	Stock-Based CompensationThe Company recognizes cost of employee services received in exchange for awards of equity instruments based on the fair value of each instrument at the date of grant. 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. 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 is estimated through the use of a Monte Carlo simulation model.
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 27% , 24% and 28% of total operating expenses in 2017 , 2016 and 2015 , respectively. The Company’s operations are largely concentrated in the southeast United States with Fort Lauderdale being the highest volume fueling point in the system. Gulf Coast Jet indexed fuel is the basis for a substantial majority of the Company’s fuel consumption. Any disruption to the oil production or refinery capacity in the Gulf Coast, as a result of weather or any other disaster, or disruptions in supply of jet fuel, dramatic escalations in the costs of jet fuel and/or the failure of fuel providers to perform under fuel arrangements for other reasons could have a material adverse effect on the Company’s financial condition and results of operations. The Company’s operations will continue to be vulnerable to weather conditions (including hurricane season or snow and severe winter weather), which could disrupt service or create air traffic control problems. These events may result in decreased revenue and/or increased costs.
Recent Accounting Developments	Recent Accounting Developments Revenue from Contracts with Customers In May 2014, the Financial Accounting Standards Board (the FASB) issued Accounting Standards Update (ASU) No. 2014-09, (ASU 2014-09), "Revenue from Contracts with Customers." The objective of ASU 2014-09 is to establish a single comprehensive model for entities to use in accounting for revenue arising from contracts with customers. The new guidance is effective for the Company in the first quarter of 2018. Entities have the option to use either a full retrospective or a modified retrospective approach to adopt ASU 2014-09. The Company currently anticipates utilizing the full retrospective method of adoption allowed by the standard, in order to provide for comparative results in all periods presented, and will adopt the standard as of January 1, 2018. While the Company is still evaluating the impact, it currently believes the most significant impact of this ASU will be the elimination of the incremental cost method for frequent flier program accounting, which will require the Company to re-value and record a liability associated with customer flight miles earned as part of the Company’s frequent flier program with a relative fair value approach. While our evaluation is ongoing, the Company currently estimates that applying a relative fair value would increase its air traffic liability by approximately $10 million at the date of adoption. The Company also expects the classification and timing of recognition of certain ancillary fees to be impacted by the adoption of ASU 2014-09. While the Company believes the adoption will not have a significant impact on earnings, the classification of certain revenues, such as bags, seats and other travel-related fees may be deemed part of the single performance obligation of providing passenger transportation. The Company expects that these revenues currently classified as non-ticket revenue, approximately $1 billion annually, will be reclassified to passenger revenue after adoption. Financial Instruments In January 2016, the FASB issued ASU 2016-01, “Financial Instruments – Overall (Subtopic 825-10).” ASU 2016-01 makes several modifications to Subtopic 825-10, including the elimination of the available-for-sale classification of equity investments, and requires equity investments with readily determinable fair values to be measured at fair value with changes in fair value recognized in net income. ASU 2016-01 is effective for interim and annual periods beginning January 1, 2018 and is not expected to have a material impact on the Company’s financial statements. Leases In February 2016, the FASB issued ASU No. 2016-02, "Leases (Topic 842)." This standard will require all leases with durations greater than twelve months to be recognized on the balance sheet and is effective for the Company in the first quarter of 2019, with early adoption permitted. The Company is currently evaluating the new guidance and believes adoption of this standard will have a significant impact on its balance sheets although adoption is not expected to significantly change the recognition, measurement or presentation of lease expenses within the statements of operations and cash flows. Refer to Note 16, Commitments and Contingencies for information regarding the Company's undiscounted future lease payments and the timing of those payments. Share-Based Compensation In March 2016, the FASB issued ASU No. 2016-09, "Improvements to Employee Share-Based Payment Accounting," which simplifies several aspects of the accounting for employee share-based payment transactions, including the accounting for income taxes, forfeitures, and statutory tax withholding requirements, as well as classification on the statement of cash flows. The Company adopted this guidance on January 1, 2017. As a result, 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, 2017 , $0.5 million of income tax deficiency related to share-based compensation was included within income tax expense on the Company's statements of operations. Additionally, excess income tax benefits and deficiencies for share-based payments are now included in net operating cash flows rather than net financing cash flows. The changes have been applied prospectively in accordance with the guidance and prior periods have not been adjusted. Accounting for Credit Losses In June 2016, the FASB issued ASU No. 2016-13, "Financial Instruments – Credit Losses." The standard requires the use of an "expected loss" model on certain types of financial instruments. The standard also amends the impairment model for available-for-sale debt securities and requires estimated credit losses to be recorded as allowances instead of reductions to amortized cost of the securities. This standard is effective for fiscal years, and interim periods within those years, beginning January 1, 2020, with early adoption permitted. The Company is evaluating the new guidance, but does not expect it to have a material impact on its financial statements. Statement of Cash Flows
Rent Expense, Deferred Gains (Losses)	Deferred losses are recognized as an increase to rent expense on a straight-line basis over the term of the respective operating leases. Deferred gains are included in deferred gains and other long-term liabilities on the accompanying balance sheet. Deferred gains are recognized as a decrease to rent expense on a straight-line basis over the term of the respective operating leases.

Summary of Significant Accoun29

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Depreciable lives used for the principal depreciable asset classifications	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 Capital leases Lease term Leasehold improvements Lesser of lease term or estimated useful life of the improvement Buildings Lesser of lease term or 30 years Year Ended December 31, 2017 2016 2015 (in thousands) Depreciation $ 86,297 $ 57,325 $ 30,797 Amortization of heavy maintenance 53,855 43,811 43,111 Total depreciation and amortization $ 140,152 $ 101,136 $ 73,908
Total cash proceeds received from the sale of mileage credit	The following table illustrates total cash proceeds received from the sale of mileage credits and the portion of such proceeds recognized in non-ticket revenue immediately as marketing component: Consideration received from credit card mile programs Portion of proceeds recognized immediately as marketing component Year Ended (in thousands) December 31, 2017 $ 49,453 $ 37,960 December 31, 2016 48,882 36,640 December 31, 2015 58,005 35,938
Primary components of non-ticket revenue and the revenue recognition method utilized for each service or product	The following table summarizes the primary components of non-ticket revenue and the revenue recognition method utilized for each service or product: Year Ended December 31, Non-ticket revenue Recognition method 2017 2016 2015 (in thousands) Baggage Time of departure $ 488,434 $ 434,269 $ 381,386 Passenger usage fee Time of departure 411,742 358,920 298,092 Advance seat selection Time of departure 131,821 110,966 97,786 Other 249,635 217,180 194,861 Non-ticket revenue $ 1,281,632 $ 1,121,335 $ 972,125
Aircraft maintenance expense	The table below summarizes the extent to which the Company’s maintenance costs are rate capped due to flight hour maintenance contracts: Year Ended December 31, 2017 2016 2015 (in thousands) Flight hour-based maintenance expense $ 54,802 $ 48,471 $ 41,818 Non-flight hour-based maintenance expense 55,637 50,116 38,630 Total maintenance, materials and repairs $ 110,439 $ 98,587 $ 80,448

Accrued Liabilities (Tables)

Accrued Liabilities (Tables)	12 Months Ended
	Dec. 31, 2017
Payables and Accruals [Abstract]
Accrued liabilities included in other current liabilities	Accrued liabilities included in other current liabilities as of December 31, 2017 and 2016 consist of the following: As of December 31, 2017 2016 (in thousands) Salaries and wages $ 54,338 $ 54,578 Airport obligations 56,299 43,989 Federal excise and other passenger taxes and fees payable 42,036 42,064 Aircraft maintenance 33,033 30,233 Fuel 25,171 14,828 Aircraft and facility lease obligations 16,992 10,378 Interest payable 11,384 8,499 Other 23,117 21,442 Other current liabilities $ 262,370 $ 226,011

