Spirit Airlines (SAVEQ) 10-K 13 Feb 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2018	Feb. 06, 2019	Jun. 30, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Entity Registrant Name	Spirit Airlines, Inc.
Amendment Flag	false
Document Fiscal Year Focus	2,018
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 Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 2.5
Entity Common Stock, Shares Outstanding		68,328,977

Statements Of Operations

Statements Of Operations - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating revenues:
Total operating revenues	$ 862,795	$ 904,330	$ 851,771	$ 704,138	$ 666,182	$ 687,227	$ 700,185	$ 589,957	$ 3,323,034	$ 2,643,552	$ 2,320,021
Operating expenses:
Aircraft fuel					175,205	158,300	142,294	139,782	939,324	615,581	447,553
Salaries, wages and benefits					136,815	134,114	129,892	127,138	719,635	527,959	472,471
Landing fees and other rents					46,117	48,498	45,592	40,448	214,677	180,655	151,679
Aircraft rent					42,820	53,396	52,566	57,070	177,641	205,852	201,675
Depreciation and amortization					36,472	36,840	35,331	31,509	176,727	140,152	101,136
Distribution					28,170	29,695	29,835	25,772	137,001	113,472	96,895
Maintenance, materials and repairs					28,966	26,176	28,985	26,312	129,078	110,439	98,587
Special charges					0	7,853	0	4,776	88,921	12,629	37,189
Loss on disposal of assets					1,054	516	1,493	1,105	9,580	4,168	4,187
Other operating					79,267	87,965	102,885	77,703	379,536	347,820	267,191
Total operating expenses					574,886	583,353	568,873	531,615	2,972,120	2,258,727	1,878,563
Operating income	136,065	145,125	108,521	(38,797)	91,296	103,874	131,312	58,342	350,914	384,825	441,458
Other (income) expense:
Interest expense					16,065	15,018	13,746	12,473	83,777	57,302	41,654
Capitalized interest					(3,668)	(3,203)	(3,342)	(3,580)	(9,841)	(13,793)	(12,705)
Interest income					(2,990)	(2,605)	(1,828)	(1,313)	(19,107)	(8,736)	(5,276)
Other expense					145	114	104	3	752	366	528
Special charges, non-operating									90,357	0	0
Total other (income) expense					9,552	9,324	8,680	7,583	145,938	35,139	24,201
Income before income taxes					81,744	94,550	122,632	50,759	204,976	349,686	417,257
Provision (benefit) for income taxes					(165,231)	34,506	45,391	19,498	49,227	(65,836)	153,774
Net income	$ 91,937	$ 97,480	$ 11,254	$ (44,922)	$ 246,975	$ 60,044	$ 77,241	$ 31,261	$ 155,749	$ 415,522	$ 263,483
Basic earnings per share (in dollars per share)	$ 1.35	$ 1.43	$ 0.16	$ (0.66)	$ 3.59	$ 0.87	$ 1.11	$ 0.45	$ 2.28	$ 6	$ 3.75
Diluted earnings per share (in dollars per share)	$ 1.34	$ 1.42	$ 0.16	$ (0.66)	$ 3.58	$ 0.86	$ 1.11	$ 0.45	$ 2.28	$ 5.99	$ 3.74
Passenger
Operating revenues:
Total operating revenues	$ 846,568	$ 887,956	$ 836,350	$ 689,141	$ 650,647	$ 669,072	$ 680,880	$ 572,287	$ 3,260,015	$ 2,572,887	$ 2,257,801
Other
Operating revenues:
Total operating revenues	$ 16,227	$ 16,374	$ 15,421	$ 14,997	$ 15,535	$ 18,155	$ 19,305	$ 17,670	$ 63,019	$ 70,665	$ 62,220

Statements of Comprehensive Inc

Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 155,749	$ 415,522	$ 263,483
Unrealized gain (loss) on short-term investment securities, net of deferred taxes of $44, ($41), and ($13)	30	(82)	(23)
Interest rate derivative loss reclassified into earnings, net of taxes of $75, $372 and $130	241	(37)	224
Other comprehensive income (loss)	271	(119)	201
Comprehensive income	$ 156,020	$ 415,403	$ 263,684

Statements of Comprehensive I_2

Statements of Comprehensive Income (Parentheticals) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Comprehensive Income [Abstract]
Deferred taxes on unrealized loss on investment securities	$ 44	$ (41)	$ (13)
Loss reclassified from AOCI into income, tax	$ 75	$ 372	$ 130

Balance Sheets

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 1,004,733	$ 800,849
Short-term investment securities	102,789	100,937
Accounts receivable, net	47,660	49,323
Aircraft maintenance deposits, net	106,901	175,615
Income tax receivable	0	69,844
Prepaid expenses and other current assets	83,383	85,542
Total current assets	1,345,466	1,282,110
Property and equipment:
Flight equipment	3,257,215	2,291,110
Ground property and equipment	191,661	155,166
Less accumulated depreciation	(332,864)	(207,808)
Total property and equipment	3,116,012	2,238,468
Pre-delivery deposits on flight equipment	236,775	253,687
Long-term aircraft maintenance deposits	138,738	150,617
Deferred heavy maintenance, net	249,010	99,915
Other long-term assets	79,456	121,003
Total assets	5,165,457	4,145,800
Current liabilities:
Accounts payable	39,320	22,822
Air traffic liability	291,981	263,711
Current maturities of long-term debt and capital leases	163,557	115,430
Other current liabilities	339,677	262,370
Total current liabilities	834,535	664,333
Long-term debt and capital leases, less current maturities	2,024,774	1,387,498
Deferred income taxes	355,141	308,814
Deferred gains and other long-term liabilities	22,503	22,581
Shareholders’ equity:
Common stock: Common stock, $0.0001 par value, 240,000,000 shares authorized at December 31, 2018 and 2017, respectively; 69,871,535 and 69,770,795 issued and 68,269,567 and 68,196,964 outstanding as of December 31, 2018 and 2017, respectively	7	7
Additional paid-in-capital	371,225	360,153
Treasury stock, at cost: 1,601,968 and 1,573,831 shares as of December 31, 2018 and 2017, respectively	(67,016)	(65,854)
Retained earnings	1,625,481	1,469,732
Accumulated other comprehensive income (loss)	(1,193)	(1,464)
Total shareholders’ equity	1,928,504	1,762,574
Total liabilities and shareholders’ equity	$ 5,165,457	$ 4,145,800

Balance Sheets (Parenthetical)

Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001	$ 0.0001
Common stock, shares authorized (in shares)	240,000,000	240,000,000	240,000,000
Common stock, shares issued (in shares)	69,871,535	69,770,795	73,549,872
Common stock, shares outstanding (in shares)	68,269,567	68,196,964	69,326,202
Treasury stock (in shares)	1,601,968	1,573,831	4,223,670
Voting Common Stock
Common stock, par value (in dollars per share)	$ 0.0001
Common stock, shares authorized (in shares)	240,000,000

Statements of Cash Flows

Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities:
Net income	$ 155,749	$ 415,522	$ 263,483
Adjustments to reconcile net income to net cash provided by operations:
Losses reclassified from other comprehensive income	315	335	354
Stock-based compensation	11,021	8,522	7,105
Allowance for doubtful accounts (recoveries)	(11)	(53)	80
Amortization of deferred gains and losses and debt issuance costs	8,819	7,944	5,732
Depreciation and amortization	176,727	140,152	101,136
Deferred income tax expense (benefit)	46,303	(492)	85,339
Loss on disposal of assets	9,580	4,168	4,187
Lease termination costs	0	12,629	37,189
Special charges, non-operating	90,357	0	0
Changes in operating assets and liabilities:
Accounts receivable	1,674	(8,134)	(12,951)
Aircraft maintenance deposits, net	14,019	(37,930)	(45,869)
Long-term deposits and other assets	(4,803)	(50,951)	(47,204)
Deferred heavy maintenance, net	(190,381)	(78,237)	(30,222)
Income tax receivable	69,844	(69,844)	0
Prepaid income taxes	0	0	72,278
Accounts payable	15,317	6,030	(6,823)
Air traffic liability	28,270	43,527	(9,455)
Other liabilities	74,038	31,672	47,200
Other	(375)	380	206
Net cash provided by operating activities	506,463	425,240	471,765
Investing activities:
Purchase of available-for-sale investment securities	(124,430)	(107,246)	(103,258)
Proceeds from the maturity of available-for-sale investment securities	122,947	105,906	2,842
Proceeds from sale of property and equipment	11,400	0	50
Pre-delivery deposits on flight equipment, net of refunds	(177,424)	(149,477)	(173,947)
Capitalized interest	(8,729)	(12,305)	(10,834)
Assets under construction for others	(501)	0	0
Purchase of property and equipment	(606,971)	(628,881)	(539,209)
Net cash used in investing activities	(783,708)	(792,003)	(824,356)
Financing activities:
Proceeds from issuance of long-term debt	832,099	629,725	417,275
Proceeds from stock options exercised	51	45	92
Payments on debt obligations	(137,275)	(102,313)	(64,371)
Payments on capital lease obligations	(205,720)	(425)	(50)
Reimbursement for assets under construction for others	501	0	0
Excess tax (deficiency) benefit from equity-based compensation	0	0	(470)
Repurchase of common stock	(1,162)	(46,580)	(102,510)
Debt issuance costs	(7,365)	(13,740)	(107)
Net cash provided by financing activities	481,129	466,712	249,859
Net increase (decrease) in cash and cash equivalents	203,884	99,949	(102,732)
Cash and cash equivalents at beginning of period	800,849	700,900	803,632
Cash and cash equivalents at end of period	1,004,733	800,849	700,900
Cash payments for:
Interest, net of capitalized interest	65,123	37,902	39,963
Income taxes paid, net of refunds	(73,489)	5,826	(5,579)
Non-cash transactions:
Capital expenditures funded by capital lease borrowings	$ (987)	$ (1,370)	$ (31)

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)	As Reported [Member]	As Reported [Member]Additional Paid-In Capital	As Reported [Member]Treasury Stock	As Reported [Member]Retained Earnings (Accumulated Deficit)	As Reported [Member]Accumulated Other Comprehensive Income (Loss)	Restatement Adjustment [Member]	Restatement Adjustment [Member]Retained Earnings (Accumulated Deficit)
Balance at Dec. 31, 2015	$ 1,217,283	$ 7	$ 544,277	$ (116,182)	$ 790,727	$ (1,546)	$ 1,225,310	$ 544,277	$ (116,182)	$ 798,754	$ (1,546)
Balance (ASU 2014-09) at Dec. 31, 2015												$ (8,027)	$ (8,027)
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 benefits from share-based compensation	(470)		(470)
Changes in comprehensive income	201					201
Net income	263,483				263,483		264,879
Net income | ASU 2014-09												(1,396)
Balance at Dec. 31, 2016	1,385,184	7	551,004	(218,692)	1,054,210	(1,345)	1,394,607
Balance (ASU 2014-09) at Dec. 31, 2016												(9,423)
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	415,522				415,522		420,606
Net income | ASU 2014-09												(5,084)
Balance at Dec. 31, 2017	1,762,574	7	360,153	(65,854)	1,469,732	(1,464)	$ 1,777,081
Balance (ASU 2014-09) at Dec. 31, 2017												$ (14,507)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	11,021		11,021
Repurchase of common stock	(1,162)			(1,162)
Proceeds from options exercised	51		51
Changes in comprehensive income	271					271
Net income	155,749				155,749
Balance at Dec. 31, 2018	$ 1,928,504	$ 7	$ 371,225	$ (67,016)	$ 1,625,481	$ (1,193)

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2018
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. In addition, certain prior period amounts have been adjusted to reflect the adoption of Accounting Standards Update ("ASU") No. 2014-09, ("ASU 2014-09") "Revenue from Contracts with Customers," completed January 1, 2018. Use of Estimates The preparation of financial statements in accordance with generally accepted accounting principles in the United States of America requires the Company's management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. The Company's estimates and assumptions are based on historical experience and changes in the business environment. However, actual results may differ from estimates under different conditions, sometimes materially. Critical accounting policies and estimates are defined as those that both (i) are most important to the portrayal of the Company's financial condition and results and (ii) require management's most subjective judgments. The Company's most critical accounting policies and estimates are described below. Cash and Cash Equivalents The Company considers all highly liquid investments with maturities of less than three months at the date of acquisition to be cash equivalents. Investments included in this category primarily consist of cash and money market funds. Cash and cash equivalents are stated at cost, which approximates fair value. Short-term Investment Securities The Company's short-term investment securities are classified as available-for-sale and consist of U.S. Treasury and U.S. government agency securities with contractual maturities of twelve months or less. These securities are stated at fair value within current assets on the Company's balance sheet. For all short-term investments, at each reset period or upon reinvestment, the Company accounts for the transaction as proceeds from the maturity of short-term investment securities for the security relinquished, and purchase of short-term investment securities for the security purchased, in the Company's statements of cash flows. Realized gains and losses on sales of investments, if any, are reflected in non-operating income (expense) in the statements of operations. Unrealized gains and losses on investment securities are reflected as a component of accumulated other comprehensive income. Accounts Receivable Accounts receivable primarily consist of amounts due from credit card processors associated with the sales of tickets and amounts due from the Internal Revenue Service related to federal excise fuel tax. The Company records an allowance for doubtful accounts for amounts not expected to be collected. The Company estimates the allowance based on historical write-offs as well as aging trends. The allowance for doubtful accounts was immaterial as of December 31, 2018 and 2017 . In addition, the provision for doubtful accounts and write-offs for 2018 , 2017 and 2016 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, 2018 , the Company had 82 aircraft, 8 spare engines and 1 flight simulator capitalized within flight equipment with depreciable lives of 25 years . As of December 31, 2018 , the Company had 46 aircraft financed through operating leases with lease terms from 8 to 18 years and 12 spare engines financed through operating leases with lease terms from 2 to 14 years . The following table illustrates the components of depreciation and amortization expense: Year Ended December 31, 2018 2017 2016 (in thousands) Depreciation $ 129,412 $ 83,154 $ 54,171 Amortization of heavy maintenance 41,286 53,855 43,811 Amortization of capitalized software 6,029 3,143 3,154 Total depreciation and amortization $ 176,727 $ 140,152 $ 101,136 The Company capitalizes certain internal and external costs associated with the acquisition and development of internal-use software for new products, and enhancements to existing products, that have reached the application development stage and meet recoverability tests. Capitalized costs include external direct costs of materials and services utilized in developing or obtaining internal-use software, and labor cost for employees who are directly associated with, and devote time, to internal-use software projects. Capitalized computer software, included as a component of ground and other equipment in the accompanying balance sheets, net of amortization, was $13.7 million and $7.7 million at December 31, 2018 and 2017 , respectively. The Company records amortization of capitalized software on a straight-line basis within depreciation and amortization expense in the accompanying statements of operations. The Company placed in service internal-use software of $12.0 million , $1.4 million and $4.1 million , during the years ended 2018 , 2017 and 2016 , respectively. Pre-Delivery Deposits on Flight Equipment The Company is required to make pre-delivery deposit payments ("PDPs") towards the purchase price of each new aircraft and engine prior to the scheduled delivery date. These deposits are initially classified as pre-delivery deposits on flight equipment on the Company's balance sheets until the aircraft or engine is delivered, at which time the related PDPs are deducted from the final purchase price of the aircraft or engine and are reclassified to flight equipment on the Company's balance sheets. In addition, the Company capitalizes the interest that is attributable to the outstanding PDP balances as a percentage of the related debt on which interest is incurred. Capitalized interest represents interest cost incurred during the acquisition period of a long-term asset, and is the amount which theoretically could have been avoided had the Company not paid PDPs for the related aircraft or engines. Related interest is capitalized and included within pre-delivery deposits on flight equipment through the acquisition period until delivery is taken of the aircraft or engine and the asset is ready for service. Once the aircraft or engine is delivered, the capitalized interest is also reclassified into flight equipment on the Company's balance sheets along with the related PDPs as they are included in the cost of the aircraft or engine. Capitalized interest for 2018, 2017 and 2016 primarily related to the interest incurred on long-term debt. Measurement of Asset Impairments The Company records impairment charges on long-lived assets used in operations when events and circumstances indicate that the assets may be impaired, the undiscounted cash flows estimated to be generated by those assets are less than the carrying amount of those assets, and the net book value of the assets exceeds their estimated fair value. In making these determinations, the Company uses certain assumptions, including, but not limited to: (i) estimated fair value of the assets; and (ii) estimated, undiscounted future cash flows expected to be generated by these assets, which are based on additional assumptions such as asset utilization, length of service the asset will be used in the Company’s operations, and estimated salvage values. Passenger Revenues Fare revenues. Tickets sold are initially deferred within air traffic liability on the Company's balance sheet. Passenger fare revenues are recognized at time of departure when transportation is provided. All tickets sold by the Company are nonrefundable. An unused ticket expires at the date of scheduled travel and is recognized as revenue at the date of scheduled travel. As of December 31, 2018 and 2017 , the Company had air traffic liability ("ATL") balances of $292.0 million and $263.7 million , respectively. As of December 31, 2018, all of the ATL balance as of December 31, 2017 has been recognized. The Company adopted ASU 2014-09 on January 1, 2018 utilizing the full retrospective method of adoption. Passenger revenues reported prior to the adoption of ASU 2014-09 are now reported as fare revenues within passenger revenues in the Company's disaggregated revenue table within Note 4, Revenue Disaggregation. Refer to Note 3, Revenue Recognition and 4, Revenue Disaggregation for information regarding the Company's adoption of ASU 2014-09. Non-fare revenues. The adoption of ASU 2014-09 impacted the classification of certain ancillary items such as bags, seats and other travel-related fees, since they are deemed part of the single performance obligation of providing passenger transportation. These ancillary items are now recognized in non-fare revenues within passenger revenues, at the time of departure, in the Company's disaggregated revenue table within Note 4, Revenue Disaggregation. The following table summarizes the primary components of the Company's non-fare revenue and the revenue recognition method utilized for each service or product: Year Ended December 31, Non-fare revenue Recognition method 2018 2017 2016 (in thousands) Baggage Time of departure $ 620,154 $ 488,434 $ 434,269 Passenger usage fee Time of departure 531,459 411,742 358,920 Advance seat selection Time of departure 180,012 131,821 110,966 Other 224,283 177,495 155,539 Non-fare revenue $ 1,555,908 $ 1,209,492 $ 1,059,694 Changes and cancellations. Customers may elect to change or cancel their itinerary prior to the date of departure. For changes, a service charge is recognized at time of departure of newly scheduled travel and is deducted from the face value of the original purchase price of the ticket, and the original ticket becomes invalid. For cancellations, a service charge is assessed and the amount remaining after deducting the service charge is called a credit shell which generally expires 60 days from the date the credit shell is created and which can be used towards the purchase of a new ticket and the Company’s other service offerings. Both the service charge and credit shell amounts are recorded as deferred revenue and amounts expected to expire unused are estimated based on historical experience. Estimating the amount of credits that will go unused involves some level of subjectivity and judgment. However, given the relatively short period of time to expiration, this does not have a significant impact on the Company's financial statements. Other Revenues Other revenues primarily consist of the marketing component of the sale of frequent flyer miles to the Company's credit card partner and commissions revenue from the sale of various items such as hotels and rental cars. Frequent Flyer Program The Company's frequent flyer program generates customer loyalty by rewarding customers with mileage credits to travel on Spirit. When traveling, customers earn redeemable mileage credits for each mile flown on Spirit. Customers can also earn mileage credits through participating companies such as the co-branded Spirit credit card. Mileage credits are redeemable by customers in future periods for air travel on Spirit. To reflect the mileage credits earned, the program includes two types of transactions that are considered revenue arrangements with multiple performance obligations: (1) mileage credits earned with travel and (2) mileage credits sold to co-branded credit card partner. The adoption of ASU 2014-09 eliminated the incremental cost method for frequent flyer program accounting, which required the Company to re-value and record a liability associated with customer flight miles earned with travel as part of the Company’s frequent flyer program with a relative fair value. Upon adoption of ASU 2014-09 on January 1, 2018, the Company recorded an increase to its air traffic liability of $12.4 million . Passenger ticket sales earning mileage credits. Passenger ticket sales earning mileage credits provide customers with (1) mileage credits earned and (2) air transportation. The Company values each performance obligation on a standalone basis. To value the mileage credits earned, the Company considers the quantitative value a passenger receives by redeeming miles for a ticket rather than paying cash, which is referred to as equivalent ticket value ("ETV"). The Company defers revenue for the mileage credits when earned and recognizes loyalty travel awards in passenger revenue as the miles are redeemed and services are provided. The Company records the air transportation portion of the passenger ticket sales in air traffic liability and recognizes passenger revenue when transportation is provided or if the ticket goes unused, at the date of scheduled travel. Sale of mileage credits. Customers may earn mileage credits based on their spending with the Company's co-branded credit card company with which the Company has an agreement to sell mileage credits. The contract to sell mileage credits under this agreement has multiple performance obligations, as discussed below. The Company's co-brand credit card agreement provides for joint marketing where cardholders earn mileage credits for making purchases using co-branded cards. During 2015, the Company extended its agreement with the administrator of the FREE SPIRIT affinity credit card program to extend through 2022. The Company accounts for this agreement consistently with the accounting method that allocates the consideration received to the individual products and services delivered. The value is allocated based on the relative selling prices of those products and services, which generally consists of (i) travel miles to be awarded, (ii) licensing of brand and access to member lists and (iii) advertising and marketing efforts. The Company determined the best estimate of the selling prices by considering discounted cash flow analysis using multiple inputs and assumptions, including: (1) the expected number of miles awarded and number of miles redeemed, (2) ETV for the award travel obligation, (3) licensing of brand and access to member lists and (4) advertising and marketing efforts. The Company defers the amount for award travel obligation as part of loyalty deferred revenue within air traffic liability on the balance sheet and recognizes loyalty travel awards in passenger revenue as the mileage credits are used for travel. Revenue allocated to the remaining performance obligations, primarily marketing components, is recorded in other revenue over time as miles are delivered. Total unrecognized revenue from future FREE SPIRIT award redemptions and the sale of mileage credits was $27.4 million and $26.6 million at December 31, 2018 and 2017 , respectively. The current portion of this balance is recorded within air traffic liability and the long-term portion of this balance is recorded within deferred gains and other long-term liabilities in the accompanying balance sheets. The following table illustrates total cash proceeds received from the sale of mileage credits and the portion of such proceeds recognized in non-ticket revenue immediately as marketing component: Consideration received from credit card mile programs Portion of proceeds recognized immediately as marketing component Year Ended (in thousands) December 31, 2018 $ 39,194 $ 30,353 December 31, 2017 49,453 37,960 December 31, 2016 48,882 36,640 Mileage breakage. For mileage credits that the Company estimates are not likely to be redeemed ("breakage"), the Company recognizes the associated value proportionally during the period in which the remaining mileage credits are redeemed. Management uses statistical models to estimate breakage based on historical redemption patterns. A change in assumptions as to the period over which mileage credits are expected to be redeemed, the actual redemption activity for mileage credits or the estimated fair value of mileage credits expected to be redeemed could have an impact on revenues in the year in which the change occurs and in future years. Current activity of frequent flyer program. Mileage credits are combined in one homogeneous pool and are not separately identifiable. As such, revenue is comprised of miles that were part of the frequent flyer deferred revenue balance at the beginning of the period as well as miles that were issued during the period. Refer to Note 3, Revenue Recognition and Note 4, Revenue Disaggregation for information regarding the Company's adoption of ASU 2014-09. 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 $41.3 million , $53.9 million and $43.8 million for the years ended 2018 , 2017 and 2016 , respectively. During the years ended 2018 , 2017 and 2016 , the Company deferred $190.5 million , $78.2 million and $35.4 million , respectively, of costs for heavy maintenance. At December 31, 2018 and 2017 , the Company had deferred heavy maintenance balance of $366.3 million and $260.5 million , and accumulated heavy maintenance amortization of $117.3 million and $160.7 million , respectively. The Company outsources certain routine, non-heavy maintenance functions under contracts that require payment on a utilization basis, such as flight hours. Costs incurred for maintenance and repair under flight hour maintenance contracts, where labor and materials price risks have been transferred to the service provider, are expensed based on contractual payment terms. All other costs for routine maintenance of the airframes and engines are charged to expense as performed. The table below summarizes the components of the Company’s maintenance cost: Year Ended December 31, 2018 2017 2016 (in thousands) Flight hour-based maintenance expense $ 68,039 $ 54,802 $ 48,471 Non-flight hour-based maintenance expense 61,039 55,637 50,116 Total maintenance, materials and repairs $ 129,078 $ 110,439 $ 98,587 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, redelivery crew 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. 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. When determining probability and estimated cost, there are various other factors which need to be considered such as the contractual terms of the lease agreement, current condition of the aircraft, the age of the aircraft at lease expiration, number of hours run on the engines, number of cycles run on the airframe, projected number of hours run on the engine at the time of return, number of projected cycles run on the airframe at the time of return, the extent of repairs needed if any at return, return locations, current configuration of the aircraft, current paint of the aircraft, estimated escalation of cost of repairs and materials at the time of return, current flight hour agreement rates and future flight hour agreement rates. 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. As a result of the different factors listed above, management assesses the need to accrue lease return costs periodically throughout the year or whenever facts and circumstances warrant an assessment. Lease return costs will generally be estimable closer to the end of the lease term but may be estimable earlier in the lease term depending on the contractual terms of the lease agreement and the timing of maintenance events for a particular aircraft. 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, 2018 , 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 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 statements 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. As of December 31, 2018 and 2017 , the Company did not have any outstanding derivative financial instruments. For additional information, refer to Note 15, Financial Instruments and Risk Management. Advertising The Company expenses advertising and the production costs of advertising as incurred. Marketing and advertising expenses of $6.3 million , $5.1 million and $3.2 million for the years ended 2018 , 2017 and 2016 , respectively, were recorded within distribution expense in the statements of operations. Income Taxes The Company accounts for income taxes using the asset and liability method. The Company records a valuation allowance to reduce the deferred tax assets reported if, based on the weight of the evidence, it is more likely than not that some portion or all of the deferred tax assets will be not realized. As of December 31, 2018 and 2017 , the Company recorded a valuation allowance of $0.3 million and $0.5 million , respectively. For additional information, refer to Note 17, Income Taxes. Stock-Based Compensation The Company recognizes cost of employee services received in exchange for awards of equity instruments based on the fair value of each instrument at the date of grant. For the majority of awards, compensation expense is recognized on a straight-line basis over the period during which an employee is required to provide service in exchange for an award. Certain awards have performance conditions that must be achieved prior to vesting and are expensed based on the expected achievement at each reporting period. The Company has issued and outstanding restricted stock awards, stock option awards and performance share awards. Restricted stock awards are valued at the fair value of the shares on the date of grant. The fair value of share option awards is estimated on the date of grant using the Black-Scholes valuation model. The fair value of performance share awards is estimated through the use of a Monte Carlo simulation model. For additional information, refer to Note 11, 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 32% , 27% and 24% of total operating expenses in 2018 , 2017 and 2016 , respectively. The Company’s operations are largely concentrated in the southeast United States with Fort Lauderdale being the highest volume fueling point in the system. Gulf Coast Jet indexed fuel is the basis for a substantial majority of the Company’s fuel consumption. Any disruption to the oil production or refinery capacity in the Gulf Coast, as a result of weather or any other disaster, or disruptions in supply of jet fuel, dramatic escalations in the costs of jet fuel and/or the failure of fuel providers to perform under fuel arrangements for other reasons could have a material adverse effect on the Company’s financial condition and results of operations. The Company’s operations will continue to be vulnerable to weather conditions (including hurricane season or snow and severe winter weather), which could disrupt service or create air traffic control problems. These events may result in decreased revenue and/or increased costs. Due to the relatively small size of the Company's fleet and high utilization rate, the unavailability of aircraft and resulting reduced capacity could have a material adverse effect on the Company’s business, results of operations and financial condition. As of December 31, 2018 , the Company had five union-represented employee groups that together represented approximately 80% of all employees. As of December 31, 2017 , the Company had four union-represented employee groups that together represented approximately 75%

Recent Accounting Developments

Recent Accounting Developments	12 Months Ended
	Dec. 31, 2018
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Recent Accounting Developments	Recent Accounting Developments Recently Adopted Accounting Pronouncements Revenue from Contracts with Customers In May 2014, the Financial Accounting Standards Board ("the FASB") issued 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 Company adopted this guidance on January 1, 2018 utilizing the full retrospective method of adoption allowed by the standard, in order to provide for comparative results in all periods presented. The most significant impact of this ASU is the elimination of the incremental cost method for frequent flyer program accounting, which requires the Company to re-value and record a liability associated with customer flight miles earned as part of the Company’s frequent flyer program with a relative fair value approach. The classification and timing of recognition of certain ancillary fees is also impacted by the adoption of ASU 2014-09. While the adoption did not have a significant impact on earnings, the classification of certain revenues, such as bags, seats and other travel-related fees are now deemed part of the single performance obligation of providing passenger transportation. Refer to Note 3, Revenue Recognition and Note 4, Revenue Disaggregation for information regarding the Company's adoption of ASU 2014-09. 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 the Company for interim and annual periods beginning January 1, 2018. The Company adopted this guidance on January 1, 2018 with no material impact on the financial statements. Statement of Cash Flows In August 2016, the FASB issued ASU No. 2016-15, "Statement of Cash Flows." The standard is intended to reduce diversity in practice in how certain transactions are classified in the statement of cash flows. This standard is effective for the Company for fiscal years, and interim periods within those years, beginning January 1, 2018. The Company adopted this guidance on January 1, 2018 with no material impact on the financial statements. Income Taxes In March 2018, the FASB issued ASU 2018-05, "Income Taxes" ("Topic 740") - Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin No. 118. The standard amends Accounting Standards Codification 740, Income Taxes ("ASC 740") to provide guidance on accounting for the tax effects of the Tax Cuts and Jobs Act (the "Tax Act") pursuant to Staff Accounting Bulletin No. 118. During the fourth quarter of 2018, the Company finalized its accounting for the tax effects of the Tax Act and had no material adjustments from the provisional amounts previously recorded by the Company. Recently Issued Accounting Pronouncements Not Yet Adopted Cloud Computing Arrangements In August 2018, the FASB issued ASU No. 2018-15, "Intangibles - Goodwill and Other - Internal-Use Software". This new standard requires a customer in a cloud computing arrangement that is a service contract to follow the internal-use software guidance in Accounting Standards Codification ("ASC") 350-40, "Accounting for Internal-Use Software", to determine which implementation costs to capitalize as assets and amortize over the term of the hosting arrangement or expense as incurred. This new standard is effective for public business entities in fiscal years beginning after December 15, 2019. Early adoption is permitted, including during an interim period. Entities have the option to apply this standard prospectively to all implementation costs incurred after the date of adoption or retrospectively. The Company is evaluating this new standard, but does not expect it to have a significant impact on its financial statement presentation or results. Leases In February 2016, the FASB issued ASU No. 2016-02, "Leases (Topic 842)." This standard will require all lessees to recognize a right-of-use asset and a lease liability, initially measured at the present value of the lease payments, for all leases with a term greater than 12 months. The standard is effective for the Company effective January 1, 2019. Early adoption is permitted. However, the Company has elected not to early adopt. The Company has established a cross functional project plan and is in the process of testing items necessary to account for the new standard. The Company expects this standard to have a significant impact of approximately $0.8 billion to $1.2 billion on its balance sheets due to the recognition of right-of-use assets and lease liabilities for certain operating leases. The adoption of Topic 842 will not have a significant impact on the Company's lease classification or a material impact on its statements of operations and liquidity. Additionally, the Company does not expect the standard to have a material impact on the Company’s debt-covenant compliance under its current agreements. Refer to Note 18, Commitments and Contingencies for information regarding the Company's undiscounted future lease payments and the timing of those payments. In July 2018, the FASB issued additional guidance on the accounting for leases. The guidance provides companies with another transition method that allows entities to recognize a cumulative-effect adjustment to the opening balance of retained earnings as of the date of adoption. Under this method, previously presented years’ financial positions and results would not be adjusted. The Company expects to use this transition method upon implementation of the guidance in 2019. The new guidance also provides lessors with a practical expedient, by class of underlying asset, to not separate non-lease components from the associated lease component if (1) the non-lease components would otherwise be accounted for under the new revenue recognition standard, (2) both the timing and pattern of transfer are the same for the non-lease components and associated lease component, and (3) if accounted for separately, the lease component would be classified as an operating lease. The Company plans to elect the package of practical expedients available under the transition provisions of Topic 842. Accounting for Credit Losses