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [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, 2017 and changes during the year ended December 31, 2017 is presented below: Number of Shares Weighted-Average Outstanding at December 31, 2016 274,645 49.55 Granted 103,030 51.68 Vested (88,580 ) 47.64 Forfeited (22,573 ) 55.79 Outstanding at December 31, 2017 266,522 50.48
Summary of share option activity	A summary of share option activity as of December 31, 2017 and changes during the year ended December 31, 2017 is presented below: Number Weighted- Average Aggregate Outstanding at December 31, 2016 16,250 8.37 3.7 804 Exercised (5,750 ) 7.80 Forfeited or expired — — Outstanding at December 31, 2017 10,500 8.68 2.7 380 Exercisable at December 31, 2017 10,500 8.68 2.7 380 Vested at December 31, 2017 10,500 8.68 2.7 380
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, 2017 Weighted-Average at Grant Date for Twelve Months Ended December 31, 2016 Expected volatility factor 0.40 0.39 Risk free interest rate 1.47 % 1.12 % Expected term (in years) 2.93 2.94 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, 2017 : Number of Awards Weighted-Average Fair Value at Grant Date ($) Outstanding at December 31, 2016 133,907 50.54 Granted 89,338 48.86 Vested (17,592 ) 50.07 Forfeited (85,294 ) 45.64 Outstanding at December 31, 2017 120,359 52.84
Performance Condition Performance Share Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance share award activity	The following table summarizes the Company’s performance condition performance share awards for the year ended December 31, 2017 : Number of Awards Weighted-Average Fair Value at Grant Date ($) Outstanding at December 31, 2016 — — Granted 37,649 46.28 Vested — — Forfeited (2,142 ) 55.50 Outstanding at December 31, 2017 35,507 45.72

Net Income per Share (Tables)

Net Income per Share (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 2016 2015 (in thousands, except per share amounts) Numerator: Net income (1) $ 420,606 $ 264,879 $ 317,220 Denominator: Weighted-average shares outstanding, basic 69,221 70,344 72,208 Effect of dilutive stock awards 156 164 218 Adjusted weighted-average shares outstanding, diluted 69,377 70,508 72,426 Net Income per Share: Basic earnings per common share (1) $ 6.08 $ 3.77 $ 4.39 Diluted earnings per common share (1) $ 6.06 $ 3.76 $ 4.38 Anti-dilutive weighted-average shares 85 66 52

Debt and Other Obligations (Tab

Debt and Other Obligations (Tables)	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Schedule of long-term debt instruments	Long-term debt is comprised of the following: As of Twelve Months Ended December 31, 2017 2016 2017 2016 (in millions) (weighted-average interest rates) Fixed-rate senior term loans due through 2027 $ 417.9 $ 451.9 4.10 % 4.10 % Fixed-rate junior term loans due through 2022 39.3 47.1 6.90 % 6.90 % Fixed-rate loans due through 2029 518.0 — 3.83 % N/A Fixed-rate class A 2015-1 EETC due through 2028 408.6 409.8 4.10 % 4.10 % Fixed-rate class B 2015-1 EETC due through 2024 92.0 103.6 4.45 % 4.45 % Fixed-rate class AA 2017-1 EETC due through 2030 37.5 — 3.38 % N/A Fixed-rate class A 2017-1 EETC due through 2030 12.5 — 3.65 % N/A Fixed-rate class B 2017-1 EETC due through 2026 13.8 — 3.80 % N/A Long-term debt $ 1,539.6 $ 1,012.4 Less current maturities 115.4 84.4 Less unamortized discount, net 36.7 30.6 Total $ 1,387.5 $ 897.4
Schedule of maturities of long-term debt	At December 31, 2017 , long-term debt principal payments for the next five years and thereafter are as follows: December 31, 2017 (in millions) 2018 $ 121.0 2019 122.3 2020 121.8 2021 121.5 2022 118.8 2023 and beyond 934.2 Total debt principal payments $ 1,539.6
Schedule of interest expense on long-term debt	Interest expense related to long-term debt consists of the following: Twelve Months Ended December 31, 2017 2016 (in thousands) Senior term loans $ 18,328 $ 19,759 Junior term loans 3,035 3,568 Fixed-rate loans 8,610 — Class A 2015-1 EETC 17,230 11,509 Class B 2015-1 EETC 4,446 3,243 Class AA 2017-1 EETC 54 — Class A 2017-1 EETC 19 — Class B 2017-1 EETC 22 — Commitment fees 124 127 Amortization of deferred financing costs 5,280 3,435 Total $ 57,148 $ 41,641

Leases and Prepaid Maintenanc34

Leases and Prepaid Maintenance Deposits (Tables)	12 Months Ended
	Dec. 31, 2017
Leases [Abstract]
Schedule of future minimum rental payments for noncancellable operating leases	Future minimum lease payments under capital leases and noncancellable operating leases with initial or remaining terms in excess of one year at December 31, 2017 were as follows: Capital Leases Operating Leases Total Operating and Capital Lease Obligations Aircraft and Spare Engine Leases Property Facility Leases (in thousands) 2018 $ 537 $ 204,292 $ 45,737 $ 250,566 2019 504 189,106 36,852 226,462 2020 188 180,842 25,870 206,900 2021 28 170,643 12,740 183,411 2022 — 150,212 11,931 162,143 2023 and thereafter — 419,908 61,211 481,119 Total minimum lease payments $ 1,257 $ 1,315,003 $ 194,341 $ 1,510,601 Less amount representing interest 94 Present value of minimum lease payments $ 1,163 Less current portion 476 Long-term portion $ 687

Financial Instruments and Ris35

Financial Instruments and Risk Management (Tables)	12 Months Ended
	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Components of aircraft fuel expense	The following table summarizes the components of aircraft fuel expense for the years ended December 31, 2017 , 2016 and 2015 : Year Ended December 31, 2017 2016 2015 (in thousands) Into-plane fuel cost $ 615,581 $ 447,553 $ 454,747 Realized losses (gains) related to fuel derivative contracts, net — — 10,580 Unrealized losses (gains) related to fuel derivative contracts, net — — (3,880 ) Aircraft fuel $ 615,581 $ 447,553 $ 461,447

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 2016 2015 (in thousands) Current: Federal $ (68,601 ) $ 60,079 $ 21,632 State and local 515 6,322 6,702 Foreign 2,742 2,034 1,235 Total current expense (benefit) (65,344 ) 68,435 29,569 Deferred: Federal (10,370 ) 82,455 149,583 State and local 8,760 3,691 6,031 Total deferred expense (benefit) (1,610 ) 86,146 155,614 Total income tax expense (benefit) $ (66,954 ) $ 154,581 $ 185,183
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, 2017 2016 2015 Expected provision at federal statutory tax rate 35.0 % 35.0 % 35.0 % State tax expense, net of federal benefit 1.7 % 1.6 % 1.7 % Income tax credits — % — % — % Revaluation of deferred taxes (56.3 )% — % — % Other 0.7 % 0.3 % 0.2 % Total income tax expense (benefit) (18.9 )% 36.9 % 36.9 %
Deferred taxes	At December 31, 2017 and 2016 , the significant components of the Company's deferred taxes consisted of the following: December 31, 2017 2016 (in thousands) Deferred tax assets: Foreign tax credits $ 5,980 $ — Deferred revenue 1,631 3,057 Nondeductible accruals 10,107 20,547 Deferred manufacturing credits 258 910 Accrued maintenance 1,991 1,854 Equity compensation 2,392 3,882 Other 5,555 4,026 Valuation allowance (454 ) — Deferred tax assets 27,460 34,276 Deferred tax liabilities: Deferred gain (loss) on leases, net 1,605 2,435 Accrued rent 12,055 14,025 Prepaid expenses 754 1,217 Property, plant and equipment 298,703 278,872 Deferred financing costs 225 5,358 Accrued aircraft and engine maintenance 27,258 40,512 Deferred tax liabilities 340,600 342,419 Net deferred tax assets (liabilities) $ (313,140 ) $ (308,143 )