Revenue Recognition

Revenue Recognition	12 Months Ended
	Dec. 31, 2018
Revenue from Contract with Customer [Abstract]
Revenue Recognition	Revenue Recognition The following tables show adjustments made due to the adoption of ASU 2014-09 on the December 31, 2017 and 2016 statements of operations. Previously reported results were derived from audited financial statements included in Company's Annual Report on Form 10-K for the fiscal years ended December 31, 2017 and December 31, 2016, as applicable. Year Ended December 31, 2017 (in thousands, except share and per-share data) As Reported Topic 606 Adjustment As Adjusted Operating revenues: Passenger $ 1,366,034 $ 1,206,853 $ 2,572,887 Other 1,281,632 (1,210,967 ) 70,665 Total operating revenues 2,647,666 (4,114 ) 2,643,552 Operating expenses: Aircraft fuel 615,581 — 615,581 Salaries, wages and benefits 527,959 — 527,959 Aircraft rent 205,852 — 205,852 Landing fees and other rents 180,655 — 180,655 Depreciation and amortization 140,152 — 140,152 Maintenance, materials and repairs 110,439 — 110,439 Distribution 113,620 (148 ) 113,472 Special charges 12,629 — 12,629 Loss on disposal of assets 4,168 — 4,168 Other operating 347,820 — 347,820 Total operating expenses 2,258,875 (148 ) 2,258,727 Operating income 388,791 (3,966 ) 384,825 Other (income) expense: Interest expense 57,302 — 57,302 Capitalized interest (13,793 ) — (13,793 ) Interest income (8,736 ) — (8,736 ) Other expense 366 — 366 Total other (income) expense 35,139 — 35,139 Income before income taxes 353,652 (3,966 ) 349,686 Provision (benefit) for income taxes (66,954 ) 1,118 (65,836 ) Net income $ 420,606 $ (5,084 ) $ 415,522 Basic earnings per share $ 6.08 $ (0.07 ) $ 6.00 Diluted earnings per share $ 6.06 $ (0.07 ) $ 5.99 Year Ended December 31, 2016 (in thousands, except share and per-share data) As Reported Topic 606 Adjustment As Adjusted Operating revenues: Passenger $ 1,200,621 $ 1,057,180 $ 2,257,801 Other 1,121,335 (1,059,115 ) 62,220 Total operating revenues 2,321,956 (1,935 ) 2,320,021 Operating expenses: Salaries, wages and benefits 472,471 — 472,471 Aircraft fuel 447,553 — 447,553 Aircraft rent 201,675 — 201,675 Landing fees and other rents 151,679 — 151,679 Depreciation and amortization 101,136 — 101,136 Maintenance, materials and repairs 98,587 — 98,587 Distribution 96,627 268 96,895 Special charges 37,189 — 37,189 Loss on disposal of assets 4,187 — 4,187 Other operating 267,191 — 267,191 Total operating expenses 1,878,295 268 1,878,563 Operating income 443,661 (2,203 ) 441,458 Other (income) expense: Interest expense 41,654 — 41,654 Capitalized interest (12,705 ) — (12,705 ) Interest income (5,276 ) — (5,276 ) Other expense 528 — 528 Total other (income) expense 24,201 — 24,201 Income before income taxes 419,460 (2,203 ) 417,257 Provision (benefit) for income taxes 154,581 (807 ) 153,774 Net income $ 264,879 $ (1,396 ) $ 263,483 Basic earnings per share $ 3.77 $ (0.02 ) $ 3.75 Diluted earnings per share $ 3.76 $ (0.02 ) $ 3.74 The following table shows adjusted balances after the adoption of ASU 2014-09 on the quarterly statements of operations for each quarter of 2017. For the Quarter Ended March 31, 2017 June 30, 2017 September 30, 2017 December 31, 2017 (unaudited) (in thousands, except share and per-share data) Operating revenues: Passenger $ 572,287 $ 680,880 $ 669,072 $ 650,647 Other 17,670 19,305 18,155 15,535 Total operating revenues 589,957 700,185 687,227 666,182 Operating expenses: Aircraft fuel 139,782 142,294 158,300 175,205 Salaries, wages and benefits 127,138 129,892 134,114 136,815 Aircraft rent 57,070 52,566 53,396 42,820 Landing fees and other rents 40,448 45,592 48,498 46,117 Depreciation and amortization 31,509 35,331 36,840 36,472 Maintenance, materials and repairs 26,312 28,985 26,176 28,966 Distribution 25,772 29,835 29,695 28,170 Special charges 4,776 — 7,853 — Loss on disposal of assets 1,105 1,493 516 1,054 Other operating 77,703 102,885 87,965 79,267 Total operating expenses 531,615 568,873 583,353 574,886 Operating income 58,342 131,312 103,874 91,296 Other (income) expense: Interest expense 12,473 13,746 15,018 16,065 Capitalized interest (3,580 ) (3,342 ) (3,203 ) (3,668 ) Interest income (1,313 ) (1,828 ) (2,605 ) (2,990 ) Other expense 3 104 114 145 Total other (income) expense 7,583 8,680 9,324 9,552 Income before income taxes 50,759 122,632 94,550 81,744 Provision (benefit) for income taxes 19,498 45,391 34,506 (165,231 ) Net income $ 31,261 $ 77,241 $ 60,044 $ 246,975 Basic earnings per share $ 0.45 $ 1.11 $ 0.87 $ 3.59 Diluted earnings per share $ 0.45 $ 1.11 $ 0.86 $ 3.58 The following table shows quarterly adjustments made due to the adoption of ASU 2014-09 on the statements of operations for 2017. Adjustments for the Quarter Ended Full Year 2017 As Reported March 31, 2017 June 30, 2017 September 30, 2017 December 31, 2017 Full Year 2017 Adjusted (unaudited) (in thousands, except share and per-share data) Operating revenues: Passenger $ 1,366,034 $ 272,525 $ 308,959 $ 312,865 $ 312,504 $ 2,572,887 Other 1,281,632 (274,314 ) (310,455 ) (312,869 ) (313,329 ) 70,665 Total operating revenues 2,647,666 (1,789 ) (1,496 ) (4 ) (825 ) 2,643,552 Operating expenses: Aircraft fuel 615,581 — — — — 615,581 Salaries, wages and benefits 527,959 — — — — 527,959 Aircraft rent 205,852 — — — — 205,852 Landing fees and other rents 180,655 — — — — 180,655 Depreciation and amortization 140,152 — — — — 140,152 Maintenance, materials and repairs 110,439 — — — — 110,439 Distribution 113,620 (726 ) (73 ) 226 425 113,472 Special charges 12,629 — — — — 12,629 Loss on disposal of assets 4,168 — — — — 4,168 Other operating 347,820 — — — — 347,820 Total operating expenses 2,258,875 (726 ) (73 ) 226 425 2,258,727 Operating income 388,791 (1,063 ) (1,423 ) (230 ) (1,250 ) 384,825 Other (income) expense: Interest expense 57,302 — — — — 57,302 Capitalized interest (13,793 ) — — — — (13,793 ) Interest income (8,736 ) — — — — (8,736 ) Other expense 366 — — — — 366 Total other (income) expense 35,139 — — — — 35,139 Income before income taxes 353,652 (1,063 ) (1,423 ) (230 ) (1,250 ) 349,686 Provision (benefit) for income taxes (66,954 ) (389 ) (522 ) (84 ) 2,113 (65,836 ) Net income $ 420,606 $ (674 ) $ (901 ) $ (146 ) $ (3,363 ) $ 415,522 Basic earnings per share $ 6.08 $ (0.01 ) $ (0.01 ) $ — $ (0.05 ) $ 6.00 Diluted earnings per share $ 6.06 $ (0.01 ) $ (0.01 ) $ — $ (0.05 ) $ 5.99 The following tables show adjustments made due to the adoption of ASU 2014-09 on the December 31, 2017 and 2016 balance sheets. Previously reported results were derived from audited financial statements included in Company's Annual Report on Form 10-K for the fiscal years ended December 31, 2017 and December 31, 2016, as applicable. December 31, 2017 (in thousands) As Reported Topic 606 Adjustment As Adjusted Assets Current assets: Cash and cash equivalents $ 800,849 $ — $ 800,849 Short-term investment securities 100,937 — 100,937 Accounts receivable, net 49,323 — 49,323 Aircraft maintenance deposits, net 175,615 — 175,615 Income tax receivable 69,844 — 69,844 Prepaid expenses and other current assets 83,692 1,850 85,542 Total current assets 1,280,260 1,850 1,282,110 Property and equipment: Flight equipment 2,291,110 — 2,291,110 Ground property and equipment 155,166 — 155,166 Less accumulated depreciation (207,808 ) — (207,808 ) 2,238,468 — 2,238,468 Deposits on flight equipment purchase contracts 253,687 — 253,687 Long-term aircraft maintenance deposits 150,617 — 150,617 Deferred heavy maintenance, net 99,915 — 99,915 Other long-term assets 121,003 — 121,003 Total assets $ 4,143,950 $ 1,850 $ 4,145,800 Liabilities and shareholders’ equity Current liabilities: Accounts payable $ 22,822 $ — $ 22,822 Air traffic liability 246,404 17,307 263,711 Current maturities of long-term debt 115,430 — 115,430 Other current liabilities 262,370 — 262,370 Total current liabilities 647,026 17,307 664,333 Long-term debt, less current maturities 1,387,498 — 1,387,498 Deferred income taxes 313,140 (4,326 ) 308,814 Deferred gains and other long-term liabilities 19,205 3,376 22,581 Shareholders’ equity: Common stock: Common stock, $0.0001 par value, 240,000,000 shares authorized at December 31, 2017; 69,770,795 issued and 68,196,964 outstanding as of December 31, 2017 7 — 7 Additional paid-in-capital 360,153 — 360,153 Treasury stock, at cost: 1,573,831 shares as of December 31, 2017 (65,854 ) — (65,854 ) Retained earnings 1,484,239 (14,507 ) 1,469,732 Accumulated other comprehensive income (loss) (1,464 ) — (1,464 ) Total shareholders’ equity 1,777,081 (14,507 ) 1,762,574 Total liabilities and shareholders’ equity $ 4,143,950 $ 1,850 $ 4,145,800 December 31, 2016 (in thousands) As Reported Topic 606 Adjustment As Adjusted Assets Current assets: Cash and cash equivalents $ 700,900 $ — $ 700,900 Short-term investment securities 100,155 — 100,155 Accounts receivable, net 41,136 — 41,136 Aircraft maintenance deposits, net 87,035 — 87,035 Income tax receivable — — — Prepaid expenses and other current assets 46,619 1,702 48,321 Total current assets 975,845 1,702 977,547 Property and equipment: Flight equipment 1,461,525 — 1,461,525 Ground property and equipment 126,206 — 126,206 Less accumulated depreciation (122,509 ) — (122,509 ) 1,465,222 — 1,465,222 Deposits on flight equipment purchase contracts 325,688 — 325,688 Long-term aircraft maintenance deposits 199,415 — 199,415 Deferred heavy maintenance, net 75,534 — 75,534 Other long-term assets 110,223 — 110,223 Total assets $ 3,151,927 $ 1,702 $ 3,153,629 Liabilities and shareholders’ equity Current liabilities: Accounts payable $ 15,193 $ — $ 15,193 Air traffic liability 206,392 13,792 220,184 Current maturities of long-term debt 84,354 — 84,354 Other current liabilities 226,011 — 226,011 Total current liabilities 531,950 13,792 545,742 Long-term debt, less current maturities 897,359 — 897,359 Deferred income taxes 308,143 (5,443 ) 302,700 Deferred gains and other long-term liabilities 19,868 2,776 22,644 Shareholders’ equity: Common stock: Common stock, $0.0001 par value, 240,000,000 shares authorized at December 31, 2016; 73,549,872 issued and 69,326,202 outstanding as of December 31, 2016 7 — 7 Additional paid-in-capital 551,004 — 551,004 Treasury stock, at cost: 4,223,670 shares as of December 31, 2016 (218,692 ) — (218,692 ) Retained earnings 1,063,633 (9,423 ) 1,054,210 Accumulated other comprehensive income (loss) (1,345 ) — (1,345 ) Total shareholders’ equity 1,394,607 (9,423 ) 1,385,184 Total liabilities and shareholders’ equity $ 3,151,927 $ 1,702 $ 3,153,629 Operating revenues is comprised of passenger revenues, which includes fare and non-fare revenues, and other revenues. The following table shows disaggregated operating revenues for each quarter of 2018 and 2017. For the Quarter Ended December 31, 2018 September 30, 2018 June 30, 2018 March 31, 2018 December 31, 2017 September 30, 2017 June 30, 2017 March 31, 2017 (unaudited, in thousands) Operating revenues: Fare $ 445,203 $ 476,660 $ 439,549 $ 342,695 $ 337,324 $ 355,593 $ 371,443 $ 299,035 Non-fare 401,365 411,296 396,801 346,446 313,323 313,479 309,437 273,252 Total passenger revenues 846,568 887,956 836,350 689,141 650,647 669,072 680,880 572,287 Other revenues 16,227 16,374 15,421 14,997 15,535 18,155 19,305 17,670 Total operating revenues $ 862,795 $ 904,330 $ 851,771 $ 704,138 $ 666,182 $ 687,227 $ 700,185 $ 589,957 The following table shows disaggregated operating revenues for years ended December 31, 2017 and 2016. Year Ended December 31, 2017 2016 (in thousands) As Reported Topic 606 Adjustment As Adjusted As Reported Topic 606 Adjustment As Adjusted Operating revenues: Fare $ 1,366,034 $ (2,639 ) $ 1,363,395 $ 1,200,621 $ (2,514 ) $ 1,198,107 Non-fare — 1,209,492 1,209,492 — 1,059,694 1,059,694 Total passenger revenues 1,366,034 1,206,853 2,572,887 1,200,621 1,057,180 2,257,801 Other revenues 1,281,632 (1,210,967 ) 70,665 1,121,335 (1,059,115 ) 62,220 Total operating revenues $ 2,647,666 $ (4,114 ) $ 2,643,552 $ 2,321,956 $ (1,935 ) $ 2,320,021

Revenue Disaggregation

Revenue Disaggregation	12 Months Ended
	Dec. 31, 2018
Revenue from Contract with Customer [Abstract]
Revenue Disaggregation	Revenue Recognition The following tables show adjustments made due to the adoption of ASU 2014-09 on the December 31, 2017 and 2016 statements of operations. Previously reported results were derived from audited financial statements included in Company's Annual Report on Form 10-K for the fiscal years ended December 31, 2017 and December 31, 2016, as applicable. Year Ended December 31, 2017 (in thousands, except share and per-share data) As Reported Topic 606 Adjustment As Adjusted Operating revenues: Passenger $ 1,366,034 $ 1,206,853 $ 2,572,887 Other 1,281,632 (1,210,967 ) 70,665 Total operating revenues 2,647,666 (4,114 ) 2,643,552 Operating expenses: Aircraft fuel 615,581 — 615,581 Salaries, wages and benefits 527,959 — 527,959 Aircraft rent 205,852 — 205,852 Landing fees and other rents 180,655 — 180,655 Depreciation and amortization 140,152 — 140,152 Maintenance, materials and repairs 110,439 — 110,439 Distribution 113,620 (148 ) 113,472 Special charges 12,629 — 12,629 Loss on disposal of assets 4,168 — 4,168 Other operating 347,820 — 347,820 Total operating expenses 2,258,875 (148 ) 2,258,727 Operating income 388,791 (3,966 ) 384,825 Other (income) expense: Interest expense 57,302 — 57,302 Capitalized interest (13,793 ) — (13,793 ) Interest income (8,736 ) — (8,736 ) Other expense 366 — 366 Total other (income) expense 35,139 — 35,139 Income before income taxes 353,652 (3,966 ) 349,686 Provision (benefit) for income taxes (66,954 ) 1,118 (65,836 ) Net income $ 420,606 $ (5,084 ) $ 415,522 Basic earnings per share $ 6.08 $ (0.07 ) $ 6.00 Diluted earnings per share $ 6.06 $ (0.07 ) $ 5.99 Year Ended December 31, 2016 (in thousands, except share and per-share data) As Reported Topic 606 Adjustment As Adjusted Operating revenues: Passenger $ 1,200,621 $ 1,057,180 $ 2,257,801 Other 1,121,335 (1,059,115 ) 62,220 Total operating revenues 2,321,956 (1,935 ) 2,320,021 Operating expenses: Salaries, wages and benefits 472,471 — 472,471 Aircraft fuel 447,553 — 447,553 Aircraft rent 201,675 — 201,675 Landing fees and other rents 151,679 — 151,679 Depreciation and amortization 101,136 — 101,136 Maintenance, materials and repairs 98,587 — 98,587 Distribution 96,627 268 96,895 Special charges 37,189 — 37,189 Loss on disposal of assets 4,187 — 4,187 Other operating 267,191 — 267,191 Total operating expenses 1,878,295 268 1,878,563 Operating income 443,661 (2,203 ) 441,458 Other (income) expense: Interest expense 41,654 — 41,654 Capitalized interest (12,705 ) — (12,705 ) Interest income (5,276 ) — (5,276 ) Other expense 528 — 528 Total other (income) expense 24,201 — 24,201 Income before income taxes 419,460 (2,203 ) 417,257 Provision (benefit) for income taxes 154,581 (807 ) 153,774 Net income $ 264,879 $ (1,396 ) $ 263,483 Basic earnings per share $ 3.77 $ (0.02 ) $ 3.75 Diluted earnings per share $ 3.76 $ (0.02 ) $ 3.74 The following table shows adjusted balances after the adoption of ASU 2014-09 on the quarterly statements of operations for each quarter of 2017. For the Quarter Ended March 31, 2017 June 30, 2017 September 30, 2017 December 31, 2017 (unaudited) (in thousands, except share and per-share data) Operating revenues: Passenger $ 572,287 $ 680,880 $ 669,072 $ 650,647 Other 17,670 19,305 18,155 15,535 Total operating revenues 589,957 700,185 687,227 666,182 Operating expenses: Aircraft fuel 139,782 142,294 158,300 175,205 Salaries, wages and benefits 127,138 129,892 134,114 136,815 Aircraft rent 57,070 52,566 53,396 42,820 Landing fees and other rents 40,448 45,592 48,498 46,117 Depreciation and amortization 31,509 35,331 36,840 36,472 Maintenance, materials and repairs 26,312 28,985 26,176 28,966 Distribution 25,772 29,835 29,695 28,170 Special charges 4,776 — 7,853 — Loss on disposal of assets 1,105 1,493 516 1,054 Other operating 77,703 102,885 87,965 79,267 Total operating expenses 531,615 568,873 583,353 574,886 Operating income 58,342 131,312 103,874 91,296 Other (income) expense: Interest expense 12,473 13,746 15,018 16,065 Capitalized interest (3,580 ) (3,342 ) (3,203 ) (3,668 ) Interest income (1,313 ) (1,828 ) (2,605 ) (2,990 ) Other expense 3 104 114 145 Total other (income) expense 7,583 8,680 9,324 9,552 Income before income taxes 50,759 122,632 94,550 81,744 Provision (benefit) for income taxes 19,498 45,391 34,506 (165,231 ) Net income $ 31,261 $ 77,241 $ 60,044 $ 246,975 Basic earnings per share $ 0.45 $ 1.11 $ 0.87 $ 3.59 Diluted earnings per share $ 0.45 $ 1.11 $ 0.86 $ 3.58 The following table shows quarterly adjustments made due to the adoption of ASU 2014-09 on the statements of operations for 2017. Adjustments for the Quarter Ended Full Year 2017 As Reported March 31, 2017 June 30, 2017 September 30, 2017 December 31, 2017 Full Year 2017 Adjusted (unaudited) (in thousands, except share and per-share data) Operating revenues: Passenger $ 1,366,034 $ 272,525 $ 308,959 $ 312,865 $ 312,504 $ 2,572,887 Other 1,281,632 (274,314 ) (310,455 ) (312,869 ) (313,329 ) 70,665 Total operating revenues 2,647,666 (1,789 ) (1,496 ) (4 ) (825 ) 2,643,552 Operating expenses: Aircraft fuel 615,581 — — — — 615,581 Salaries, wages and benefits 527,959 — — — — 527,959 Aircraft rent 205,852 — — — — 205,852 Landing fees and other rents 180,655 — — — — 180,655 Depreciation and amortization 140,152 — — — — 140,152 Maintenance, materials and repairs 110,439 — — — — 110,439 Distribution 113,620 (726 ) (73 ) 226 425 113,472 Special charges 12,629 — — — — 12,629 Loss on disposal of assets 4,168 — — — — 4,168 Other operating 347,820 — — — — 347,820 Total operating expenses 2,258,875 (726 ) (73 ) 226 425 2,258,727 Operating income 388,791 (1,063 ) (1,423 ) (230 ) (1,250 ) 384,825 Other (income) expense: Interest expense 57,302 — — — — 57,302 Capitalized interest (13,793 ) — — — — (13,793 ) Interest income (8,736 ) — — — — (8,736 ) Other expense 366 — — — — 366 Total other (income) expense 35,139 — — — — 35,139 Income before income taxes 353,652 (1,063 ) (1,423 ) (230 ) (1,250 ) 349,686 Provision (benefit) for income taxes (66,954 ) (389 ) (522 ) (84 ) 2,113 (65,836 ) Net income $ 420,606 $ (674 ) $ (901 ) $ (146 ) $ (3,363 ) $ 415,522 Basic earnings per share $ 6.08 $ (0.01 ) $ (0.01 ) $ — $ (0.05 ) $ 6.00 Diluted earnings per share $ 6.06 $ (0.01 ) $ (0.01 ) $ — $ (0.05 ) $ 5.99 The following tables show adjustments made due to the adoption of ASU 2014-09 on the December 31, 2017 and 2016 balance sheets. Previously reported results were derived from audited financial statements included in Company's Annual Report on Form 10-K for the fiscal years ended December 31, 2017 and December 31, 2016, as applicable. December 31, 2017 (in thousands) As Reported Topic 606 Adjustment As Adjusted Assets Current assets: Cash and cash equivalents $ 800,849 $ — $ 800,849 Short-term investment securities 100,937 — 100,937 Accounts receivable, net 49,323 — 49,323 Aircraft maintenance deposits, net 175,615 — 175,615 Income tax receivable 69,844 — 69,844 Prepaid expenses and other current assets 83,692 1,850 85,542 Total current assets 1,280,260 1,850 1,282,110 Property and equipment: Flight equipment 2,291,110 — 2,291,110 Ground property and equipment 155,166 — 155,166 Less accumulated depreciation (207,808 ) — (207,808 ) 2,238,468 — 2,238,468 Deposits on flight equipment purchase contracts 253,687 — 253,687 Long-term aircraft maintenance deposits 150,617 — 150,617 Deferred heavy maintenance, net 99,915 — 99,915 Other long-term assets 121,003 — 121,003 Total assets $ 4,143,950 $ 1,850 $ 4,145,800 Liabilities and shareholders’ equity Current liabilities: Accounts payable $ 22,822 $ — $ 22,822 Air traffic liability 246,404 17,307 263,711 Current maturities of long-term debt 115,430 — 115,430 Other current liabilities 262,370 — 262,370 Total current liabilities 647,026 17,307 664,333 Long-term debt, less current maturities 1,387,498 — 1,387,498 Deferred income taxes 313,140 (4,326 ) 308,814 Deferred gains and other long-term liabilities 19,205 3,376 22,581 Shareholders’ equity: Common stock: Common stock, $0.0001 par value, 240,000,000 shares authorized at December 31, 2017; 69,770,795 issued and 68,196,964 outstanding as of December 31, 2017 7 — 7 Additional paid-in-capital 360,153 — 360,153 Treasury stock, at cost: 1,573,831 shares as of December 31, 2017 (65,854 ) — (65,854 ) Retained earnings 1,484,239 (14,507 ) 1,469,732 Accumulated other comprehensive income (loss) (1,464 ) — (1,464 ) Total shareholders’ equity 1,777,081 (14,507 ) 1,762,574 Total liabilities and shareholders’ equity $ 4,143,950 $ 1,850 $ 4,145,800 December 31, 2016 (in thousands) As Reported Topic 606 Adjustment As Adjusted Assets Current assets: Cash and cash equivalents $ 700,900 $ — $ 700,900 Short-term investment securities 100,155 — 100,155 Accounts receivable, net 41,136 — 41,136 Aircraft maintenance deposits, net 87,035 — 87,035 Income tax receivable — — — Prepaid expenses and other current assets 46,619 1,702 48,321 Total current assets 975,845 1,702 977,547 Property and equipment: Flight equipment 1,461,525 — 1,461,525 Ground property and equipment 126,206 — 126,206 Less accumulated depreciation (122,509 ) — (122,509 ) 1,465,222 — 1,465,222 Deposits on flight equipment purchase contracts 325,688 — 325,688 Long-term aircraft maintenance deposits 199,415 — 199,415 Deferred heavy maintenance, net 75,534 — 75,534 Other long-term assets 110,223 — 110,223 Total assets $ 3,151,927 $ 1,702 $ 3,153,629 Liabilities and shareholders’ equity Current liabilities: Accounts payable $ 15,193 $ — $ 15,193 Air traffic liability 206,392 13,792 220,184 Current maturities of long-term debt 84,354 — 84,354 Other current liabilities 226,011 — 226,011 Total current liabilities 531,950 13,792 545,742 Long-term debt, less current maturities 897,359 — 897,359 Deferred income taxes 308,143 (5,443 ) 302,700 Deferred gains and other long-term liabilities 19,868 2,776 22,644 Shareholders’ equity: Common stock: Common stock, $0.0001 par value, 240,000,000 shares authorized at December 31, 2016; 73,549,872 issued and 69,326,202 outstanding as of December 31, 2016 7 — 7 Additional paid-in-capital 551,004 — 551,004 Treasury stock, at cost: 4,223,670 shares as of December 31, 2016 (218,692 ) — (218,692 ) Retained earnings 1,063,633 (9,423 ) 1,054,210 Accumulated other comprehensive income (loss) (1,345 ) — (1,345 ) Total shareholders’ equity 1,394,607 (9,423 ) 1,385,184 Total liabilities and shareholders’ equity $ 3,151,927 $ 1,702 $ 3,153,629 Operating revenues is comprised of passenger revenues, which includes fare and non-fare revenues, and other revenues. The following table shows disaggregated operating revenues for each quarter of 2018 and 2017. For the Quarter Ended December 31, 2018 September 30, 2018 June 30, 2018 March 31, 2018 December 31, 2017 September 30, 2017 June 30, 2017 March 31, 2017 (unaudited, in thousands) Operating revenues: Fare $ 445,203 $ 476,660 $ 439,549 $ 342,695 $ 337,324 $ 355,593 $ 371,443 $ 299,035 Non-fare 401,365 411,296 396,801 346,446 313,323 313,479 309,437 273,252 Total passenger revenues 846,568 887,956 836,350 689,141 650,647 669,072 680,880 572,287 Other revenues 16,227 16,374 15,421 14,997 15,535 18,155 19,305 17,670 Total operating revenues $ 862,795 $ 904,330 $ 851,771 $ 704,138 $ 666,182 $ 687,227 $ 700,185 $ 589,957 The following table shows disaggregated operating revenues for years ended December 31, 2017 and 2016. Year Ended December 31, 2017 2016 (in thousands) As Reported Topic 606 Adjustment As Adjusted As Reported Topic 606 Adjustment As Adjusted Operating revenues: Fare $ 1,366,034 $ (2,639 ) $ 1,363,395 $ 1,200,621 $ (2,514 ) $ 1,198,107 Non-fare — 1,209,492 1,209,492 — 1,059,694 1,059,694 Total passenger revenues 1,366,034 1,206,853 2,572,887 1,200,621 1,057,180 2,257,801 Other revenues 1,281,632 (1,210,967 ) 70,665 1,121,335 (1,059,115 ) 62,220 Total operating revenues $ 2,647,666 $ (4,114 ) $ 2,643,552 $ 2,321,956 $ (1,935 ) $ 2,320,021

Special Charges

Special Charges	12 Months Ended
	Dec. 31, 2018
Special Charges and Credits [Abstract]
Special Charges	Special Charges Special Charges, Operating During the twelve months ended December 31, 2018 , the Company negotiated and amended the collective bargaining agreement with the Air Line Pilots Association, International ("ALPA"), under the guidance of the National Mediation Board ("NMB"). In connection with the new agreement, the Company incurred a one-time ratification incentive of $80.2 million , including payroll taxes, and an $8.5 million adjustment related to other contractual provisions. As a result, the Company recorded $88.7 million in special charges within operating expenses in the statement of operations for the twelve months ended December 31, 2018 . As of December 31, 2018 , the Company has paid the full ratification incentive related to the Company's new collective bargaining agreement with its pilots. During the twelve months ended December 31, 2017 , the Company purchased one engine and one aircraft which were previously financed under operating lease agreements. The purchase price of the one engine and one aircraft was $8.1 million and $20.0 million , respectively, comprised of a cash payment of $3.8 million and $12.6 million , respectively, and the non-cash application of maintenance and security deposits held by the previous lessor of $4.3 million and $7.4 million , respectively. The Company estimated the fair value of the engine and aircraft to be $3.1 million and $11.9 million , respectively, and has recorded the one purchased engine and one aircraft at fair value within flight equipment on the balance sheets. The Company determined the valuation of the engine and aircraft based on a third-party appraisal considering the condition of the engine and aircraft (a Level 3 measurement). The Company recognized $4.8 million and $7.9 million as a cost of terminating the lease within special charges on the statement of operations, respectively, made up of the excess of the purchase price paid over the fair value of the engine and the aircraft, less other non-cash items of $0.2 million and $0.2 million , respectively. 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 . Special Charges, Non-Operating During the twelve months ended December 31, 2018 , the Company recorded $90.4 million , in special charges, non-operating within other (income) expense in the statement of operations. During the first quarter of 2018, the Company entered into an aircraft purchase agreement for the purchase of 14 A319 aircraft previously operated under operating leases by the Company. The aggregate gross purchase price for the 14 aircraft was $285.0 million , and the price for each aircraft at the time of the sale comprised a cash payment net of the amount of maintenance reserves and security deposits for such aircraft held by the applicable lessor pursuant to the lease for such aircraft. The contract was deemed a lease modification which resulted in a change of classification from operating leases to capital leases for the 14 aircraft. During the first quarter of 2018, the capital lease assets were recorded at the lower of cost or fair value of the aircraft within flight equipment on the Company's balance sheets. During the second quarter of 2018, the purchase of the 14 aircraft was completed and the obligation was accreted up to the net cash payment price with interest charges recognized in special charges, non-operating in the statement of operations. The Company determined the valuation of the aircraft based on third-party appraisals considering the condition of the aircraft (a Level 3 measurement). During the twelve months ended December 31, 2017 and 2016

Letters of Credit

Letters of Credit	12 Months Ended
	Dec. 31, 2018
Financial Instruments Pledged as Collateral [Abstract]
Letters of Credit	Letters of Credit As of December 31, 2018 and 2017 , the Company had a $35.0 million and $35.0 million unsecured standby letter of credit facility, of which $18.1 million and $17.5 million

Credit Card Processing Arrangem

Credit Card Processing Arrangements	12 Months Ended
	Dec. 31, 2018
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, 2018 and 2017 , 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, 2018 and 2017 , was $321.0 million and $286.3 million

Short-term Investment Securitie

Short-term Investment Securities	12 Months Ended
	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Short-term Investment Securities	Short-term Investment Securities The Company's short-term investment securities are classified as available-for-sale and consist of U.S. Treasury and U.S. government agency securities with contractual maturities of twelve months or less. These securities are stated at fair value within current assets on the Company's balance sheet. Realized gains and losses on sales of investments, if any, are reflected in non-operating income (expense) in the statements of operations. Unrealized gains and losses on investment securities are reflected as a component of accumulated other comprehensive income, ("AOCI"). As of December 31, 2018 and December 31, 2017 , the Company had $102.8 million and $100.9 million in short-term available-for-sale investment securities, respectively. During the twelve months ended December 31, 2018 and December 31, 2017 , these investments earned interest income at a weighted-average fixed rate of approximately 1.6% and 1.4% , respectively. For the twelve months ended December 31, 2018 and December 31, 2017 , an unrealized gain of $30 thousand and an unrealized loss of $82 thousand , net of deferred taxes of $44 thousand and $41 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, 2018 and December 31, 2017 , $74 thousand and $105 thousand

Accrued Liabilities

Accrued Liabilities	12 Months Ended
	Dec. 31, 2018
Payables and Accruals [Abstract]
Accrued Liabilities	Accrued Liabilities Accrued liabilities included in other current liabilities as of December 31, 2018 and 2017 consist of the following: As of December 31, 2018 2017 (in thousands) Salaries and wages $ 82,900 $ 54,338 Federal excise and other passenger taxes and fees payable 60,604 42,036 Aircraft maintenance 59,805 33,033 Airport obligations 52,029 56,299 Fuel 25,368 25,171 Interest payable 18,086 11,384 Aircraft and facility lease obligations 15,149 16,992 Other 25,736 23,117 Other current liabilities $ 339,677 $ 262,370

Common Stock and Preferred Stoc

Common Stock and Preferred Stock	12 Months Ended
	Dec. 31, 2018
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, 2018 and 2017 , 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, 2018 and 2017 , there were no

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2018
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, performance share awards and other equity-based instruments as a means of promoting the Company’s long-term growth and profitability. The plans are intended to encourage participants to contribute to, and participate in, the success of the Company. On December 16, 2014, the Company's Board of Directors approved the 2015 Incentive Award Plan, or 2015 Plan, which was subsequently approved by the Company's stockholders on June 16, 2015. 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, 2018 and December 31, 2017 , 2,110,598 and 2,377,650 shares of the Company’s common stock, respectively, remained available for future issuance under the 2015 Plan. Stock-based compensation cost amounted to $11.0 million , $8.5 million and $7.1 million for 2018 , 2017 and 2016 , respectively. During 2018 , 2017 and 2016 there was $2.6 million , $1.6 million and $2.6 million tax benefit recognized in income related to stock-based compensation. Restricted Stock and Restricted Stock Units Restricted stock and restricted stock unit awards are valued at the fair value of the shares on the date of grant. Generally, granted shares and units vest over a three 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, 2018 and changes during the year ended December 31, 2018 is presented below: Number of Shares Weighted-Average Outstanding at December 31, 2017 266,522 50.48 Granted 185,054 46.90 Vested (141,089 ) 51.09 Forfeited (12,690 ) 50.87 Outstanding at December 31, 2018 297,797 47.95 There were 185,054 and 103,030 restricted stock shares granted during the years ended December 31, 2018 and December 31, 2017 , respectively. As of December 31, 2018 and December 31, 2017 , there was $9.1 million and $7.5 million , respectively, of total unrecognized compensation cost related to nonvested restricted stock to be recognized over 2.5 years and 2.0 years , respectively. The weighted-average fair value of restricted stock granted during the years ended December 31, 2018 , 2017 and 2016 was $46.90 , $51.68 and $42.91 , respectively. The total fair value of restricted stock shares vested during the years ended December 31, 2018 , 2017 and 2016 was $6.5 million , $4.3 million and $6.6 million respectively. Performance Share Awards The Company grants certain senior-level executives performance stock units that vest based on either market and time-based service conditions or performance and time-based service conditions as part of a long-term incentive plan, which are referred to herein as performance share awards. The number of shares of common stock underlying each award is determined at the end of a three-year performance period. In order to vest, the senior level executive must still be employed by the Company, with certain contractual exclusions, at the end of the performance period. Depending on the type of performance stock unit, at the end of the performance period, the percentage of the stock units that will vest will be determined by ranking the Company’s total shareholder return compared to the total shareholder return of the peer companies identified in the plan or by ranking the Company's adjusted operating margin percentage compared to the adjusted operating margin percentage of the peer company's identified in the plan. Based on the level of performance, between 0% and 200% of the award may vest. Within 60 days after vesting, the shares underlying the award will be issued to the participant. In the event of a change in control of the Company or the death or permanent disability of a participant, the payout of any award is limited to a pro-rated portion of such award based upon a performance assessment prior to the change-in-control date or date of death or permanent disability. The grant date fair value of the performance share awards based on total shareholder return (market condition) is determined through the use of a Monte Carlo simulation model. The market condition requirements are reflected in the grant date fair value of the award, and the compensation expense, net of forfeitures, for the award is recognized assuming that the requisite service is rendered regardless of whether the market conditions are achieved. Compensation expense is recognized on a straight-line basis over the requisite service period. The Monte Carlo simulation model used for valuation of these awards utilizes multiple input variables that determine the probability of satisfying the market condition requirements applicable to each award. The inputs utilized for the performance share awards based on total shareholder return are as follows: Weighted-Average at Grant Date for Twelve Months Ended December 31, 2018 Weighted-Average at Grant Date for Twelve Months Ended December 31, 2017 Expected volatility factor 0.39 0.40 Risk free interest rate 2.11 % 1.47 % Expected term (in years) 2.96 2.93 Expected dividend yield — % — % For grants awarded in 2018, 2017 and 2016, 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, 2018 : Number of Awards Weighted-Average Fair Value at Grant Date ($) Outstanding at December 31, 2017 120,359 52.84 Granted 52,074 52.07 Vested (83,977 ) 50.88 Forfeited (5,549 ) 59.01 Outstanding at December 31, 2018 82,907 53.92 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, 2018 : Number of Awards Weighted-Average Fair Value at Grant Date ($) Outstanding at December 31, 2017 35,507 45.72 Granted 26,037 46.21 Vested (18,717 ) 39.18 Forfeited (1,373 ) 51.04 Outstanding at December 31, 2018 41,454 49.81 As of December 31, 2018 and 2017 , there was $3.4 million and $3.7 million , respectively, of total unrecognized compensation cost related to performance share awards expected to be recognized over 1.80 years and 1.52 years , respectively. Stock Appreciation Rights During 2018, the Company issued stock appreciation awards to certain senior-level executives. These awards have a four -year service requisite period from January 1, 2018 through December 31, 2021 and a two -year performance period from January 1, 2018 through December 31, 2019. This is a market-condition performance award based on the appreciation of the Company's stock price over the two -year performance period. Issuance of the award on January 1, 2018 represents a right to receive shares of the Company's common stock upon achievement of certain performance goals. The number of shares of common stock to be granted under each award is determined at grant date of December 31, 2019. The grant-date fair value of the stock appreciation award is derived using 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 the awards. Expense is recognized over the four -year service requisite period commencing on January 1, 2018 and ending on December 31, 2021. Since the service inception date of January 1, 2018 precedes the grant date of December 31, 2019, the Company will revalue the awards at each reporting date within the two -year period from the service inception date to the grant date and adjust the expense to reflect the valuation as of period end. Beginning on the grant date, once the number of shares are determined, and through the end of the vesting period, expense will be recognized on a straight-line basis over the remaining two -year service requisite period ending on vest date of December 31, 2021. Based on the level of performance, between 0% and 370% of the award may vest. 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. 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. During the twelve months ended December 31, 2018 , the Company recognized $1.2 million of stock-based compensation cost related to the stock appreciation awards granted during 2018. Treasury Stock During the year ended December 31, 2018 and 2017 , the Company repurchased 28 thousand and 1.2 million shares, respectively, for $1.2 million and $46.6 million , respectively. Repurchases made during the twelve months ended December 31, 2018 include repurchases made from employees who received restricted stock. Repurchases made during the twelve months ended December 31, 2017 include open market repurchases made under our stock repurchase program as well as repurchases made from employees who received restricted stock. During the year ended December 31, 2018 , the Company did not retire any treasury shares. During the year ended December 31, 2017 , the Company retired 3.9 million treasury shares in a total aggregate amount of $199.4 million

Earnings per Share

Earnings per Share	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Earnings per Share	Earnings per Share The following table sets forth the computation of basic and diluted earnings per common share: Year Ended December 31, 2018 2017 2016 (in thousands, except per-share amounts) Numerator: Net income (1) $ 155,749 $ 415,522 $ 263,483 Denominator: Weighted-average shares outstanding, basic 68,249 69,221 70,344 Effect of dilutive stock awards 182 156 164 Adjusted weighted-average shares outstanding, diluted 68,431 69,377 70,508 Earnings per Share: Basic earnings per common share (1) $ 2.28 $ 6.00 $ 3.75 Diluted earnings per common share (1) $ 2.28 $ 5.99 $ 3.74 Anti-dilutive weighted-average shares 145 85 66 (1) During the twelve months ended December 31, 2017 , the Company recorded a non-recurring income tax benefit of $196.7 million ( $2.84 and $2.84