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Schedule of Future Operating Aircraft Leases	As of December 31, 2017 , the Company's firm aircraft orders consisted of the following: Airbus A320ceo A320neo A321ceo Total 2018 5 5 10 2019 1 14 15 2020 16 16 2021 18 18 6 48 5 59
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, 2017 . Employee Groups Representative Amendable Date Percentage of Workforce Pilots Air Line Pilots Association, International (ALPA) August 2015 26% Flight Attendants Association of Flight Attendants (AFA-CWA) May 2021 44% Dispatchers Transport Workers Union (TWU) August 2018 1% Ramp Service Agents International Association of Machinists and Aerospace Workers (IAMAW) June 2020 4%

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 and December 31, 2016 , were as follows: As of December 31, 2017 2016 Carrying Value Estimated Fair Value Carrying Value Estimated Fair Value Fair value level hierarchy (in millions) Senior term loans $ 417.9 $ 435.3 $ 451.9 463.9 Level 3 Junior term loans 39.3 40.4 47.1 48.1 Level 3 Fixed-rate loans 518.0 528.6 — N/A Level 3 2015-1 EETC Class A 408.6 420.9 409.8 416.0 Level 2 2015-1 EETC Class B 92.0 94.2 103.6 105.7 Level 2 2017-1 EETC Class AA 37.5 37.4 — N/A Level 2 2017-1 EETC Class A 12.5 12.6 — N/A Level 2 2017-1 EETC Class B 13.8 13.8 — N/A Level 2 Total long-term debt $ 1,539.6 $ 1,583.2 $ 1,012.4 $ 1,033.7
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, 2017 Total Level Level Level (in millions) Cash and cash equivalents $ 800.8 $ 800.8 $ — $ — Short-term investment securities 100.9 100.9 — — Total assets $ 901.7 $ 901.7 $ — $ — Total liabilities $ — $ — $ — $ — Fair Value Measurements as of December 31, 2016 Total Level Level Level (in millions) Cash and cash equivalents $ 700.9 $ 700.9 $ — $ — Short-term investment securities 100.2 100.2 — — Total assets $ 801.1 $ 801.1 $ — $ — Total liabilities $ — $ — $ — $ —

Operating Segments and Relate39

Operating Segments and Related Disclosures (Tables)	12 Months Ended
	Dec. 31, 2017
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: 2017 2016 2015 (in millions) DOT—Domestic $ 2,435.9 $ 2,136.2 $ 1,940.2 DOT—Latin America 211.8 185.8 201.3 Total $ 2,647.7 $ 2,322.0 $ 2,141.5

Quarterly Financial Data (Una40

Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Quarterly results of operations	Quarterly results of operations for the years ended December 31, 2017 and 2016 are summarized below: Three Months Ended March 31 June 30 September 30 December 31 (in thousands, except per share amounts) 2017 Operating revenue $ 591,746 $ 701,682 $ 687,231 $ 667,007 Operating income 59,405 132,736 104,104 92,546 Net income (1) 31,935 78,143 60,190 250,338 Basic earnings per share (1) 0.46 1.13 0.87 3.64 Diluted earnings per share (1) 0.46 1.12 0.87 3.63 2016 Operating revenue $ 538,143 $ 584,133 $ 621,329 $ 578,351 Operating income 101,299 121,835 135,216 85,311 Net income 61,920 73,084 81,382 48,493 Basic earnings per share 0.87 1.03 1.17 0.70 Diluted earnings per share 0.86 1.03 1.17 0.70

Summary of Significant Accoun41

Summary of Significant Accounting Policies - Reportable Segments (Details)	12 Months Ended
	Dec. 31, 2017segment
Accounting Policies [Abstract]
Number of reportable segments	1

Summary of Significant Accoun42

Summary of Significant Accounting Policies - Property and Equipment (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)aircraft_engineaircraft	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Property, Plant and Equipment [Line Items]
Number of Spare Engines Capitalized | aircraft	4
Number of Flight Simulators Capitalized | aircraft	1
Depreciation, Depletion and Amortization, Nonproduction [Abstract]
Depreciation	$ 86,297	$ 57,325	$ 30,797
Amortization of heavy maintenance	53,855	43,811	43,111
Total depreciation and amortization	140,152	101,136	73,908
Amortization of capitalized software costs	$ 3,100	3,200	3,100
Spare engines
Property, Plant and Equipment [Line Items]
Number of aircraft under operating lease | aircraft_engine	11
Aircrafts, Major Spare Rotable Parts, Avionics, and Assemblies [Member]
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	$ 7,700	9,400
Capitalized software costs during the year	$ 1,400	$ 4,100	$ 4,300
Internal use software | Minimum
Property, Plant and Equipment [Line Items]
Depreciable lives used for the principal depreciable asset classifications	3 years
Internal use software | Maximum
Property, Plant and Equipment [Line Items]
Depreciable lives used for the principal depreciable asset classifications	10 years
Spare engines | Minimum
Property, Plant and Equipment [Line Items]
Operating leases, term	2 years
Spare engines | Maximum
Property, Plant and Equipment [Line Items]
Operating leases, term	13 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

Summary of Significant Accoun43

Summary of Significant Accounting Policies - Frequent Flier Program (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)unit_of_accounting	Dec. 31, 2015USD ($)	Dec. 31, 2016USD ($)
Accounting Policies [Abstract]
Credit shell, term of expiration	60 days
Signing bonus		$ 10.7
Number of units of accounting | unit_of_accounting	3
Affinity Card Program, deferred fair value of the transportation component, recognition period	17 months
Liability for future FREE SPIRIT award redemptions and unrecognized revenue from the sale of mileage credits	$ 13.8		$ 15.3

Summary of Significant Accoun44

Summary of Significant Accounting Policies - Cash Proceeds Received from the Sale of Mileage Credits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Consideration received from credit card mile programs	$ 49,453	$ 48,882	$ 58,005
Portion of proceeds recognized immediately as marketing component	$ 37,960	$ 36,640	$ 35,938

Summary of Significant Accoun45

Summary of Significant Accounting Policies - Primary Components of Non-Ticket Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Product Information [Line Items]
Non-ticket revenue	$ 1,281,632	$ 1,121,335	$ 972,125
Baggage
Product Information [Line Items]
Non-ticket revenue	488,434	434,269	381,386
Passenger usage fee
Product Information [Line Items]
Non-ticket revenue	411,742	358,920	298,092
Advance seat selection
Product Information [Line Items]
Non-ticket revenue	131,821	110,966	97,786
Other
Product Information [Line Items]
Non-ticket revenue	$ 249,635	$ 217,180	$ 194,861

Summary of Significant Accoun46

Summary of Significant Accounting Policies - Maintenance Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Amortization of heavy maintenance	$ 53,855	$ 43,811	$ 43,111
Deferred costs for heavy maintenance	78,200	35,400	9,100
Deferred heavy maintenance, gross	260,500	238,300
Accumulated heavy maintenance amortization	207,808	122,509
Airframe and Engine Maintenance Costs [Abstract]
Flight hour-based maintenance expense	54,802	48,471	41,818
Non-flight hour-based maintenance expense	55,637	50,116	38,630
Total maintenance, materials and repairs	110,439	98,587	$ 80,448
Heavy Maintenance
Property, Plant and Equipment [Line Items]
Accumulated heavy maintenance amortization	$ 160,700	$ 162,800