Debt and Other Obligations

Debt and Other Obligations	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Debt and Other Obligations	Debt and Other Obligations Long-term debt As of December 31, 2018 , the Company had outstanding non-public and public debt instruments. During 2018 , the Company acquired additional debt through the 2015-1 and 2017-1 EETCs, fixed-rate term loan facility agreements and revolving credit facility described below. 2017-1 Class AA, Class A and Class B 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 is 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 commenced on August 15, 2018 in the case of five new Airbus A321 delivered from February 2018 to March 2018 and three Airbus A320 delivered from December 2017 to January 2018 and will commence on February 15, 2019 for four Airbus A320 aircraft delivered 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, 2018 , all of the proceeds from the sale of the Series 2017-1 Class AA, Class A and Class B EETCs had been used to purchase equipment notes in connection with the financing of five Airbus A321 aircraft and seven Airbus A320 aircraft. Equipment notes that are issued are reported as long-term debt on the Company's balance sheets. 2015-1C and 2017-1C EETCs In May 2018, the Company completed a private placement of an aggregate amount of $115.2 million pass-through certificates, Series 2015-1C. The Company entered into 15 separate participation agreement amendments to existing participation agreements that were entered into by the Company during the period from October 2015 to February 2017 under the existing pass through trust formed by the Company on August 11, 2015. The Series 2015-1C equipment notes are secured by 12 Airbus A321 aircraft previously delivered from October 2015 to January 2017 and 3 Airbus A320 aircraft previously delivered from March 2016 to June 2016. The Series 2015-1C equipment notes mature in April 2023 and accrue interest at a rate of 4.93% . Principal and interest on the issued and outstanding Series 2015-1C equipment notes is payable semiannually on April 1 and October 1 of each year, commencing on October 1, 2018. Equipment notes that are issued are reported as long-term debt on the Company's balance sheets. In May 2018, the Company also completed a private placement of an aggregate amount of $85.5 million pass-through certificates, Series 2017-1C. The Company entered into 9 separate participation agreement amendments to existing participation agreements that were entered into by the Company during the period from December 2017 to April 2018 under the existing pass through trust formed by the Company on November 28, 2017. The participation agreement amendments provide for the issuance of Series 2017-1C equipment notes, in the aggregate principal amount of $65.7 million in connection with previously delivered aircraft. The 2017-1C equipment notes are secured by five Airbus A321 aircraft previously delivered from February 2018 to March 2018 and four Airbus A320 aircraft previously delivered from December 2017 to April 2018. The Series 2017-1C equipment notes mature in February 2023 and accrue interest at a rate of 5.11% . Interest on the Class C 2017-1 issued and outstanding equipment notes are payable semiannually on February 15 and August 15 of each year, commencing on August 15, 2018. The entire principal on the issued and outstanding Series 2017-1C equipment notes is scheduled for payment on February 15, 2023. As of December 31, 2018 , the remaining $19.8 million of the proceeds from the sale of the Series 2017-1 Class C had been used to purchase equipment notes in connection with the financing of three Airbus A320 aircraft delivered from August 2018 to October 2018. Equipment notes that are issued are reported as long-term debt on the Company's 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. Fixed-rate term loans During 2018, the Company entered into facility agreements with banks, which as of December 31, 2018 provided $139.5 million of debt financing for 4 Airbus A320 aircraft delivered during the fourth quarter of 2018. Each loan extended under the facility agreements was funded on or near the delivery date of each aircraft and is secured by a first-priority security interest on the individual aircraft. Each loan has a term life ranging from 10 to 12 years and amortizes on a mortgage-style basis, which requires quarterly principal and interest payments. Loans bear interest on a fixed-rate basis with interest rates ranging between 4.06% and 4.10% . As of December 31, 2018 , the Company has taken delivery of all 4 Airbus A320 aircraft financed through these facility agreements. Revolving credit facility During the fourth quarter of 2018, the Company entered into a revolving credit facility for up to $160 million secured by the collateral assignment of certain of the Company's rights under the purchase agreement with Airbus, related to 43 Airbus A320neo aircraft scheduled to be delivered between August 2019 and December 2021. The final maturity of the facility is December 30, 2020. As of December 31, 2018 , the Company had drawn $135.3 million on the facility which is included in long-term debt and capital leases, less current maturities on the Company's balance sheets. The revolving credit facility bears variable interest based on LIBOR. Long-term debt is comprised of the following: As of Year Ended December 31, 2018 2017 2018 2017 (in millions) (weighted-average interest rates) Fixed-rate senior term loans due through 2027 $ 382.4 $ 417.9 4.10 % 4.10 % Fixed-rate junior term loans due through 2022 31.1 39.3 6.90 % 6.90 % Fixed-rate loans due through 2030 625.1 518.0 3.88 % 3.83 % Fixed-rate class A 2015-1 EETC due through 2028 378.6 408.6 4.10 % 4.10 % Fixed-rate class B 2015-1 EETC due through 2024 80.0 92.0 4.45 % 4.45 % Fixed-rate class C 2015-1 EETC due through 2023 109.5 — 4.93 % N/A Fixed-rate class AA 2017-1 EETC due through 2030 242.5 37.5 3.38 % 3.38 % Fixed-rate class A 2017-1 EETC due through 2030 80.8 12.5 3.65 % 3.65 % Fixed-rate class B 2017-1 EETC due through 2026 83.7 13.8 3.80 % 3.80 % Fixed-rate class C 2017-1 EETC due through 2023 85.5 — 5.11 % N/A Revolving credit facility due in 2020 135.3 — 3.72 % N/A Long-term debt $ 2,234.5 $ 1,539.6 Less current maturities 162.8 115.4 Less unamortized discount, net 47.7 36.7 Total $ 2,024.0 $ 1,387.5 The Company's debt financings are collateralized by first priority security interest in the individual aircraft being financed with the exception of the Company's revolving credit facility secured by the Company's rights under the purchase agreement with Airbus related to certain A320neo aircraft. During the year ended December 31, 2018 and 2017 , the Company made scheduled principal payments of $137.3 million and $102.3 million on its outstanding debt obligations, respectively. At December 31, 2018 , long-term debt principal payments for the next five years and thereafter are as follows: December 31, 2018 (in millions) 2019 $ 171.3 2020 303.6 2021 162.1 2022 159.5 2023 298.0 2024 and beyond 1,140.0 Total debt principal payments $ 2,234.5 Interest Expense Interest expense related to long-term debt and capital leases consists of the following: Year Ended December 31, 2018 2017 (in thousands) Fixed-rate senior term loans $ 16,869 $ 18,328 Fixed-rate junior term loans 2,475 3,035 Fixed-rate term loans 19,615 8,610 Class A 2015-1 EETC 16,138 17,230 Class B 2015-1 EETC 3,781 4,446 Class C 2015-1 EETC 3,575 — Class AA 2017-1 EETC 6,026 54 Class A 2017-1 EETC 2,172 19 Class B 2017-1 EETC 2,420 22 Class C 2017-1 EETC 2,478 — Revolving credit facility 793 — Capital leases 104 154 Commitment fees 262 124 Amortization of deferred financing costs 7,069 5,280 Total $ 83,777 $ 57,302 As of December 31, 2018 and 2017 , the Company had a line of credit for $33.6 million and $33.6 million related to corporate credit cards. Respectively, the Company had drawn $3.5 million and $1.7 million as of December 31, 2018 and 2017 , which is included in accounts payable. As of December 31, 2018 and 2017 , the Company had lines of credit with counterparties for derivatives and physical fuel delivery in the amount of $41.5 million and $51.5 million , respectively. As of December 31, 2018 and 2017 , the Company had drawn $23.0 million and $24.2 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, 2018 and 2017

Leases and Prepaid Maintenance

Leases and Prepaid Maintenance Deposits	12 Months Ended
	Dec. 31, 2018
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 2018 , 2017 and 2016 was $312.0 million , $309.8 million and $283.9 million , respectively. Total rental expense charged to operations for aircraft and engine operating leases for the years ended December 31, 2018 , 2017 and 2016 was $177.6 million , $205.9 million and $201.7 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 $5.8 million in 2019 , $5.6 million in 2020 , $5.7 million in 2021 , $4.9 million in 2022 , $4.1 million in 2023 , and $8.8 million in 2024 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, 2018 , 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 $3.4 million , $8.4 million and $9.0 million of supplemental rent recorded within aircraft rent during 2018 , 2017 and 2016 , respectively. These amounts include $1.3 million , $0.4 million and $2.2 million of paid maintenance reserves expensed as supplemental rent during 2018 , 2017 and 2016 , respectively. As of December 31, 2018 , the Company had $245.6 million ( $106.9 million in aircraft maintenance deposits and $138.7 million in long-term aircraft maintenance deposits) on the accompanying balance sheet. As of December 31, 2018 , the Company had a fleet consisting of 128 A320 family aircraft. As of December 31, 2018 , the Company had 46 aircraft financed under operating leases with lease term expirations between 2021 and 2030 and owned 82 aircraft of which 22 were purchased off lease and are currently unencumbered. In addition, as of December 31, 2018 , the Company had 12 spare engines financed under operating leases with lease term expiration dates ranging from 2019 to 2027 and owned 8 spare engines of which 1 was purchased off lease and all 8 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 its 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, 2018 , the Company took delivery of 14 aircraft under secured debt arrangements, purchased 14 previously leased aircraft, and took delivery of 2 aircraft under operating leases. The Company also purchased 10 engines, and took delivery of 1 engine under an operating lease. In addition, during 2018, the Company sold 6 used engines for $11.4 million at a loss of $5.2 million which is recorded within loss on disposal of assets in the statement of operations. 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 Company's capital lease obligations relate to the lease of computer equipment used by the Company's flight crew and office equipment. Payments under the Company's capital lease agreements are fixed for terms ranging from 3 to 5 years. Future minimum lease payments under capital leases and noncancellable operating leases with initial or remaining terms in excess of one year at December 31, 2018 were as follows: Capital Leases Operating Leases Total Operating and Capital Lease Obligations Aircraft and Spare Engine Leases Property Facility Leases (in thousands) 2019 $ 810 $ 176,256 $ 52,627 $ 229,693 2020 408 170,106 29,962 200,476 2021 186 168,416 17,004 185,606 2022 158 155,135 14,718 170,011 2023 145 133,915 13,787 147,847 2024 and thereafter — 350,266 89,536 439,802 Total minimum lease payments $ 1,707 $ 1,154,094 $ 217,634 $ 1,373,435 Less amount representing interest 152 Present value of minimum lease payments $ 1,555 Less current portion 721 Long-term portion $ 834

Financial Instruments and Risk

Financial Instruments and Risk Management	12 Months Ended
	Dec. 31, 2018
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, 2018 and 2017 , 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 jet fuel swaps and 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 has not historically elected hedge accounting on its fuel derivative instruments. As of December 31, 2018 and 2017 , the Company did not have any outstanding fuel derivatives and had no fuel hedging activity for the twelve months ended December 31, 2018 , 2017 and 2016 . Interest Rate Swaps From time to time, the Company may enter into interest rate swaps to fix the benchmark interest rate component of interest payments or for other reasons. These instruments limit the Company's exposure to changes in the benchmark interest rate in the period from the trade date through the date of maturity. Interest rate swaps may be designated as cash flow hedges. The Company generally 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 AOCI if designated as cash flow hedges. As of December 31, 2018 and 2017 , the Company did not have any outstanding interest rate swaps. Realized gains and losses from cash flow hedges are recorded in the statements 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, 2018 and December 31, 2017 , 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, 2018 , the Company reclassified interest rate swap losses of $316 thousand , into earnings, resulting in a gain of $241 thousand , net of tax of $75 thousand . 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 . As of December 31, 2018 and December 31, 2017 , $1.1 million and $1.4 million

Defined Contribution 401(k) Pla

Defined Contribution 401(k) Plan	12 Months Ended
	Dec. 31, 2018
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, dispatchers and mechanics 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, dispatcher and mechanic'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. This plan is subject to the 2018 annual IRS elective deferral limits of $18,500 for employee contributions and $55,000 for both employee and employer contributions combined. The second plan is for the Company’s pilots, and contains the same service requirements as the first plan. Prior to March 1, 2018, the Company matched 100% of the pilot's contribution, up to 9% of the individual pilot's annual compensation. Beginning on March 1, 2018, the Company contributes 11% of the individual pilot's annual compensation, regardless of the pilot's contributions to the plan. The Company's contribution will increase by 1% on an annual basis each March until 2022 at which time the contribution will be 15% . In 2018, the Company's contributions to the plan are limited by the IRS annual compensation limit of $275,000 . In addition, this plan is subject to the 2018 annual IRS pre-tax elective deferral limits of $18,500 for employee contributions and $55,000 for both employee and employer contributions combined. 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, prior to March 1, 2018, the Company matched 100% of their contribution, up to 9% of the individual pilot's annual compensation. Beginning on March 1, 2018, the Company contributes 11% of the individual pilot's annual compensation, regardless of the pilot's contributions to the plan. The Company's contribution will increase by 1% on an annual basis each March until 2022 at which time the contribution will be 15% . 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. In 2018, the Company's contributions to the plan are limited by the IRS annual compensation limit of $275,000 . In addition, this plan is subject to the 2018 annual Puerto Rico pre-tax elective deferral limits of $15,000 for employee contributions and $55,000 for both employee and employer contributions combined. Employer contributions made to all plans were $36.7 million , $19.6 million and $16.2 million in 2018 , 2017 and 2016

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
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, 2018 2017 2016 (in thousands) Current: Federal $ (2,178 ) $ (68,601 ) $ 60,079 State and local 410 515 6,322 Foreign 4,692 2,742 2,034 Total current expense (benefit) 2,924 (65,344 ) 68,435 Deferred: Federal 42,246 (9,349 ) 81,682 State and local 4,057 8,857 3,657 Total deferred expense (benefit) 46,303 (492 ) 85,339 Total income tax expense (benefit) $ 49,227 $ (65,836 ) $ 153,774 The income tax provision differs from that computed at the federal statutory corporate tax rate as follows: Year Ended December 31, 2018 2017 2016 Expected provision at federal statutory tax rate 21.0 % 35.0 % 35.0 % State tax expense, net of federal benefit 1.7 % 1.7 % 1.6 % Revaluation of deferred taxes — % (56.3 )% — % Other 1.3 % 0.7 % 0.3 % Total income tax expense (benefit) 24.0 % (18.9 )% 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, 2018 and 2017 , the significant components of the Company's deferred taxes consisted of the following: December 31, 2018 2017 (in thousands) Deferred tax assets: Income tax credits $ 10,004 $ 5,980 Net operating losses 155,670 871 Deferred revenue 6,824 5,957 Nondeductible accruals 14,691 10,107 Deferred manufacturing credits — 258 Accrued maintenance 2,168 1,991 Equity compensation 2,592 2,392 Other 5,262 4,684 Valuation allowance (254 ) (454 ) Deferred tax assets 196,957 31,786 Deferred tax liabilities: Deferred gain (loss) on leases, net 1,672 1,605 Accrued rent 6,068 12,055 Prepaid expenses 793 754 Property, plant and equipment 481,847 298,703 Deferred financing costs 189 225 Accrued aircraft and engine maintenance 61,529 27,258 Deferred tax liabilities 552,098 340,600 Net deferred tax assets (liabilities) $ (355,141 ) $ (308,814 ) 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 reduction in net deferred tax liabilities of $196.0 million upon enactment of the TCJA. As of December 31, 2018 , the Company has completed its accounting for all of the enactment date income tax effects of the TCJA. In assessing the realizability of the deferred tax assets, management considered whether it is more likely than not that some or all of the deferred tax assets would be realized. In evaluating the Company’s ability to utilize its deferred tax assets, it considered all available evidence, both positive and negative, in determining future taxable income on a jurisdiction by jurisdiction basis. As of December 31, 2018 and 2017 , the Company had a valuation allowance of $0.3 million and $0.5 million , respectively, against certain deferred tax assets related to equity compensation for executives due to changes in tax law resulting from the TCJA. At December 31, 2018 , the Company had $8.6 million of foreign tax credits and $1.4 million of general business tax credits, $703.0 million of federal net operating loss and $114.5 million of state net operating loss available, that may be applied against future tax liabilities. The foreign tax credits will begin to expire in 2025 , the state net operating losses will begin to expire in 2027 , the general business credits will begin to expire in 2038 and there is no expiration of federal net operating losses. In addition, as of December 31, 2018 , the Company had $1.1 million of alternative minimum tax credits, which were recorded within other long-term assets on the balance sheet. 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, 2018 and 2017, $0.4 million and $0.5 million of income tax deficiency related to share-based compensation was included within income tax expense, respectively. Prior to the adoption of ASU No. 2016-09, the excess tax benefit/(deficiency) was recorded as a reduction/(increase) to income tax payable and a corresponding entry to additional paid in capital. The Company recognized an excess tax deficiency of $0.5 million for the tax year ended December 31, 2016 . For tax years ended December 31, 2018 , 2017 and 2016 , the Company did not recognize any liabilities for uncertain tax positions nor any interest and penalties on unrecognized tax benefits. For tax years 2018 , 2017 and 2016 , all income for the Company is subject to domestic income taxes. The Company files its tax returns as prescribed by the tax laws of the jurisdictions in which it operates. The Company's federal income tax returns for 2015 through 2017

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
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 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 revisions, 5 of the 14 scheduled A320neo aircraft were converted to A320ceo aircraft, of which 3 were delivered in the fourth quarter of 2018 and the remaining 2 are scheduled to be delivered in 2019. As of December 31, 2018 , the Company's firm aircraft orders consisted of the following: Airbus Third-Party Lessor A320ceo A320neo A320neo Total 2019 2 9 5 16 2020 16 16 2021 18 18 2 43 5 50 During the first quarter of 2018, the Company entered into an aircraft purchase agreement for the purchase of 14 A319 aircraft, which were previously financed under operating lease agreements. The contract was deemed a lease modification which resulted in a change in classification from operating leases to capital leases for the 14 aircraft. As a result, the Company recorded a short-term capital lease asset of $236.7 million within flight equipment and a short-term capital lease obligation of $143.8 million , net of the related maintenance reserves and security deposits, within current maturities of long-term debt and capital leases on the Company's balance sheet as of March 31, 2018. The purchase of all 14 aircraft was completed during the second quarter of 2018 for an aggregate gross purchase price of $285.0 million , which was comprised of cash payments, net of the application of cash maintenance and security deposits held by the previous lessor. For additional information, refer to Note 5, Special Charges. The Company also has two spare engine orders for V2500 SelectTwo engines with IAE and eight spare engine orders for PurePower PW 1100G-JM engines with Pratt & Whitney. Spare engines are scheduled for delivery from 2019 through 2024 . Purchase commitments for these aircraft and engines, including estimated amounts for contractual price escalations and pre-delivery payments, are expected to be $583.8 million in 2019 , $811.4 million in 2020 , $775.6 million in 2021 , $17.7 million in 2022 , $8.4 million in 2023 , and $0.0 million in 2024 and beyond . As of December 31, 2018 , the Company had secured debt financing commitments of $70.0 million for 2 aircraft, scheduled for delivery in 2019. In addition, as of December 31, 2018 , the Company had secured financing for five aircraft to be leased directly from a third-party lessor, scheduled for delivery in 2019. Aircraft rent commitments for these 5 aircraft are expected to be approximately $18.2 million in 2019, $20.2 million in 2020, $20.2 million in 2021, $20.2 million in 2022, $20.2 million in 2023, and $143.3 million in 2024 and beyond. The Company did not have financing commitments in place for the remaining 43 Airbus aircraft currently on firm order, which are scheduled for delivery in 2019 through 2021 . Interest commitments related to the secured debt financing of 60 delivered aircraft as of December 31, 2018 are $83.3 million in 2019 , $76.2 million in 2020 , $69.2 million in 2021 , $62.4 million in 2022 , $52.3 million in 2023 , and $150.3 million in 2024 and beyond . For principal commitments related to these financed aircraft, refer to Note 13, Debt and Other Obligations. As of December 31, 2018 , principal and interest commitments related to the Company's future secured debt financing of two undelivered aircraft are approximately $5.7 million in 2019 , $9.4 million in 2020 , $8.1 million in 2021 , $7.1 million in 2022 , $7.1 million in 2023 , and $52.4 million in 2024 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 (the "reconfiguration"). The reconfiguration of the aircraft commenced in the first quarter of 2016 and was completed in the second quarter of 2018. As of December 31, 2018 , the Company had no further commitments related to this agreement. The amounts related to the reconfiguration are capitalized within flight equipment on the Company's balance sheet. The Company is contractually obligated to pay the following minimum guaranteed payments for its reservation system and other miscellaneous subscriptions and services as of December 31, 2018 : $14.4 million in 2019 , $14.3 million in 2020 , $11.3 million in 2021 , $11.2 million in 2022 , $11.3 million in 2023 , and $56.0 million in 2024 and beyond . During the first quarter of 2018, the Company entered into a contract renewal with its reservation system provider which expires in 2028. Litigation The Company is subject to commercial litigation claims and to administrative and regulatory proceedings and reviews that may be asserted or maintained from time to time. The Company believes the ultimate outcome of such lawsuits, proceedings and reviews will not, individually or in the aggregate, have a material adverse effect on its financial position, liquidity or results of operations. Employees The Company has five union-represented employee groups that together represent approximately 80% of all employees at December 31, 2018 . The Company had four union-represented employee groups that together represented approximately 75% of all employees at December 31, 2017 . The table below sets forth the Company's employee groups and status of the collective bargaining agreements as of December 31, 2018 . Employee Groups Representative Amendable Date Percentage of Workforce Pilots Air Line Pilots Association, International (ALPA) February 2023 27% Flight Attendants Association of Flight Attendants (AFA-CWA) May 2021 45% Dispatchers Professional Airline Flight Control Association (PAFCA) October 2023 1% Ramp Service Agents International Association of Machinists and Aerospace Workers (IAMAW) June 2020 4% Passenger Service Agents Transport Workers Union of America (TWU) NA 3% 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 agreement. In February 2018 , the pilot group voted to approve the new five -year agreement with the Company. The new agreement includes a one-time ratification incentive of $80.2 million , including payroll taxes, and an $8.5 million adjustment related to other contractual provisions which was recorded in special charges within operating expenses in the statement of operations for the year ended 2018. For additional information, refer to Note 5, Special Charges. 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 , PAFCA filed an application with the NMB seeking to represent our dispatchers, who were previously represented by the TWU. In January 2018 , the NMB determined that a representation election would be held. The voting period for the representation election took place through February 20, 2018 and the dispatchers elected to be represented by the PAFCA. In June 2018, the Company commenced negotiations with PAFCA for an amended agreement with its dispatchers. In October 2018, PAFCA and the Company reached a tentative agreement for a new five -year agreement, which was ratified by the PAFCA members in October 2018. In June 2018, the NMB notified the Company that the TWU filed an application seeking a representation election for the Company's passenger service agents. The NMB determined that a representation election would be held and the voting period for the election took place through September 4, 2018. The Company's passenger service agents voted to be represented by the TWU, but the representation applies only to the Company's Fort Lauderdale station where the Company has direct employees in the passenger service classification. The Company and the TWU began meeting in late October 2018 to negotiate an initial collective bargaining agreement. As of December 31, 2018, the Company continued to negotiate with the TWU. The Company is self-insured for health care claims, subject to a stop-loss policy, for eligible participating employees and qualified dependent medical claims, subject to deductibles and limitations. The Company’s liabilities for claims incurred but not reported are determined based on an estimate of the ultimate aggregate liability for claims incurred. The estimate is calculated from actual claim rates and adjusted periodically as necessary. The Company has accrued $4.4 million and $3.9 million , for health care claims as of December 31, 2018 , and 2017