Summary of Significant Accoun47

Summary of Significant Accounting Policies - Derivative Instruments (Details) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2015USD ($)aircraftderivative_instrument	Dec. 31, 2015aircraftderivative_instrument
Airbus A321
Derivative [Line Items]
Number of aircraft protected by interest rate derivatives scheduled for delivery		3
2015 | Airbus A321
Derivative [Line Items]
Number of aircraft protected by interest rate derivatives scheduled for delivery	3
Interest rate swap
Derivative [Line Items]
Number of interest rate derivatives settled | derivative_instrument	6	6
Notional amount | $	$ 120

Summary of Significant Accoun48

Summary of Significant Accounting Policies - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)employee_group	Dec. 31, 2016USD ($)employee_group	Dec. 31, 2015USD ($)
Concentration Risk [Line Items]
Marketing and advertising expenses	$ 5,100	$ 3,200	$ 3,500
Valuation allowance	$ 454	$ 0
Aircraft fuel expenditure concentration risk | Total operating expenses
Concentration Risk [Line Items]
Concentration of risk	27.00%	24.00%	28.00%
Unionized employees concentration risk | Number of employees, total
Concentration Risk [Line Items]
Concentration of risk	75.00%	73.00%
Union-represented employee groups | employee_group	4	4

Recent Accounting Developments

Recent Accounting Developments Recent Accounting Developments (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)
Adjustments for New Accounting Pronouncements [Line Items]
Income tax deficiency	$ 0.5
ASU 2014-09
Adjustments for New Accounting Pronouncements [Line Items]
Expected increase in customer flight miles liability	10
Expected reclassification of passenger revenue	$ 1,000

Special Charges (Details)

Special Charges (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)aircraft	Dec. 31, 2016USD ($)aircraft	Dec. 31, 2015USD ($)
Property, Plant and Equipment [Line Items]
Purchase of property and equipment	$ 628,881	$ 541,122	$ 548,800
Special charges	$ 12,629	$ 37,189	673
Restructuring Costs			$ 700
Aircraft
Property, Plant and Equipment [Line Items]
Number of previously leased aircraft purchased | 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]
Number of previously leased aircraft purchased | aircraft		7
Purchase price	$ 20,000	$ 147,700
Purchase of property and equipment		107,100
Value of noncash consideration		40,600
Fair value of assets acquired		95,700
Special charges		37,200
Airbus A319 | Aircraft | Other Non-Cash Items
Property, Plant and Equipment [Line Items]
Value of noncash consideration		$ 14,800
Spare engines
Property, Plant and Equipment [Line Items]
Number of previously leased aircraft purchased | aircraft	1
Spare engines | Spare engines
Property, Plant and Equipment [Line Items]
Number of previously leased aircraft purchased | aircraft	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

Letters of Credit (Details)

Letters of Credit (Details) - Unsecured, standby letter of credit facility - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Line of Credit Facility [Line Items]
Letters of credit, limit, amount	$ 35	$ 25.2
Letter of credit facility, amount outstanding	$ 17.5	$ 19.7

Credit Card Processing Arrang52

Credit Card Processing Arrangements (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Credit Card Processing Arrangements [Abstract]
Cash holdbacks	$ 0	$ 0
Maximum potential exposure to cash holdbacks by the credit card processors	$ 286,300,000	$ 234,600,000

Short-term Investment Securit53

Short-term Investment Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Short-term investment securities	$ 100,937	$ 100,155
Unrealized loss on investment securities, net of deferred taxes	82	23	$ 0
Deferred taxes on unrealized loss on investment securities	41	13	$ 0
Accumulated other comprehensive loss	$ (1,464)	$ (1,345)
Available-for-sale Securities
Schedule of Available-for-sale Securities [Line Items]
Weighted-average fixed rate	1.40%	1.10%
Accumulated other comprehensive loss	$ (105)	$ (23)

Accrued Liabilities (Details)

Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables and Accruals [Abstract]
Salaries and wages	$ 54,338	$ 54,578
Airport obligations	56,299	43,989
Federal excise and other passenger taxes and fees payable	42,036	42,064
Aircraft maintenance	33,033	30,233
Fuel	25,171	14,828
Aircraft and facility lease obligations	16,992	10,378
Interest payable	11,384	8,499
Other	23,117	21,442
Other current liabilities	$ 262,370	$ 226,011

Common Stock and Preferred St55

Common Stock and Preferred Stock (Details) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Common stock, shares authorized	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	68,196,964	69,326,202
Preferred stock, shares outstanding	0	0
Voting Common Stock
Class of Stock [Line Items]
Common stock, shares authorized	240,000,000
Common stock, par value (in dollars per share)	$ 0.0001
Non-Voting Common Stock
Class of Stock [Line Items]
Common stock, shares authorized	50,000,000
Common stock, par value (in dollars per share)	$ 0.0001
Common stock, shares outstanding	0	0

Stock-Based Compensation - Narr

Stock-Based Compensation - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation cost	$ 8,500	$ 7,100	$ 9,200
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense	$ (1,600)	$ 2,600	3,400
Shares repurchased (in shares)	1,200,000	2,400,000
Repurchase of common stock	$ 46,580	$ 102,510	$ 112,261
Number of treasury shares retired (in shares)	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	$ 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)	2,377,650	2,358,283

Stock-Based Compensation - Rest

Stock-Based Compensation - Restricted Stock Activity (Details) - Restricted Stock [Member] - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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)	274,645
Granted (in shares)	103,030	237,001
Vested (in shares)	(88,580)
Forfeited (in shares)	(22,573)
Outstanding (in shares)	266,522	274,645
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)	$ 49.55
Granted, weighted-average grant date fair value (in dollars per share)	51.68	$ 42.91	$ 75.40
Vested, weighted-average grant date fair value (in dollars per share)	47.64
Forfeited, weighted-average grant date fair value (in dollars per share)	55.79
Outstanding, weighted-average grant date fair value (in dollars per share)	$ 50.48	$ 49.55
Total compensation cost not yet recognized	$ 7.5	$ 9.2
Total compensation cost not yet recognized, period for recognition	2 years	2 years 6 months
Total fair value of shares vested	$ 4.3	$ 6.6	$ 9.2
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	3 years
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	4 years

Stock-Based Compensation - Shar

Stock-Based Compensation - Share Option Activity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding (in shares)	16,250
Exercised (in shares)	(5,750)
Forfeited or expired (in shares)	0
Outstanding (in shares)	10,500	16,250
Exercisable (in shares)	10,500
Vested (in shares)	10,500
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Roll Forward]
Outstanding, weighted-average exercise price (in dollars per share)	$ 8.37
Exercised, weighted-average exercise price (in dollars per share)	7.80
Forfeited or expired, weighted-average exercise price (in dollars per share)	0
Outstanding, weighted-average exercise price (in dollars per share)	8.68	$ 8.37
Exercisable, weighted-average exercise price (in dollars per share)	8.68
Vested, weighted-average exercise price (in dollars per share)	$ 8.68
Outstanding, average remaining contractual term (in years)	2 years 8 months 12 days	3 years 8 months 12 days
Exercisable, average remaining contractual term (in years)	2 years 8 months 12 days
Vested, average remaining contractual term	2 years 8 months 12 days
Outstanding, aggregate intrinsic value	$ 380,000	$ 804,000
Exercisable, aggregate intrinsic value	380,000
Vested, aggregate intrinsic value	380,000
Total intrinsic value of share options exercised	200,000	400,000	$ 200,000
Options, vested in period, fair value	$ 0	$ 0	$ 4,000
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	4 years
Contractual term	10 years
Grants in period (in shares)	0	0	0
Expected volatility factor				46.25%
Expected dividend yield (percent)				0.00%
Expected term (in years)				6 years 3 months
Risk free interest rate (percent)				2.03%
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Roll Forward]
Total compensation cost not yet recognized	$ 0	$ 0	$ 0

Stock-Based Compensation - Simu

Stock-Based Compensation - Simulation Model Input Variables (Details) - Market Condition Performance Share Awards	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility factor	0.40%	0.39%
Risk free interest rate (percent)	1.47%	1.12%
Expected term (in years)	2 years 11 months 4 days	2 years 11 months 8 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	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Performance Shares Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award requisite performance 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	$ 3.7	$ 4.3
Total compensation cost not yet recognized, period for recognition	1 year 6 months 7 days	1 year 10 months 24 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)	133,907
Granted (in shares)	89,338
Vested (in shares)	(17,592)
Forfeited (in shares)	(85,294)
Outstanding (in shares)	120,359	133,907
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)	$ 50.54
Granted, weighted-average grant date fair value (in dollars per share)	48.86
Vested, weighted-average grant date fair value (in dollars per share)	50.07
Forfeited, weighted-average grant date fair value (in dollars per share)	45.64
Outstanding, weighted-average grant date fair value (in dollars per share)	$ 52.84	$ 50.54
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)	0
Granted (in shares)	37,649
Vested (in shares)	0
Forfeited (in shares)	(2,142)
Outstanding (in shares)	35,507	0
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)	$ 0
Granted, weighted-average grant date fair value (in dollars per share)	46.28
Vested, weighted-average grant date fair value (in dollars per share)	0
Forfeited, weighted-average grant date fair value (in dollars per share)	55.50
Outstanding, weighted-average grant date fair value (in dollars per share)	$ 45.72	$ 0

Net Income per Share (Details)

Net Income per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net income	$ 250,338	$ 60,190	$ 78,143	$ 31,935	$ 48,493	$ 81,382	$ 73,084	$ 61,920	$ 420,606	$ 264,879	$ 317,220
Denominator:
Weighted-average shares outstanding, basic (in shares)									69,221	70,344	72,208
Effect of dilutive stock awards (in shares)									156	164	218
Adjusted weighted-average shares outstanding, diluted (in shares)									69,377	70,508	72,426
Net Income per Share:
Basic earnings per common share (in dollars per share)	$ 3.64	$ 0.87	$ 1.13	$ 0.46	$ 0.70	$ 1.17	$ 1.03	$ 0.87	$ 6.08	$ 3.77	$ 4.39
Diluted earnings per common share (in dollars per share)	$ 3.63	$ 0.87	$ 1.12	$ 0.46	$ 0.70	$ 1.17	$ 1.03	$ 0.86	$ 6.06	$ 3.76	$ 4.38
Antidilutive awards excluded from computation of earnings per common share (in shares)									85	66	52
Non-recurring income tax benefit due to the Tax Cuts and Jobs Act of 2017	$ 199,300
Non-recurring income tax benefit due to the Tax Cuts and Jobs Act of 2017 per basic share (in dollars per share)	$ 2.90								$ 2.88
Non-recurring income tax benefit due to the Tax Cuts and Jobs Act of 2017 per diluted share (in dollars per share)	$ 2.89								$ 2.87

Debt and Other Obligations - Na

Debt and Other Obligations - Narrative (Details) $ in Millions	1 Months Ended	12 Months Ended
	Nov. 30, 2017USD ($)aircrafttrust	Dec. 31, 2017USD ($)aircraft	Dec. 31, 2016USD ($)
Line of Credit Facility [Line Items]
Repayments of principal | $		$ 102.3	$ 64.4
Corporate credit cards
Line of Credit Facility [Line Items]
Maximum borrowing capacity | $		33.6	23.6
Line of credit, outstanding | $		1.7	9.9
Aircraft fuel
Line of Credit Facility [Line Items]
Maximum borrowing capacity | $		51.5	46.5
Line of credit, outstanding | $		$ 24.2	$ 8
Airbus
Line of Credit Facility [Line Items]
Number of delivered aircraft with secured debt financing commitments		46
Airbus | 2018
Line of Credit Facility [Line Items]
Number of aircraft with secured debt financing commitments scheduled for delivery		10
Airbus | A320 | December 2017 To January 2018
Line of Credit Facility [Line Items]
Number of aircraft with secured debt financing commitments scheduled for delivery	3
Airbus | A320 | April 2018 To October 2018
Line of Credit Facility [Line Items]
Number of aircraft with secured debt financing commitments scheduled for delivery	4
Airbus | A320 | Enhanced Equipment Trust Certificate
Line of Credit Facility [Line Items]
Number of aircraft with secured debt financing commitments scheduled for delivery	7
Airbus | A321 | February 2018 To March 2018
Line of Credit Facility [Line Items]
Number of aircraft with secured debt financing commitments scheduled for delivery	5
Airbus | A321 | Enhanced Equipment Trust Certificate
Line of Credit Facility [Line Items]
Number of aircraft with secured debt financing commitments scheduled for delivery	5
Fixed-rate loans
Line of Credit Facility [Line Items]
Long-term debt outstanding | $		$ 527.3
Fixed-rate loans | Minimum
Line of Credit Facility [Line Items]
Debt instrument, term		10 years
Fixed-rate loans | Maximum
Line of Credit Facility [Line Items]
Debt instrument, term		12 years
Enhanced Equipment Trust Certificate
Line of Credit Facility [Line Items]
Long-term debt outstanding | $		$ 63.9
Number of delivered aircraft with secured debt financing commitments		2
Number of trusts | trust	3
Face amount, commitment for future issuance | $	$ 420.5
Enhanced Equipment Trust Certificate | Aircraft and Related Flight Equipment
Line of Credit Facility [Line Items]
Face amount, commitment for future issuance | $		$ 356.6
Enhanced Equipment Trust Certificate | 2017-1 EETC Class AA
Line of Credit Facility [Line Items]
State interest rate percentage		3.375%
Enhanced Equipment Trust Certificate | 2017-1 EETC Class A
Line of Credit Facility [Line Items]
State interest rate percentage		3.65%
Enhanced Equipment Trust Certificate | 2017-1 EETC Class B
Line of Credit Facility [Line Items]
State interest rate percentage		3.80%
Enhanced Equipment Trust Certificate | Airbus | 2018
Line of Credit Facility [Line Items]
Number of aircraft with secured debt financing commitments scheduled for delivery		10
Fixed-rate loans | Airbus | A320
Line of Credit Facility [Line Items]
Number of delivered aircraft with secured debt financing commitments		4
Fixed-rate loans | Airbus | A321
Line of Credit Facility [Line Items]
Number of delivered aircraft with secured debt financing commitments		10

Debt and Other Obligations - Lo

Debt and Other Obligations - Long-term Debt (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Long-term debt, gross	$ 1,539,600	$ 1,012,400
Less current maturities	115,430	84,354
Less unamortized discount, net	36,700	30,600
Total	1,387,498	897,359
Senior loans
Debt Instrument [Line Items]
Long-term debt, gross	$ 417,900	$ 451,900
Weighted-average interest rates	4.10%	4.10%
Junior loans
Debt Instrument [Line Items]
Long-term debt, gross	$ 39,300	$ 47,100
Weighted-average interest rates	6.90%	6.90%
Fixed-rate loans
Debt Instrument [Line Items]
Long-term debt, gross	$ 518,000	$ 0
Weighted-average interest rates	3.83%
2015-1 EETC Class A
Debt Instrument [Line Items]
Long-term debt, gross	$ 408,600	$ 409,800
Weighted-average interest rates	4.10%	4.10%
2015-1 EETC Class B
Debt Instrument [Line Items]
Long-term debt, gross	$ 92,000	$ 103,600
Weighted-average interest rates	4.45%	4.45%
2017-1 EETC Class AA
Debt Instrument [Line Items]
Long-term debt, gross	$ 37,500	$ 0
Weighted-average interest rates	3.38%
2017-1 EETC Class A
Debt Instrument [Line Items]
Long-term debt, gross	$ 12,500	0
Weighted-average interest rates	3.65%
2017-1 EETC Class B
Debt Instrument [Line Items]
Long-term debt, gross	$ 13,800	$ 0
Weighted-average interest rates	3.80%
Minimum | Fixed-rate loans
Debt Instrument [Line Items]
Debt instrument, term	10 years
Maximum | Fixed-rate loans
Debt Instrument [Line Items]
Debt instrument, term	12 years