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2018
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, if any, 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 has not historically elected hedge accounting on its fuel derivative instruments. As of December 31, 2018 and 2017 , the Company had no outstanding jet fuel derivatives. Long-term Debt The estimated fair value of the Company's term loan 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 and non-publicly held EETC debt agreements has been determined to be Level 2 as the Company utilizes quoted market prices in markets with low trading volumes to estimate the fair value of its Level 2 long-term debt. The carrying amounts and estimated fair values of the Company's long-term debt at December 31, 2018 and December 31, 2017 , were as follows: As of December 31, 2018 2017 Carrying Value Estimated Fair Value Carrying Value Estimated Fair Value Fair value level hierarchy (in millions) Fixed-rate senior term loans $ 382.4 $ 373.6 $ 417.9 $ 435.3 Level 3 Fixed-rate junior term loans 31.1 31.1 39.3 40.4 Level 3 Fixed-rate term loans 625.1 600.1 518.0 528.6 Level 3 2015-1 EETC Class A 378.6 374.8 408.6 420.9 Level 2 2015-1 EETC Class B 80.0 78.1 92.0 94.2 Level 2 2015-1 EETC Class C 109.5 107.9 — — Level 2 2017-1 EETC Class AA 242.5 228.8 37.5 37.4 Level 2 2017-1 EETC Class A 80.8 76.6 12.5 12.6 Level 2 2017-1 EETC Class B 83.7 79.1 13.8 13.8 Level 2 2017-1 EETC Class C 85.5 84.2 — — Level 2 Revolving credit facility 135.3 135.3 — — Level 3 Total long-term debt $ 2,234.5 $ 2,169.6 $ 1,539.6 $ 1,583.2 Cash and Cash Equivalents Cash and cash equivalents at December 31, 2018 and December 31, 2017 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, 2018 and December 31, 2017 are classified as available-for-sale and consist of U.S. Treasury and U.S. government agency securities with contractual maturities of twelve months or less. The Company's short-term investment securities are categorized as Level 1 instruments, as the Company uses quoted market prices in active markets when determining the fair value of these securities. For additional information, refer to Note 8, Short-term Investment Securities. Assets and liabilities measured at gross fair value on a recurring basis are summarized below: Fair Value Measurements as of December 31, 2018 Total Level Level Level (in millions) Cash and cash equivalents $ 1,004.7 $ 1,004.7 $ — $ — Short-term investment securities 102.8 102.8 — — Total assets $ 1,107.5 $ 1,107.5 $ — $ — Total liabilities $ — $ — $ — $ — 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 $ — $ — $ — $ — The Company had no transfers of assets or liabilities between any of the above levels during the years ended December 31, 2018 or 2017

Operating Segments and Related

Operating Segments and Related Disclosures	12 Months Ended
	Dec. 31, 2018
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: 2018 2017 2016 (in millions) DOT—Domestic $ 2,990.7 $ 2,432.1 $ 2,134.4 DOT—Latin America and Caribbean 332.3 211.5 185.6 Total $ 3,323.0 $ 2,643.6 $ 2,320.0 During 2018 , 2017 and 2016 , no revenue from any one foreign country represented greater than 4%

Quarterly Financial Data (Unaud

Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Data (Unaudited)	Quarterly Financial Data (Unaudited) Quarterly results of operations for the years ended December 31, 2018 and 2017 are summarized below: Three Months Ended March 31 June 30 September 30 December 31 (in thousands, except per-share amounts) 2018 Operating revenue $ 704,138 $ 851,771 $ 904,330 $ 862,795 Operating income (loss) (38,797 ) 108,521 145,125 136,065 Net income (loss) (44,922 ) 11,254 97,480 91,937 Basic earnings (loss) per share (0.66 ) 0.16 1.43 1.35 Diluted earnings (loss) per share (0.66 ) 0.16 1.42 1.34 2017 Operating revenue $ 589,957 $ 700,185 $ 687,227 $ 666,182 Operating income 58,342 131,312 103,874 91,296 Net income (1) 31,261 77,241 60,044 246,975 Basic earnings per share (1) 0.45 1.11 0.87 3.59 Diluted earnings per share (1) 0.45 1.11 0.86 3.58 (1) During the fourth quarter of 2017, the Company recorded a non-recurring income tax benefit of $196.7 million ( $2.86 and $2.85

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
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 are classified as available-for-sale and consist of U.S. Treasury and U.S. government agency securities with contractual maturities of twelve months or less. These securities are stated at fair value within current assets on the Company's balance sheet. For all short-term investments, at each reset period or upon reinvestment, the Company accounts for the transaction as proceeds from the maturity of short-term investment securities for the security relinquished, and purchase of short-term investment securities for the security purchased, in the Company's statements of cash flows. Realized gains and losses on sales of investments, if any, are reflected in non-operating income (expense) in the statements of operations. Unrealized gains and losses on investment securities are reflected as a component of accumulated other comprehensive income.
Accounts Receivable	Accounts 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	The Company records amortization of capitalized software on a straight-line basis within depreciation and amortization expense in the accompanying statements of operations.The Company 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. 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, 2018 , the Company had 82 aircraft, 8 spare engines and 1 flight simulator capitalized within flight equipment with depreciable lives of 25 years . As of December 31, 2018 , the Company had 46 aircraft financed through operating leases with lease terms from 8 to 18 years and 12 spare engines financed through operating leases with lease terms from 2 to 14 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.
Revenue from Contract with Customer	The Company's co-brand credit card agreement provides for joint marketing where cardholders earn mileage credits for making purchases using co-branded cards. During 2015, the Company extended its agreement with the administrator of the FREE SPIRIT affinity credit card program to extend through 2022. The Company accounts for this agreement consistently with the accounting method that allocates the consideration received to the individual products and services delivered. The value is allocated based on the relative selling prices of those products and services, which generally consists of (i) travel miles to be awarded, (ii) licensing of brand and access to member lists and (iii) advertising and marketing efforts. The Company determined the best estimate of the selling prices by considering discounted cash flow analysis using multiple inputs and assumptions, including: (1) the expected number of miles awarded and number of miles redeemed, (2) ETV for the award travel obligation, (3) licensing of brand and access to member lists and (4) advertising and marketing efforts. Other Revenues Fare revenues. Tickets sold are initially deferred within air traffic liability on the Company's balance sheet. Passenger fare revenues are recognized at time of departure when transportation is provided. All tickets sold by the Company are nonrefundable. An unused ticket expires at the date of scheduled travel and is recognized as revenue at the date of scheduled travel. As of December 31, 2018 and 2017 , the Company had air traffic liability ("ATL") balances of $292.0 million and $263.7 million , respectively. As of December 31, 2018, all of the ATL balance as of December 31, 2017 has been recognized. The Company adopted ASU 2014-09 on January 1, 2018 utilizing the full retrospective method of adoption. Passenger revenues reported prior to the adoption of ASU 2014-09 are now reported as fare revenues within passenger revenues in the Company's disaggregated revenue table within Note 4, Revenue Disaggregation. Refer to Note 3, Revenue Recognition and 4, Revenue Disaggregation for information regarding the Company's adoption of ASU 2014-09. Non-fare revenues.
Frequent Flier Program	Frequent Flyer Program The Company's frequent flyer program generates customer loyalty by rewarding customers with mileage credits to travel on Spirit. When traveling, customers earn redeemable mileage credits for each mile flown on Spirit. Customers can also earn mileage credits through participating companies such as the co-branded Spirit credit card. Mileage credits are redeemable by customers in future periods for air travel on Spirit. To reflect the mileage credits earned, the program includes two types of transactions that are considered revenue arrangements with multiple performance obligations: (1) mileage credits earned with travel and (2) mileage credits sold to co-branded credit card partner. The adoption of ASU 2014-09 eliminated the incremental cost method for frequent flyer program accounting, which required the Company to re-value and record a liability associated with customer flight miles earned with travel as part of the Company’s frequent flyer program with a relative fair value. Upon adoption of ASU 2014-09 on January 1, 2018, the Company recorded an increase to its air traffic liability of $12.4 million . Passenger ticket sales earning mileage credits. Passenger ticket sales earning mileage credits provide customers with (1) mileage credits earned and (2) air transportation. The Company values each performance obligation on a standalone basis. To value the mileage credits earned, the Company considers the quantitative value a passenger receives by redeeming miles for a ticket rather than paying cash, which is referred to as equivalent ticket value ("ETV"). The Company defers revenue for the mileage credits when earned and recognizes loyalty travel awards in passenger revenue as the miles are redeemed and services are provided. The Company records the air transportation portion of the passenger ticket sales in air traffic liability and recognizes passenger revenue when transportation is provided or if the ticket goes unused, at the date of scheduled travel. Sale of mileage credits. Customers may earn mileage credits based on their spending with the Company's co-branded credit card company with which the Company has an agreement to sell mileage credits. The contract to sell mileage credits under this agreement has multiple performance obligations, as discussed below. The Company's co-brand credit card agreement provides for joint marketing where cardholders earn mileage credits for making purchases using co-branded cards. During 2015, the Company extended its agreement with the administrator of the FREE SPIRIT affinity credit card program to extend through 2022. The Company accounts for this agreement consistently with the accounting method that allocates the consideration received to the individual products and services delivered. The value is allocated based on the relative selling prices of those products and services, which generally consists of (i) travel miles to be awarded, (ii) licensing of brand and access to member lists and (iii) advertising and marketing efforts. The Company determined the best estimate of the selling prices by considering discounted cash flow analysis using multiple inputs and assumptions, including: (1) the expected number of miles awarded and number of miles redeemed, (2) ETV for the award travel obligation, (3) licensing of brand and access to member lists and (4) advertising and marketing efforts. The Company defers the amount for award travel obligation as part of loyalty deferred revenue within air traffic liability on the balance sheet and recognizes loyalty travel awards in passenger revenue as the mileage credits are used for travel. Revenue allocated to the remaining performance obligations, primarily marketing components, is recorded in other revenue over time as miles are delivered. Total unrecognized revenue from future FREE SPIRIT award redemptions and the sale of mileage credits was $27.4 million and $26.6 million at December 31, 2018 and 2017 For mileage credits that the Company estimates are not likely to be redeemed ("breakage"), the Company recognizes the associated value proportionally during the period in which the remaining mileage credits are redeemed. Management uses statistical models to estimate breakage based on historical redemption patterns. A change in assumptions as to the period over which mileage credits are expected to be redeemed, the actual redemption activity for mileage credits or the estimated fair value of mileage credits expected to be redeemed could have an impact on revenues in the year in which the change occurs and in future years. Current activity of frequent flyer program.
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 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, redelivery crew 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. 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. When determining probability and estimated cost, there are various other factors which need to be considered such as the contractual terms of the lease agreement, current condition of the aircraft, the age of the aircraft at lease expiration, number of hours run on the engines, number of cycles run on the airframe, projected number of hours run on the engine at the time of return, number of projected cycles run on the airframe at the time of return, the extent of repairs needed if any at return, return locations, current configuration of the aircraft, current paint of the aircraft, estimated escalation of cost of repairs and materials at the time of return, current flight hour agreement rates and future flight hour agreement rates. In addition, typically near the
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 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 statements 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. As of December 31, 2018 and 2017
Advertising	AdvertisingThe Company expenses advertising and the production costs of advertising as incurred.
Income Taxes	Income TaxesThe Company accounts for income taxes using the asset and liability method. The Company records a valuation allowance to reduce the deferred tax assets reported if, based on the weight of the evidence, it is more likely than not that some portion or all of the deferred tax assets will be not realized.
Stock-Based Compensation	Stock-Based CompensationThe Company recognizes cost of employee services received in exchange for awards of equity instruments based on the fair value of each instrument at the date of grant. For the majority of awards, compensation expense is recognized on a straight-line basis over the period during which an employee is required to provide service in exchange for an award. Certain awards have performance conditions that must be achieved prior to vesting and are expensed based on the expected achievement at each reporting period. The Company has issued and outstanding restricted stock awards, stock option awards and performance share awards. Restricted stock awards are valued at the fair value of the shares on the date of grant. The fair value of share option awards is estimated on the date of grant using the Black-Scholes valuation model. The fair value of performance share awards 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 32% , 27% and 24% of total operating expenses in 2018 , 2017 and 2016 , 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 Recently Adopted Accounting Pronouncements Revenue from Contracts with Customers In May 2014, the Financial Accounting Standards Board ("the FASB") issued 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 Company adopted this guidance on January 1, 2018 utilizing the full retrospective method of adoption allowed by the standard, in order to provide for comparative results in all periods presented. The most significant impact of this ASU is the elimination of the incremental cost method for frequent flyer program accounting, which requires the Company to re-value and record a liability associated with customer flight miles earned as part of the Company’s frequent flyer program with a relative fair value approach. The classification and timing of recognition of certain ancillary fees is also impacted by the adoption of ASU 2014-09. While the adoption did not have a significant impact on earnings, the classification of certain revenues, such as bags, seats and other travel-related fees are now deemed part of the single performance obligation of providing passenger transportation. Refer to Note 3, Revenue Recognition and Note 4, Revenue Disaggregation for information regarding the Company's adoption of ASU 2014-09. 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 the Company for interim and annual periods beginning January 1, 2018. The Company adopted this guidance on January 1, 2018 with no material impact on the financial statements. Statement of Cash Flows In August 2016, the FASB issued ASU No. 2016-15, "Statement of Cash Flows." The standard is intended to reduce diversity in practice in how certain transactions are classified in the statement of cash flows. This standard is effective for the Company for fiscal years, and interim periods within those years, beginning January 1, 2018. The Company adopted this guidance on January 1, 2018 with no material impact on the financial statements. Income Taxes In March 2018, the FASB issued ASU 2018-05, "Income Taxes" ("Topic 740") - Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin No. 118. The standard amends Accounting Standards Codification 740, Income Taxes ("ASC 740") to provide guidance on accounting for the tax effects of the Tax Cuts and Jobs Act (the "Tax Act") pursuant to Staff Accounting Bulletin No. 118. During the fourth quarter of 2018, the Company finalized its accounting for the tax effects of the Tax Act and had no material adjustments from the provisional amounts previously recorded by the Company. Recently Issued Accounting Pronouncements Not Yet Adopted Cloud Computing Arrangements In August 2018, the FASB issued ASU No. 2018-15, "Intangibles - Goodwill and Other - Internal-Use Software". This new standard requires a customer in a cloud computing arrangement that is a service contract to follow the internal-use software guidance in Accounting Standards Codification ("ASC") 350-40, "Accounting for Internal-Use Software", to determine which implementation costs to capitalize as assets and amortize over the term of the hosting arrangement or expense as incurred. This new standard is effective for public business entities in fiscal years beginning after December 15, 2019. Early adoption is permitted, including during an interim period. Entities have the option to apply this standard prospectively to all implementation costs incurred after the date of adoption or retrospectively. The Company is evaluating this new standard, but does not expect it to have a significant impact on its financial statement presentation or results. Leases In February 2016, the FASB issued ASU No. 2016-02, "Leases (Topic 842)." This standard will require all lessees to recognize a right-of-use asset and a lease liability, initially measured at the present value of the lease payments, for all leases with a term greater than 12 months. The standard is effective for the Company effective January 1, 2019. Early adoption is permitted. However, the Company has elected not to early adopt. The Company has established a cross functional project plan and is in the process of testing items necessary to account for the new standard. The Company expects this standard to have a significant impact of approximately $0.8 billion to $1.2 billion on its balance sheets due to the recognition of right-of-use assets and lease liabilities for certain operating leases. The adoption of Topic 842 will not have a significant impact on the Company's lease classification or a material impact on its statements of operations and liquidity. Additionally, the Company does not expect the standard to have a material impact on the Company’s debt-covenant compliance under its current agreements. Refer to Note 18, Commitments and Contingencies for information regarding the Company's undiscounted future lease payments and the timing of those payments. In July 2018, the FASB issued additional guidance on the accounting for leases. The guidance provides companies with another transition method that allows entities to recognize a cumulative-effect adjustment to the opening balance of retained earnings as of the date of adoption. Under this method, previously presented years’ financial positions and results would not be adjusted. The Company expects to use this transition method upon implementation of the guidance in 2019. The new guidance also provides lessors with a practical expedient, by class of underlying asset, to not separate non-lease components from the associated lease component if (1) the non-lease components would otherwise be accounted for under the new revenue recognition standard, (2) both the timing and pattern of transfer are the same for the non-lease components and associated lease component, and (3) if accounted for separately, the lease component would be classified as an operating lease. The Company plans to elect the package of practical expedients available under the transition provisions of Topic 842. Accounting for Credit Losses
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 Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 2017 2016 (in thousands) Depreciation $ 129,412 $ 83,154 $ 54,171 Amortization of heavy maintenance 41,286 53,855 43,811 Amortization of capitalized software 6,029 3,143 3,154 Total depreciation and amortization $ 176,727 $ 140,152 $ 101,136
Primary components of non-fare revenue and the revenue recognition method utilized for each service or product	The following table summarizes the primary components of the Company's non-fare revenue and the revenue recognition method utilized for each service or product: Year Ended December 31, Non-fare revenue Recognition method 2018 2017 2016 (in thousands) Baggage Time of departure $ 620,154 $ 488,434 $ 434,269 Passenger usage fee Time of departure 531,459 411,742 358,920 Advance seat selection Time of departure 180,012 131,821 110,966 Other 224,283 177,495 155,539 Non-fare revenue $ 1,555,908 $ 1,209,492 $ 1,059,694
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, 2018 $ 39,194 $ 30,353 December 31, 2017 49,453 37,960 December 31, 2016 48,882 36,640
Aircraft maintenance expense	The table below summarizes the components of the Company’s maintenance cost: Year Ended December 31, 2018 2017 2016 (in thousands) Flight hour-based maintenance expense $ 68,039 $ 54,802 $ 48,471 Non-flight hour-based maintenance expense 61,039 55,637 50,116 Total maintenance, materials and repairs $ 129,078 $ 110,439 $ 98,587