Debt and Other Obligations - Fu

Debt and Other Obligations - Future Maturities (Details) $ in Millions	Dec. 31, 2017USD ($)
Debt Disclosure [Abstract]
2,018	$ 121
2,019	122.3
2,020	121.8
2,021	121.5
2,022	118.8
2023 and beyond	934.2
Long-term debt	$ 1,539.6

Debt and Other Obligations - In

Debt and Other Obligations - Interest Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Commitment fees	$ 124	$ 127
Amortization of deferred financing costs	5,280	3,435
Total	57,148	41,641
Senior loans
Debt Instrument [Line Items]
Interest expense, debt	18,328	19,759
Junior loans
Debt Instrument [Line Items]
Interest expense, debt	3,035	3,568
Fixed-rate loans
Debt Instrument [Line Items]
Interest expense, debt	8,610	0
2015-1 EETC Class A
Debt Instrument [Line Items]
Interest expense, debt	17,230	11,509
2015-1 EETC Class B
Debt Instrument [Line Items]
Interest expense, debt	4,446	3,243
2017-1 EETC Class AA
Debt Instrument [Line Items]
Interest expense, debt	54	0
2017-1 EETC Class A
Debt Instrument [Line Items]
Interest expense, debt	19	0
2017-1 EETC Class B
Debt Instrument [Line Items]
Interest expense, debt	$ 22	$ 0

Leases and Prepaid Maintenanc66

Leases and Prepaid Maintenance Deposits - Operating Leases (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)aircraft_engineaircraft	Dec. 31, 2016USD ($)aircraft	Dec. 31, 2015USD ($)
Operating Leased Assets [Line Items]
Total rental expense | $	$ 309,800	$ 283,900	$ 282,700
Rental expense charged to operations for aircraft and engine operating leases | $	205,852	201,675	211,531
Maintenance reserves, supplemental rent | $	400	2,200	2,300
Aircraft maintenance deposits | $	$ 326,200	286,400
Number of Spare Engines Capitalized	4
Number of leased engines delivered | aircraft_engine	1
Lease term	3 years
Spare engines
Operating Leased Assets [Line Items]
Number of leased assets financed under operating leases | aircraft_engine	11
A320 Family
Operating Leased Assets [Line Items]
Number of Aircraft Held	112
A320 Family | Aircraft
Operating Leased Assets [Line Items]
Number of leased assets financed under operating leases	58
Fixed Maintenance Reserve Payments
Operating Leased Assets [Line Items]
Committed expenditures, 2018 | $	$ 8,400
Committed expenditures, 2019 | $	5,900
Committed expenditures, 2020 | $	5,600
Committed expenditures, 2021 | $	5,700
Committed expenditures, 2022 | $	4,900
Committed expenditures, 2023 and beyond | $	12,900
Aircraft Supplemental Rent
Operating Leased Assets [Line Items]
Supplemental rent expense | $	$ 8,400	$ 9,000	$ 7,700
Aircraft
Operating Leased Assets [Line Items]
Number of previously leased aircraft purchased	1
Number of aircraft and spare engines financed under operating leases with a variable rate	1
Number of aircrafts delivered under secured debt arrangements	17
Aircraft | A320 Family
Operating Leased Assets [Line Items]
Number of aircraft capitalized	54
Number of previously leased aircraft purchased	8
Aircraft | Airbus A319
Operating Leased Assets [Line Items]
Number of previously leased aircraft purchased		7
Aircraft | Minimum
Operating Leased Assets [Line Items]
Operating leases, term	8 years
Aircraft | Maximum
Operating Leased Assets [Line Items]
Operating leases, term	18 years
Other leased equipment and property | Maximum
Operating Leased Assets [Line Items]
Operating leases, term	30 years
Spare engines
Operating Leased Assets [Line Items]
Number of spare engines purchased | aircraft_engine	2
Spare engines | Minimum
Operating Leased Assets [Line Items]
Operating leases, term	2 years
Spare engines | Maximum
Operating Leased Assets [Line Items]
Operating leases, term	13 years
Third Party Lessor
Operating Leased Assets [Line Items]
Number of direct lease aircraft from a third-party lessor	2
Third Party Lessor | Aircraft
Operating Leased Assets [Line Items]
Number of aircrafts returned	2
Spare engines
Operating Leased Assets [Line Items]
Number of previously leased aircraft purchased	1
Spare engines | Spare engines
Operating Leased Assets [Line Items]
Number of previously leased aircraft purchased	1

Leases and Prepaid Maintenanc67

Leases and Prepaid Maintenance Deposits - Future Minimum Lease Payments Under Noncancelable Operating Leases (Details) $ in Thousands	Dec. 31, 2017USD ($)
Total Operating and Capital Lease Obligations
2,018	$ 250,566
2,019	226,462
2,020	206,900
2,021	183,411
2,022	162,143
2023 and thereafter	481,119
Total minimum lease payments	1,510,601
Office Equipment
Capital Leases
2,018	537
2,019	504
2,020	188
2,021	28
2,022	0
2023 and thereafter	0
Total minimum lease payments	1,257
Less amount representing interest	94
Present value of minimum lease payments	1,163
Less current portion	476
Long-term portion	687
Aircraft and Spare Engine Leases
Operating Leases
2,018	204,292
2,019	189,106
2,020	180,842
2,021	170,643
2,022	150,212
2023 and thereafter	419,908
Total minimum lease payments	1,315,003
Property Facility Leases
Operating Leases
2,018	45,737
2,019	36,852
2,020	25,870
2,021	12,740
2,022	11,931
2023 and thereafter	61,211
Total minimum lease payments	$ 194,341

Financial Instruments and Ris68

Financial Instruments and Risk Management Financial Instruments and Risk Management (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Into-plane fuel cost	$ 615,581	$ 447,553	$ 454,747
Realized losses (gains) related to fuel derivative contracts, net	0	0	10,580
Unrealized losses (gains) related to fuel derivative contracts, net	0	0	(3,880)
Aircraft fuel	$ 615,581	$ 447,553	$ 461,447

Financial Instruments and Ris69

Financial Instruments and Risk Management - Narrative (Details)	3 Months Ended	12 Months Ended
	Sep. 30, 2015derivative_instrument	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)aircraftderivative_instrument
Derivative [Line Items]
Unrealized gain (loss) on interest rate derivative instruments		$ 0	$ 0	$ (910,000)
Interest rate derivative losses reclassified into earnings, net of tax		(37,000)	224,000	82,000
Loss reclassified from AOCI into income, tax		372,000	130,000	$ 50,000
Accumulated other comprehensive loss		1,464,000	1,345,000
Airbus A321
Derivative [Line Items]
Number of aircraft protected by interest rate derivatives delivered | aircraft				3
Interest rate swap
Derivative [Line Items]
Number of interest rate derivatives settled | derivative_instrument	6			6
Accumulated other comprehensive loss		1,400,000	1,300,000
Interest rate swap | Cash flow hedge
Derivative [Line Items]
Unrealized gain (loss) on interest rate derivative instruments		0	0
Loss reclassified from AOCI into earnings		335,000	354,000
Interest rate derivative losses reclassified into earnings, net of tax		$ (37,000)	$ 224,000

Defined Contribution 401(k) P70

Defined Contribution 401(k) Plan (Details) $ in Millions	4 Months Ended	12 Months Ended
	Apr. 30, 2016	Dec. 31, 2017USD ($)defined_contribution_plan	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
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 | $		$ 19.6	$ 16.2	$ 12.5
Employee Retirement Savings Plan | Flight Attendants and Dispatchers
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, employer matching contribution, percent of match	50.00%	100.00%
Employee Retirement Savings Plan | Other Employees
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, employer matching contribution, percent of match		50.00%
Employee Retirement Savings Plan | Maximum | Flight Attendants and Dispatchers
Defined Contribution Plan Disclosure [Line Items]
Defined contribution 401(k) plan, maximum employer contribution percent of the employee’s annual compensation	6.00%	6.00%
Employee Retirement Savings Plan | Maximum | Other Employees
Defined Contribution Plan Disclosure [Line Items]
Defined contribution 401(k) plan, maximum employer contribution percent of the employee’s annual compensation		6.00%
Pilots’ Retirement Savings Plan
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, employer matching contribution, percent of match		100.00%
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%
Puerto Rico Retirement Savings Plan | Pilots
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, employer matching contribution, percent of match		100.00%
Puerto Rico Retirement Savings Plan | Other Employees
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, employer matching contribution, percent of match		50.00%
Puerto Rico Retirement Savings Plan | Maximum | Pilots
Defined Contribution Plan Disclosure [Line Items]
Defined contribution 401(k) plan, maximum employer contribution percent of the employee’s annual compensation		9.00%
Puerto Rico Retirement Savings Plan | Maximum | Other Employees
Defined Contribution Plan Disclosure [Line Items]
Defined contribution 401(k) plan, maximum employer contribution percent of the employee’s annual compensation		6.00%

Income Taxes - Components of th

Income Taxes - Components of the Provision for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current:
Federal	$ (68,601)	$ 60,079	$ 21,632
State and local	515	6,322	6,702
Foreign	2,742	2,034	1,235
Total current expense (benefit)	(65,344)	68,435	29,569
Deferred:
Federal	(10,370)	82,455	149,583
State and local	8,760	3,691	6,031
Total deferred expense (benefit)	(1,610)	86,146	155,614
Total income tax expense (benefit)	$ (66,954)	$ 154,581	$ 185,183

Income Taxes - Income Tax Recon

Income Taxes - Income Tax Reconciliation (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Income Tax Expense
Expected provision at federal statutory tax rate	35.00%	35.00%	35.00%
State tax expense, net of federal benefit	1.70%	1.60%	1.70%
Income tax credits	0.00%	0.00%	0.00%
Revaluation of deferred taxes	(56.30%)	0.00%	0.00%
Other	0.70%	0.30%	0.20%
Total income tax expense (benefit)	(18.90%)	36.90%	36.90%

Income Taxes - Deferred Taxes (

Income Taxes - Deferred Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Foreign tax credits	$ 5,980	$ 0
Deferred revenue	1,631	3,057
Nondeductible accruals	10,107	20,547
Deferred manufacturing credits	258	910
Accrued maintenance	1,991	1,854
Equity compensation	2,392	3,882
Other	5,555	4,026
Valuation allowance	(454)	0
Deferred tax assets	27,460	34,276
Deferred tax liabilities:
Deferred gain (loss) on leases, net	1,605	2,435
Accrued rent	12,055	14,025
Prepaid expenses	754	1,217
Property, plant and equipment	298,703	278,872
Deferred financing costs	225	5,358
Accrued aircraft and engine maintenance	27,258	40,512
Deferred tax liabilities	340,600	342,419
Net deferred tax assets (liabilities)	$ (313,140)	$ (308,143)

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary Of Income Taxes [Line Items]
Expected provision at federal statutory tax rate		35.00%	35.00%	35.00%
Reduction in net deferred tax liabilities		$ 196,000
Income tax deficiency		500
Alternative minimum tax		6,300
Valuation allowance		454	$ 0
Foreign tax credits		6,000
State net operating losses		14,800
Excess tax (deficiency) benefit from equity-based compensation		$ 0	$ (470)	$ 8,850
Forecast
Summary Of Income Taxes [Line Items]
Expected provision at federal statutory tax rate	21.00%

Commitments and Contingencies -

Commitments and Contingencies - Schedule of Future Aircraft Operating Leases (Details)	Dec. 31, 2017aircraft
Long-term Purchase Commitment [Line Items]
2,018	10
2,019	15
2,020	16
2,021	18
Total future aircraft to be received	59
Airbus | A320
Long-term Purchase Commitment [Line Items]
2,018	5
2,019	1
2,020
Total future aircraft to be received	6
Airbus | A320NEO
Long-term Purchase Commitment [Line Items]
2,019	14
2,020	16
2,021	18
Total future aircraft to be received	48
Airbus | A321
Long-term Purchase Commitment [Line Items]
2,018	5
2,019
Total future aircraft to be received	5

Commitments and Contingencies76

Commitments and Contingencies - Aircraft-Related Commitments and Financing Arrangements (Details) ft² in Thousands, $ in Millions	1 Months Ended
	Sep. 30, 2015aft²extension_option	Jul. 31, 2015aircraftseat	Dec. 31, 2017USD ($)aircraft_engineaircraft	Nov. 30, 2017USD ($)
Unrecorded Unconditional Purchase Obligation [Line Items]
Future Aircraft To Be Received | aircraft			59
Hangar Facility | Operating Lease
Principal and Interest Commitments
Area of Land | a	10
Real estate property (in square feet) | ft²	126
Operating leases, term	30 years
Number of renewal options | extension_option	2
Aircraft operating leases, renewal term	10 years
Airbus
Committed Expenditures
Number of delivered aircraft with secured debt financing commitments | aircraft			46
Secured Debt
Interest Commitments
Interest commitments, 2018			$ 60.8
Interest commitments, 2019			56.2
Interest commitments, 2020			51
Interest commitments, 2021			45.8
Interest commitments, 2022			40.7
Interest commitments, 2023 and beyond			133.3
Principal and Interest Commitments
Principal and interest commitments, 2018			19.6
Principal and interest commitments, 2019			39.4
Principal and interest commitments, 2020			35.8
Principal and interest commitments, 2021			30.3
Principal and interest commitments, 2022			29
Principal and interest commitments, 2023 and beyond			$ 301.4
Enhanced Equipment Trust Certificate
Committed Expenditures
Face amount, commitment for future issuance				$ 420.5
Number of delivered aircraft with secured debt financing commitments | aircraft			2
Aircraft and Related Flight Equipment
Principal and Interest Commitments
Estimated future project construction costs			$ 0.6
2018 | Airbus
Committed Expenditures
Number of aircraft with secured debt financing commitments scheduled for delivery | aircraft			10
2018 | Enhanced Equipment Trust Certificate | Airbus
Committed Expenditures
Number of aircraft with secured debt financing commitments scheduled for delivery | aircraft			10
2019-2021 | Airbus
Committed Expenditures
Number of aircraft without secured financing commitments scheduled for delivery | aircraft			49
Aircraft and Related Flight Equipment
Committed Expenditures
Committed expenditures, 2018			$ 528.4
Committed expenditures, 2019			774.8
Committed expenditures, 2020			820.9
Committed expenditures, 2021			785.1
Committed expenditures, 2022			16.8
Committed expenditures, 2023 and beyond			7.9
Aircraft and Related Flight Equipment | Enhanced Equipment Trust Certificate
Committed Expenditures
Face amount, commitment for future issuance			356.6
Non-aircraft Related Commitments
Committed Expenditures
Committed expenditures, 2018			5.9
Committed expenditures, 2019			1.8
Committed expenditures, 2020			1.3
Committed expenditures, 2021			0.7
Committed expenditures, 2022			0.4
Committed expenditures, 2023 and beyond			$ 0
V2500 SelectOne Engine
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of spare aircraft engines ordered | aircraft_engine			4
PurePower PW1100G-JM Engine
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of spare aircraft engines ordered | aircraft_engine			9
A320NEO | Airbus
Unrecorded Unconditional Purchase Obligation [Line Items]
Future Aircraft To Be Received | aircraft			48
A320NEO | 2018 | Airbus
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of Aircraft Order Conversions | aircraft			4
A320NEO | 2019 | Airbus
Unrecorded Unconditional Purchase Obligation [Line Items]
Future Aircraft To Be Received | aircraft			14
A320 | Airbus
Unrecorded Unconditional Purchase Obligation [Line Items]
Future Aircraft To Be Received | aircraft			6
Principal and Interest Commitments
Number of aircrafts with increased seating capacity | aircraft		40
A320 | Airbus | Minimum
Principal and Interest Commitments
Number of seats | seat		178
A320 | Airbus | Maximum
Principal and Interest Commitments
Number of seats | seat		182
Aircraft order contract renegotiation | A320NEO | 2019 | Airbus
Unrecorded Unconditional Purchase Obligation [Line Items]
Future Aircraft To Be Received | aircraft			2
Aircraft order contract renegotiation | Airbus A320 | 2018 | Airbus
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of Aircraft Order Conversions | aircraft			2
Aircraft order contract renegotiation | Airbus A320 | 2019 | Airbus
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of Aircraft Order Conversions | aircraft			5