Revenue Recognition (Tables)

Revenue Recognition (Tables)	12 Months Ended
	Dec. 31, 2018
Revenue from Contract with Customer [Abstract]
Schedule of New Accounting Pronouncements and Changes in Accounting Principles [Table Text Block]	The following tables show adjustments made due to the adoption of ASU 2014-09 on the December 31, 2017 and 2016 statements of operations. Previously reported results were derived from audited financial statements included in Company's Annual Report on Form 10-K for the fiscal years ended December 31, 2017 and December 31, 2016, as applicable. Year Ended December 31, 2017 (in thousands, except share and per-share data) As Reported Topic 606 Adjustment As Adjusted Operating revenues: Passenger $ 1,366,034 $ 1,206,853 $ 2,572,887 Other 1,281,632 (1,210,967 ) 70,665 Total operating revenues 2,647,666 (4,114 ) 2,643,552 Operating expenses: Aircraft fuel 615,581 — 615,581 Salaries, wages and benefits 527,959 — 527,959 Aircraft rent 205,852 — 205,852 Landing fees and other rents 180,655 — 180,655 Depreciation and amortization 140,152 — 140,152 Maintenance, materials and repairs 110,439 — 110,439 Distribution 113,620 (148 ) 113,472 Special charges 12,629 — 12,629 Loss on disposal of assets 4,168 — 4,168 Other operating 347,820 — 347,820 Total operating expenses 2,258,875 (148 ) 2,258,727 Operating income 388,791 (3,966 ) 384,825 Other (income) expense: Interest expense 57,302 — 57,302 Capitalized interest (13,793 ) — (13,793 ) Interest income (8,736 ) — (8,736 ) Other expense 366 — 366 Total other (income) expense 35,139 — 35,139 Income before income taxes 353,652 (3,966 ) 349,686 Provision (benefit) for income taxes (66,954 ) 1,118 (65,836 ) Net income $ 420,606 $ (5,084 ) $ 415,522 Basic earnings per share $ 6.08 $ (0.07 ) $ 6.00 Diluted earnings per share $ 6.06 $ (0.07 ) $ 5.99 Year Ended December 31, 2016 (in thousands, except share and per-share data) As Reported Topic 606 Adjustment As Adjusted Operating revenues: Passenger $ 1,200,621 $ 1,057,180 $ 2,257,801 Other 1,121,335 (1,059,115 ) 62,220 Total operating revenues 2,321,956 (1,935 ) 2,320,021 Operating expenses: Salaries, wages and benefits 472,471 — 472,471 Aircraft fuel 447,553 — 447,553 Aircraft rent 201,675 — 201,675 Landing fees and other rents 151,679 — 151,679 Depreciation and amortization 101,136 — 101,136 Maintenance, materials and repairs 98,587 — 98,587 Distribution 96,627 268 96,895 Special charges 37,189 — 37,189 Loss on disposal of assets 4,187 — 4,187 Other operating 267,191 — 267,191 Total operating expenses 1,878,295 268 1,878,563 Operating income 443,661 (2,203 ) 441,458 Other (income) expense: Interest expense 41,654 — 41,654 Capitalized interest (12,705 ) — (12,705 ) Interest income (5,276 ) — (5,276 ) Other expense 528 — 528 Total other (income) expense 24,201 — 24,201 Income before income taxes 419,460 (2,203 ) 417,257 Provision (benefit) for income taxes 154,581 (807 ) 153,774 Net income $ 264,879 $ (1,396 ) $ 263,483 Basic earnings per share $ 3.77 $ (0.02 ) $ 3.75 Diluted earnings per share $ 3.76 $ (0.02 ) $ 3.74 The following table shows adjusted balances after the adoption of ASU 2014-09 on the quarterly statements of operations for each quarter of 2017. For the Quarter Ended March 31, 2017 June 30, 2017 September 30, 2017 December 31, 2017 (unaudited) (in thousands, except share and per-share data) Operating revenues: Passenger $ 572,287 $ 680,880 $ 669,072 $ 650,647 Other 17,670 19,305 18,155 15,535 Total operating revenues 589,957 700,185 687,227 666,182 Operating expenses: Aircraft fuel 139,782 142,294 158,300 175,205 Salaries, wages and benefits 127,138 129,892 134,114 136,815 Aircraft rent 57,070 52,566 53,396 42,820 Landing fees and other rents 40,448 45,592 48,498 46,117 Depreciation and amortization 31,509 35,331 36,840 36,472 Maintenance, materials and repairs 26,312 28,985 26,176 28,966 Distribution 25,772 29,835 29,695 28,170 Special charges 4,776 — 7,853 — Loss on disposal of assets 1,105 1,493 516 1,054 Other operating 77,703 102,885 87,965 79,267 Total operating expenses 531,615 568,873 583,353 574,886 Operating income 58,342 131,312 103,874 91,296 Other (income) expense: Interest expense 12,473 13,746 15,018 16,065 Capitalized interest (3,580 ) (3,342 ) (3,203 ) (3,668 ) Interest income (1,313 ) (1,828 ) (2,605 ) (2,990 ) Other expense 3 104 114 145 Total other (income) expense 7,583 8,680 9,324 9,552 Income before income taxes 50,759 122,632 94,550 81,744 Provision (benefit) for income taxes 19,498 45,391 34,506 (165,231 ) Net income $ 31,261 $ 77,241 $ 60,044 $ 246,975 Basic earnings per share $ 0.45 $ 1.11 $ 0.87 $ 3.59 Diluted earnings per share $ 0.45 $ 1.11 $ 0.86 $ 3.58 The following table shows quarterly adjustments made due to the adoption of ASU 2014-09 on the statements of operations for 2017. Adjustments for the Quarter Ended Full Year 2017 As Reported March 31, 2017 June 30, 2017 September 30, 2017 December 31, 2017 Full Year 2017 Adjusted (unaudited) (in thousands, except share and per-share data) Operating revenues: Passenger $ 1,366,034 $ 272,525 $ 308,959 $ 312,865 $ 312,504 $ 2,572,887 Other 1,281,632 (274,314 ) (310,455 ) (312,869 ) (313,329 ) 70,665 Total operating revenues 2,647,666 (1,789 ) (1,496 ) (4 ) (825 ) 2,643,552 Operating expenses: Aircraft fuel 615,581 — — — — 615,581 Salaries, wages and benefits 527,959 — — — — 527,959 Aircraft rent 205,852 — — — — 205,852 Landing fees and other rents 180,655 — — — — 180,655 Depreciation and amortization 140,152 — — — — 140,152 Maintenance, materials and repairs 110,439 — — — — 110,439 Distribution 113,620 (726 ) (73 ) 226 425 113,472 Special charges 12,629 — — — — 12,629 Loss on disposal of assets 4,168 — — — — 4,168 Other operating 347,820 — — — — 347,820 Total operating expenses 2,258,875 (726 ) (73 ) 226 425 2,258,727 Operating income 388,791 (1,063 ) (1,423 ) (230 ) (1,250 ) 384,825 Other (income) expense: Interest expense 57,302 — — — — 57,302 Capitalized interest (13,793 ) — — — — (13,793 ) Interest income (8,736 ) — — — — (8,736 ) Other expense 366 — — — — 366 Total other (income) expense 35,139 — — — — 35,139 Income before income taxes 353,652 (1,063 ) (1,423 ) (230 ) (1,250 ) 349,686 Provision (benefit) for income taxes (66,954 ) (389 ) (522 ) (84 ) 2,113 (65,836 ) Net income $ 420,606 $ (674 ) $ (901 ) $ (146 ) $ (3,363 ) $ 415,522 Basic earnings per share $ 6.08 $ (0.01 ) $ (0.01 ) $ — $ (0.05 ) $ 6.00 Diluted earnings per share $ 6.06 $ (0.01 ) $ (0.01 ) $ — $ (0.05 ) $ 5.99 The following tables show adjustments made due to the adoption of ASU 2014-09 on the December 31, 2017 and 2016 balance sheets. Previously reported results were derived from audited financial statements included in Company's Annual Report on Form 10-K for the fiscal years ended December 31, 2017 and December 31, 2016, as applicable. December 31, 2017 (in thousands) As Reported Topic 606 Adjustment As Adjusted Assets Current assets: Cash and cash equivalents $ 800,849 $ — $ 800,849 Short-term investment securities 100,937 — 100,937 Accounts receivable, net 49,323 — 49,323 Aircraft maintenance deposits, net 175,615 — 175,615 Income tax receivable 69,844 — 69,844 Prepaid expenses and other current assets 83,692 1,850 85,542 Total current assets 1,280,260 1,850 1,282,110 Property and equipment: Flight equipment 2,291,110 — 2,291,110 Ground property and equipment 155,166 — 155,166 Less accumulated depreciation (207,808 ) — (207,808 ) 2,238,468 — 2,238,468 Deposits on flight equipment purchase contracts 253,687 — 253,687 Long-term aircraft maintenance deposits 150,617 — 150,617 Deferred heavy maintenance, net 99,915 — 99,915 Other long-term assets 121,003 — 121,003 Total assets $ 4,143,950 $ 1,850 $ 4,145,800 Liabilities and shareholders’ equity Current liabilities: Accounts payable $ 22,822 $ — $ 22,822 Air traffic liability 246,404 17,307 263,711 Current maturities of long-term debt 115,430 — 115,430 Other current liabilities 262,370 — 262,370 Total current liabilities 647,026 17,307 664,333 Long-term debt, less current maturities 1,387,498 — 1,387,498 Deferred income taxes 313,140 (4,326 ) 308,814 Deferred gains and other long-term liabilities 19,205 3,376 22,581 Shareholders’ equity: Common stock: Common stock, $0.0001 par value, 240,000,000 shares authorized at December 31, 2017; 69,770,795 issued and 68,196,964 outstanding as of December 31, 2017 7 — 7 Additional paid-in-capital 360,153 — 360,153 Treasury stock, at cost: 1,573,831 shares as of December 31, 2017 (65,854 ) — (65,854 ) Retained earnings 1,484,239 (14,507 ) 1,469,732 Accumulated other comprehensive income (loss) (1,464 ) — (1,464 ) Total shareholders’ equity 1,777,081 (14,507 ) 1,762,574 Total liabilities and shareholders’ equity $ 4,143,950 $ 1,850 $ 4,145,800 December 31, 2016 (in thousands) As Reported Topic 606 Adjustment As Adjusted Assets Current assets: Cash and cash equivalents $ 700,900 $ — $ 700,900 Short-term investment securities 100,155 — 100,155 Accounts receivable, net 41,136 — 41,136 Aircraft maintenance deposits, net 87,035 — 87,035 Income tax receivable — — — Prepaid expenses and other current assets 46,619 1,702 48,321 Total current assets 975,845 1,702 977,547 Property and equipment: Flight equipment 1,461,525 — 1,461,525 Ground property and equipment 126,206 — 126,206 Less accumulated depreciation (122,509 ) — (122,509 ) 1,465,222 — 1,465,222 Deposits on flight equipment purchase contracts 325,688 — 325,688 Long-term aircraft maintenance deposits 199,415 — 199,415 Deferred heavy maintenance, net 75,534 — 75,534 Other long-term assets 110,223 — 110,223 Total assets $ 3,151,927 $ 1,702 $ 3,153,629 Liabilities and shareholders’ equity Current liabilities: Accounts payable $ 15,193 $ — $ 15,193 Air traffic liability 206,392 13,792 220,184 Current maturities of long-term debt 84,354 — 84,354 Other current liabilities 226,011 — 226,011 Total current liabilities 531,950 13,792 545,742 Long-term debt, less current maturities 897,359 — 897,359 Deferred income taxes 308,143 (5,443 ) 302,700 Deferred gains and other long-term liabilities 19,868 2,776 22,644 Shareholders’ equity: Common stock: Common stock, $0.0001 par value, 240,000,000 shares authorized at December 31, 2016; 73,549,872 issued and 69,326,202 outstanding as of December 31, 2016 7 — 7 Additional paid-in-capital 551,004 — 551,004 Treasury stock, at cost: 4,223,670 shares as of December 31, 2016 (218,692 ) — (218,692 ) Retained earnings 1,063,633 (9,423 ) 1,054,210 Accumulated other comprehensive income (loss) (1,345 ) — (1,345 ) Total shareholders’ equity 1,394,607 (9,423 ) 1,385,184 Total liabilities and shareholders’ equity $ 3,151,927 $ 1,702 $ 3,153,629

Revenue Disaggregation (Tables)

Revenue Disaggregation (Tables)	12 Months Ended
	Dec. 31, 2018
Revenue from Contract with Customer [Abstract]
Schedule of Revenue Disaggregation	The following table shows disaggregated operating revenues for each quarter of 2018 and 2017. For the Quarter Ended December 31, 2018 September 30, 2018 June 30, 2018 March 31, 2018 December 31, 2017 September 30, 2017 June 30, 2017 March 31, 2017 (unaudited, in thousands) Operating revenues: Fare $ 445,203 $ 476,660 $ 439,549 $ 342,695 $ 337,324 $ 355,593 $ 371,443 $ 299,035 Non-fare 401,365 411,296 396,801 346,446 313,323 313,479 309,437 273,252 Total passenger revenues 846,568 887,956 836,350 689,141 650,647 669,072 680,880 572,287 Other revenues 16,227 16,374 15,421 14,997 15,535 18,155 19,305 17,670 Total operating revenues $ 862,795 $ 904,330 $ 851,771 $ 704,138 $ 666,182 $ 687,227 $ 700,185 $ 589,957 The following table shows disaggregated operating revenues for years ended December 31, 2017 and 2016. Year Ended December 31, 2017 2016 (in thousands) As Reported Topic 606 Adjustment As Adjusted As Reported Topic 606 Adjustment As Adjusted Operating revenues: Fare $ 1,366,034 $ (2,639 ) $ 1,363,395 $ 1,200,621 $ (2,514 ) $ 1,198,107 Non-fare — 1,209,492 1,209,492 — 1,059,694 1,059,694 Total passenger revenues 1,366,034 1,206,853 2,572,887 1,200,621 1,057,180 2,257,801 Other revenues 1,281,632 (1,210,967 ) 70,665 1,121,335 (1,059,115 ) 62,220 Total operating revenues $ 2,647,666 $ (4,114 ) $ 2,643,552 $ 2,321,956 $ (1,935 ) $ 2,320,021

Accrued Liabilities (Tables)

Accrued Liabilities (Tables)	12 Months Ended
	Dec. 31, 2018
Payables and Accruals [Abstract]
Accrued liabilities included in other current liabilities	Accrued liabilities included in other current liabilities as of December 31, 2018 and 2017 consist of the following: As of December 31, 2018 2017 (in thousands) Salaries and wages $ 82,900 $ 54,338 Federal excise and other passenger taxes and fees payable 60,604 42,036 Aircraft maintenance 59,805 33,033 Airport obligations 52,029 56,299 Fuel 25,368 25,171 Interest payable 18,086 11,384 Aircraft and facility lease obligations 15,149 16,992 Other 25,736 23,117 Other current liabilities $ 339,677 $ 262,370

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 and changes during the year ended December 31, 2018 is presented below: Number of Shares Weighted-Average Outstanding at December 31, 2017 266,522 50.48 Granted 185,054 46.90 Vested (141,089 ) 51.09 Forfeited (12,690 ) 50.87 Outstanding at December 31, 2018 297,797 47.95
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, 2018 Weighted-Average at Grant Date for Twelve Months Ended December 31, 2017 Expected volatility factor 0.39 0.40 Risk free interest rate 2.11 % 1.47 % Expected term (in years) 2.96 2.93 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, 2018 : Number of Awards Weighted-Average Fair Value at Grant Date ($) Outstanding at December 31, 2017 120,359 52.84 Granted 52,074 52.07 Vested (83,977 ) 50.88 Forfeited (5,549 ) 59.01 Outstanding at December 31, 2018 82,907 53.92
Performance Condition Performance Share Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance share award activity	The following table summarizes the Company’s performance condition performance share awards for the year ended December 31, 2018 : Number of Awards Weighted-Average Fair Value at Grant Date ($) Outstanding at December 31, 2017 35,507 45.72 Granted 26,037 46.21 Vested (18,717 ) 39.18 Forfeited (1,373 ) 51.04 Outstanding at December 31, 2018 41,454 49.81

Earnings per Share (Tables)

Earnings per Share (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 2017 2016 (in thousands, except per-share amounts) Numerator: Net income (1) $ 155,749 $ 415,522 $ 263,483 Denominator: Weighted-average shares outstanding, basic 68,249 69,221 70,344 Effect of dilutive stock awards 182 156 164 Adjusted weighted-average shares outstanding, diluted 68,431 69,377 70,508 Earnings per Share: Basic earnings per common share (1) $ 2.28 $ 6.00 $ 3.75 Diluted earnings per common share (1) $ 2.28 $ 5.99 $ 3.74 Anti-dilutive weighted-average shares 145 85 66