Commitments and Contingencies77

Commitments and Contingencies - Employees (Details) $ in Millions	1 Months Ended			12 Months Ended
	Jan. 31, 2018USD ($)	May 31, 2016	Mar. 31, 2016	Dec. 31, 2017USD ($)employee_group	Dec. 31, 2016USD ($)employee_group	Dec. 31, 2015USD ($)
Health Insurance Product Line
Concentration Risk [Line Items]
Accrued health care claims				$ 3.9	$ 5.7
Association of Flight Attendants (AFA-CWA)
Concentration Risk [Line Items]
Tentative collective bargaining agreement, contract term			5 years
Ratified collective bargaining agreement, contract term		5 years
Ratified Collective Bargaining Agreement, Incentive Payment				$ 9.6
Increase (Decrease) in Employee Related Liabilities to Ratified Collective Bargaining Agreement, Incentive Payment						$ 1.2
Association of Flight Attendants (AFA-CWA) | Salaries, Wages, and Benefits
Concentration Risk [Line Items]
Increase (Decrease) in Employee Related Liabilities to Ratified Collective Bargaining Agreement, Incentive Payment					$ 8.4
Unionized employees concentration risk | Number of employees (percent)
Concentration Risk [Line Items]
Union-represented employee groups | employee_group				4	4
Concentration of risk				75.00%	73.00%
Unionized employees concentration risk | Number of employees (percent) | Air Line Pilots Association, International (ALPA)
Concentration Risk [Line Items]
Concentration of risk				26.00%
Unionized employees concentration risk | Number of employees (percent) | Association of Flight Attendants (AFA-CWA)
Concentration Risk [Line Items]
Concentration of risk				44.00%
Unionized employees concentration risk | Number of employees (percent) | Transport Workers Union (TWU)
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				4.00%
Subsequent Event
Concentration Risk [Line Items]
Ratification Incentive	$ 75
Subsequent Event | Air Line Pilots Association, International (ALPA)
Concentration Risk [Line Items]
Tentative collective bargaining agreement, contract term	5 years

Fair Value Measurements - Long-

Fair Value Measurements - Long-term Debt (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	$ 1,539.6	$ 1,012.4
Senior loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	417.9	451.9
Junior loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	39.3	47.1
Fixed-rate loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	518	0
2015-1 EETC Class A
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	408.6	409.8
2015-1 EETC Class B
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	92	103.6
2017-1 EETC Class AA
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	37.5	0
2017-1 EETC Class A
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	12.5	0
2017-1 EETC Class B
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	13.8	0
Estimate of Fair Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	1,583.2	1,033.7
Estimate of Fair Value Measurement [Member] | Level 3 | Senior loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	435.3	463.9
Estimate of Fair Value Measurement [Member] | Level 3 | Junior loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	40.4	48.1
Estimate of Fair Value Measurement [Member] | Level 3 | Fixed-rate loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	528.6
Estimate of Fair Value Measurement [Member] | Level 2 | 2015-1 EETC Class A
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	420.9	416
Estimate of Fair Value Measurement [Member] | Level 2 | 2015-1 EETC Class B
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	94.2	$ 105.7
Estimate of Fair Value Measurement [Member] | Level 2 | 2017-1 EETC Class AA
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	37.4
Estimate of Fair Value Measurement [Member] | Level 2 | 2017-1 EETC Class A
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	12.6
Estimate of Fair Value Measurement [Member] | Level 2 | 2017-1 EETC Class B
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	$ 13.8

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, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 800.8	$ 700.9
Short-term investment securities	100.9	100.2
Total assets	901.7	801.1
Total liabilities	0	0
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	800.8	700.9
Short-term investment securities	100.9	100.2
Total assets	901.7	801.1
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
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
Total assets	0	0
Total liabilities	$ 0	$ 0

Operating Segments and Relate80

Operating Segments and Related Disclosures - Geographic Revenues (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenue	$ 667,007	$ 687,231	$ 701,682	$ 591,746	$ 578,351	$ 621,329	$ 584,133	$ 538,143	$ 2,647,666	$ 2,321,956	$ 2,141,463
DOT—Domestic
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenue									2,435,900	2,136,200	1,940,200
DOT—Latin America
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenue									$ 211,800	$ 185,800	$ 201,300

Operating Segments and Relate81

Operating Segments and Related Disclosures - Foreign Revenues (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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 (Una82

Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Operating revenue	$ 667,007	$ 687,231	$ 701,682	$ 591,746	$ 578,351	$ 621,329	$ 584,133	$ 538,143	$ 2,647,666	$ 2,321,956	$ 2,141,463
Operating income	92,546	104,104	132,736	59,405	85,311	135,216	121,835	101,299	388,791	443,661	509,122
Net income	$ 250,338	$ 60,190	$ 78,143	$ 31,935	$ 48,493	$ 81,382	$ 73,084	$ 61,920	$ 420,606	$ 264,879	$ 317,220
Basic earnings per share (in dollars per share)	$ 3.64	$ 0.87	$ 1.13	$ 0.46	$ 0.70	$ 1.17	$ 1.03	$ 0.87	$ 6.08	$ 3.77	$ 4.39
Diluted earnings per share (in dollars per share)	$ 3.63	$ 0.87	$ 1.12	$ 0.46	$ 0.70	$ 1.17	$ 1.03	$ 0.86	6.06	$ 3.76	$ 4.38
Non-recurring income tax benefit due to the Tax Cuts and Jobs Act of 2017	$ 199,300
Non-recurring income tax benefit due to the Tax Cuts and Jobs Act of 2017 per basic share (in dollars per share)	$ 2.90								2.88
Non-recurring income tax benefit due to the Tax Cuts and Jobs Act of 2017 per diluted share (in dollars per share)	$ 2.89								$ 2.87