Debt and Other Obligations (Tab

Debt and Other Obligations (Tables)	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Schedule of long-term debt instruments	Long-term debt is comprised of the following: As of Year Ended December 31, 2018 2017 2018 2017 (in millions) (weighted-average interest rates) Fixed-rate senior term loans due through 2027 $ 382.4 $ 417.9 4.10 % 4.10 % Fixed-rate junior term loans due through 2022 31.1 39.3 6.90 % 6.90 % Fixed-rate loans due through 2030 625.1 518.0 3.88 % 3.83 % Fixed-rate class A 2015-1 EETC due through 2028 378.6 408.6 4.10 % 4.10 % Fixed-rate class B 2015-1 EETC due through 2024 80.0 92.0 4.45 % 4.45 % Fixed-rate class C 2015-1 EETC due through 2023 109.5 — 4.93 % N/A Fixed-rate class AA 2017-1 EETC due through 2030 242.5 37.5 3.38 % 3.38 % Fixed-rate class A 2017-1 EETC due through 2030 80.8 12.5 3.65 % 3.65 % Fixed-rate class B 2017-1 EETC due through 2026 83.7 13.8 3.80 % 3.80 % Fixed-rate class C 2017-1 EETC due through 2023 85.5 — 5.11 % N/A Revolving credit facility due in 2020 135.3 — 3.72 % N/A Long-term debt $ 2,234.5 $ 1,539.6 Less current maturities 162.8 115.4 Less unamortized discount, net 47.7 36.7 Total $ 2,024.0 $ 1,387.5
Schedule of maturities of long-term debt	At December 31, 2018 , long-term debt principal payments for the next five years and thereafter are as follows: December 31, 2018 (in millions) 2019 $ 171.3 2020 303.6 2021 162.1 2022 159.5 2023 298.0 2024 and beyond 1,140.0 Total debt principal payments $ 2,234.5
Schedule of interest expense on long-term debt and capital leases	Interest expense related to long-term debt and capital leases consists of the following: Year Ended December 31, 2018 2017 (in thousands) Fixed-rate senior term loans $ 16,869 $ 18,328 Fixed-rate junior term loans 2,475 3,035 Fixed-rate term loans 19,615 8,610 Class A 2015-1 EETC 16,138 17,230 Class B 2015-1 EETC 3,781 4,446 Class C 2015-1 EETC 3,575 — Class AA 2017-1 EETC 6,026 54 Class A 2017-1 EETC 2,172 19 Class B 2017-1 EETC 2,420 22 Class C 2017-1 EETC 2,478 — Revolving credit facility 793 — Capital leases 104 154 Commitment fees 262 124 Amortization of deferred financing costs 7,069 5,280 Total $ 83,777 $ 57,302

Leases and Prepaid Maintenanc_2

Leases and Prepaid Maintenance Deposits (Tables)	12 Months Ended
	Dec. 31, 2018
Leases [Abstract]
Schedule of future minimum rental payments for capital 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, 2018 were as follows: Capital Leases Operating Leases Total Operating and Capital Lease Obligations Aircraft and Spare Engine Leases Property Facility Leases (in thousands) 2019 $ 810 $ 176,256 $ 52,627 $ 229,693 2020 408 170,106 29,962 200,476 2021 186 168,416 17,004 185,606 2022 158 155,135 14,718 170,011 2023 145 133,915 13,787 147,847 2024 and thereafter — 350,266 89,536 439,802 Total minimum lease payments $ 1,707 $ 1,154,094 $ 217,634 $ 1,373,435 Less amount representing interest 152 Present value of minimum lease payments $ 1,555 Less current portion 721 Long-term portion $ 834
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, 2018 were as follows: Capital Leases Operating Leases Total Operating and Capital Lease Obligations Aircraft and Spare Engine Leases Property Facility Leases (in thousands) 2019 $ 810 $ 176,256 $ 52,627 $ 229,693 2020 408 170,106 29,962 200,476 2021 186 168,416 17,004 185,606 2022 158 155,135 14,718 170,011 2023 145 133,915 13,787 147,847 2024 and thereafter — 350,266 89,536 439,802 Total minimum lease payments $ 1,707 $ 1,154,094 $ 217,634 $ 1,373,435 Less amount representing interest 152 Present value of minimum lease payments $ 1,555 Less current portion 721 Long-term portion $ 834

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 2017 2016 (in thousands) Current: Federal $ (2,178 ) $ (68,601 ) $ 60,079 State and local 410 515 6,322 Foreign 4,692 2,742 2,034 Total current expense (benefit) 2,924 (65,344 ) 68,435 Deferred: Federal 42,246 (9,349 ) 81,682 State and local 4,057 8,857 3,657 Total deferred expense (benefit) 46,303 (492 ) 85,339 Total income tax expense (benefit) $ 49,227 $ (65,836 ) $ 153,774
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, 2018 2017 2016 Expected provision at federal statutory tax rate 21.0 % 35.0 % 35.0 % State tax expense, net of federal benefit 1.7 % 1.7 % 1.6 % Revaluation of deferred taxes — % (56.3 )% — % Other 1.3 % 0.7 % 0.3 % Total income tax expense (benefit) 24.0 % (18.9 )% 36.9 %
Deferred taxes	At December 31, 2018 and 2017 , the significant components of the Company's deferred taxes consisted of the following: December 31, 2018 2017 (in thousands) Deferred tax assets: Income tax credits $ 10,004 $ 5,980 Net operating losses 155,670 871 Deferred revenue 6,824 5,957 Nondeductible accruals 14,691 10,107 Deferred manufacturing credits — 258 Accrued maintenance 2,168 1,991 Equity compensation 2,592 2,392 Other 5,262 4,684 Valuation allowance (254 ) (454 ) Deferred tax assets 196,957 31,786 Deferred tax liabilities: Deferred gain (loss) on leases, net 1,672 1,605 Accrued rent 6,068 12,055 Prepaid expenses 793 754 Property, plant and equipment 481,847 298,703 Deferred financing costs 189 225 Accrued aircraft and engine maintenance 61,529 27,258 Deferred tax liabilities 552,098 340,600 Net deferred tax assets (liabilities) $ (355,141 ) $ (308,814 )

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Schedule of Future Operating Aircraft Leases	As of December 31, 2018 , the Company's firm aircraft orders consisted of the following: Airbus Third-Party Lessor A320ceo A320neo A320neo Total 2019 2 9 5 16 2020 16 16 2021 18 18 2 43 5 50
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, 2018 . Employee Groups Representative Amendable Date Percentage of Workforce Pilots Air Line Pilots Association, International (ALPA) February 2023 27% Flight Attendants Association of Flight Attendants (AFA-CWA) May 2021 45% Dispatchers Professional Airline Flight Control Association (PAFCA) October 2023 1% Ramp Service Agents International Association of Machinists and Aerospace Workers (IAMAW) June 2020 4% Passenger Service Agents Transport Workers Union of America (TWU) NA 3%

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 and December 31, 2017 , were as follows: As of December 31, 2018 2017 Carrying Value Estimated Fair Value Carrying Value Estimated Fair Value Fair value level hierarchy (in millions) Fixed-rate senior term loans $ 382.4 $ 373.6 $ 417.9 $ 435.3 Level 3 Fixed-rate junior term loans 31.1 31.1 39.3 40.4 Level 3 Fixed-rate term loans 625.1 600.1 518.0 528.6 Level 3 2015-1 EETC Class A 378.6 374.8 408.6 420.9 Level 2 2015-1 EETC Class B 80.0 78.1 92.0 94.2 Level 2 2015-1 EETC Class C 109.5 107.9 — — Level 2 2017-1 EETC Class AA 242.5 228.8 37.5 37.4 Level 2 2017-1 EETC Class A 80.8 76.6 12.5 12.6 Level 2 2017-1 EETC Class B 83.7 79.1 13.8 13.8 Level 2 2017-1 EETC Class C 85.5 84.2 — — Level 2 Revolving credit facility 135.3 135.3 — — Level 3 Total long-term debt $ 2,234.5 $ 2,169.6 $ 1,539.6 $ 1,583.2
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, 2018 Total Level Level Level (in millions) Cash and cash equivalents $ 1,004.7 $ 1,004.7 $ — $ — Short-term investment securities 102.8 102.8 — — Total assets $ 1,107.5 $ 1,107.5 $ — $ — Total liabilities $ — $ — $ — $ — 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 $ — $ — $ — $ —

Operating Segments and Relate_2

Operating Segments and Related Disclosures (Tables)	12 Months Ended
	Dec. 31, 2018
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: 2018 2017 2016 (in millions) DOT—Domestic $ 2,990.7 $ 2,432.1 $ 2,134.4 DOT—Latin America and Caribbean 332.3 211.5 185.6 Total $ 3,323.0 $ 2,643.6 $ 2,320.0

Quarterly Financial Data (Una_2

Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Quarterly results of operations	Quarterly results of operations for the years ended December 31, 2018 and 2017 are summarized below: Three Months Ended March 31 June 30 September 30 December 31 (in thousands, except per-share amounts) 2018 Operating revenue $ 704,138 $ 851,771 $ 904,330 $ 862,795 Operating income (loss) (38,797 ) 108,521 145,125 136,065 Net income (loss) (44,922 ) 11,254 97,480 91,937 Basic earnings (loss) per share (0.66 ) 0.16 1.43 1.35 Diluted earnings (loss) per share (0.66 ) 0.16 1.42 1.34 2017 Operating revenue $ 589,957 $ 700,185 $ 687,227 $ 666,182 Operating income 58,342 131,312 103,874 91,296 Net income (1) 31,261 77,241 60,044 246,975 Basic earnings per share (1) 0.45 1.11 0.87 3.59 Diluted earnings per share (1) 0.45 1.11 0.86 3.58

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Reportable Segments (Details)	12 Months Ended
	Dec. 31, 2018segment
Accounting Policies [Abstract]
Number of reportable segments	1

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Property and Equipment (Details) $ in Thousands	3 Months Ended				12 Months Ended
	Dec. 31, 2017USD ($)	Sep. 30, 2017USD ($)	Jun. 30, 2017USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2018USD ($)aircraft_engineaircraft	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Property, Plant and Equipment [Line Items]
Number of Spare Engines Capitalized | aircraft					8
Number of Flight Simulators Capitalized | aircraft					1
Depreciation, Depletion and Amortization, Nonproduction [Abstract]
Depreciation					$ 129,412	$ 83,154	$ 54,171
Amortization of capitalized software					6,029	3,143	3,154
Total depreciation and amortization	$ 36,472	$ 36,840	$ 35,331	$ 31,509	$ 176,727	140,152	101,136
Spare engines
Property, Plant and Equipment [Line Items]
Number of aircraft under operating lease | aircraft_engine					12
Aircrafts, Major Spare Rotable Parts, Avionics, and Assemblies
Property, Plant and Equipment [Line Items]
Residual value, percentage					10.00%
Aircraft
Property, Plant and Equipment [Line Items]
Depreciable lives used for the principal depreciable asset classifications					25 years
Spare rotables and flight assemblies | Minimum
Property, Plant and Equipment [Line Items]
Depreciable lives used for the principal depreciable asset classifications					7 years
Spare rotables and flight assemblies | Maximum
Property, Plant and Equipment [Line Items]
Depreciable lives used for the principal depreciable asset classifications					15 years
Other equipment and vehicles | Minimum
Property, Plant and Equipment [Line Items]
Depreciable lives used for the principal depreciable asset classifications					5 years
Other equipment and vehicles | Maximum
Property, Plant and Equipment [Line Items]
Depreciable lives used for the principal depreciable asset classifications					7 years
Internal use software
Depreciation, Depletion and Amortization, Nonproduction [Abstract]
Capitalized computer software, net	$ 7,700				$ 13,700	7,700
Capitalized software costs during the year					$ 12,000	1,400	4,100
Internal use software | Minimum
Property, Plant and Equipment [Line Items]
Depreciable lives used for the principal depreciable asset classifications					3 years
Internal use software | Maximum
Property, Plant and Equipment [Line Items]
Depreciable lives used for the principal depreciable asset classifications					10 years
Heavy maintenance and major overhaul
Depreciation, Depletion and Amortization, Nonproduction [Abstract]
Amortization of heavy maintenance					$ 41,286	$ 53,855	$ 43,811
Spare engines | Minimum
Property, Plant and Equipment [Line Items]
Operating leases, term					2 years
Spare engines | Maximum
Property, Plant and Equipment [Line Items]
Operating leases, term					14 years
Buildings
Property, Plant and Equipment [Line Items]
Depreciable lives used for the principal depreciable asset classifications					30 years
Aircraft | Minimum
Property, Plant and Equipment [Line Items]
Operating leases, term					8 years
Aircraft | Maximum
Property, Plant and Equipment [Line Items]
Operating leases, term					18 years
A320 Family | Aircraft
Property, Plant and Equipment [Line Items]
Number of aircraft under operating lease | aircraft					46
A320 Family | Aircraft
Property, Plant and Equipment [Line Items]
Number of aircraft capitalized | aircraft					82

Summary of Significant Accoun_6

Summary of Significant Accounting Policies Summary of Significant Accounting Policies - Passenger Revenues (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Credit Shell, Term of Expiration									60 days
Total operating revenues	$ 862,795	$ 904,330	$ 851,771	$ 704,138	$ 666,182	$ 687,227	$ 700,185	$ 589,957	$ 3,323,034	$ 2,643,552	$ 2,320,021
Baggage
Disaggregation of Revenue [Line Items]
Total operating revenues									620,154	488,434	434,269
Passenger usage fee
Disaggregation of Revenue [Line Items]
Total operating revenues									531,459	411,742	358,920
Advance seat selection
Disaggregation of Revenue [Line Items]
Total operating revenues									180,012	131,821	110,966
Other
Disaggregation of Revenue [Line Items]
Total operating revenues									224,283	177,495	155,539
Non-fare revenue
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 401,365	$ 411,296	$ 396,801	$ 346,446	$ 313,323	$ 313,479	$ 309,437	$ 273,252	$ 1,555,908	$ 1,209,492	$ 1,059,694

Summary of Significant Accoun_7

Summary of Significant Accounting Policies - Frequent Flier Program (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)revenue_arrangement	Jan. 01, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Frequent flyer program, number of types of revenue arrangements | revenue_arrangement	2
Unrecognized revenue from future FREE SPIRIT award redemptions and the sale of mileage credits	$ 27,400		$ 26,600
Air traffic liability	$ 291,981		$ 263,711	$ 220,184
Difference between Revenue Guidance in Effect before and after Topic 606 | ASU 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Air traffic liability		$ 12,400

Summary of Significant Accoun_8

Summary of Significant Accounting Policies - Cash Proceeds Received from the Sale of Mileage Credits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Consideration received from credit card mile programs	$ 39,194	$ 49,453	$ 48,882
Portion of proceeds recognized immediately as marketing component	$ 30,353	$ 37,960	$ 36,640

Summary of Significant Accoun_9

Summary of Significant Accounting Policies - Maintenance Costs (Details) - USD ($) $ in Thousands	3 Months Ended				12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Deferred costs for heavy maintenance					$ 190,500	$ 78,200	$ 35,400
Deferred heavy maintenance, gross	$ 260,500				366,300	260,500
Accumulated heavy maintenance amortization	207,808				332,864	207,808	122,509
Airframe and Engine Maintenance Costs [Abstract]
Flight hour-based maintenance expense					68,039	54,802	48,471
Non-flight hour-based maintenance expense					61,039	55,637	50,116
Total maintenance, materials and repairs	28,966	$ 26,176	$ 28,985	$ 26,312	129,078	110,439	98,587
Heavy maintenance and major overhaul
Property, Plant and Equipment [Line Items]
Amortization of heavy maintenance					41,286	53,855	$ 43,811
Heavy Maintenance
Property, Plant and Equipment [Line Items]
Accumulated heavy maintenance amortization	$ 160,700				$ 117,300	$ 160,700

Summary of Significant Accou_10

Summary of Significant Accounting Policies - Derivative Instruments (Details) $ in Millions	9 Months Ended
	Sep. 30, 2015USD ($)aircraftderivative_instrument
2015 | Airbus A321
Derivative [Line Items]
Number of aircraft protected by interest rate derivatives scheduled for delivery | aircraft	3
Interest rate swap
Derivative [Line Items]
Number of interest rate derivatives settled | derivative_instrument	6
Notional amount | $	$ 120

Summary of Significant Accou_11

Summary of Significant Accounting Policies - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)employee_group	Dec. 31, 2017USD ($)employee_group	Dec. 31, 2016USD ($)
Accounting Policies [Abstract]
Marketing and advertising expenses	$ 6,300	$ 5,100	$ 3,200
Valuation allowance	$ 254	$ 454
Aircraft fuel expenditure concentration risk | Total operating expenses
Concentration Risk [Line Items]
Concentration of risk	32.00%	27.00%	24.00%
Unionized employees concentration risk | Number of employees, total
Concentration Risk [Line Items]
Concentration of risk	80.00%	75.00%
Union-represented employee groups | employee_group	5	4

Recent Accounting Developments

Recent Accounting Developments Recent Accounting Developments (Details) - Scenario, Forecast - Accounting Standards Update 2016-02 $ in Billions	Jan. 01, 2019USD ($)
Minimum
Adjustments for New Accounting Pronouncements [Line Items]
Operating lease, right-of-use asset	$ 0.8
Operating lease, liability	0.8
Maximum
Adjustments for New Accounting Pronouncements [Line Items]
Operating lease, right-of-use asset	1.2
Operating lease, liability	$ 1.2

Revenue Recognition - Impact on

Revenue Recognition - Impact on Statement of Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Common stock, par value (in dollars per share)	$ 0.0001				$ 0.0001				$ 0.0001	$ 0.0001	$ 0.0001
Operating revenues:
Total operating revenues	$ 862,795	$ 904,330	$ 851,771	$ 704,138	$ 666,182	$ 687,227	$ 700,185	$ 589,957	$ 3,323,034	$ 2,643,552	$ 2,320,021
Operating expenses:
Aircraft fuel					175,205	158,300	142,294	139,782	939,324	615,581	447,553
Salaries, wages and benefits					136,815	134,114	129,892	127,138	719,635	527,959	472,471
Aircraft rent					42,820	53,396	52,566	57,070	177,641	205,852	201,675
Landing fees and other rents					46,117	48,498	45,592	40,448	214,677	180,655	151,679
Depreciation and amortization					36,472	36,840	35,331	31,509	176,727	140,152	101,136
Maintenance, materials and repairs					28,966	26,176	28,985	26,312	129,078	110,439	98,587
Distribution					28,170	29,695	29,835	25,772	137,001	113,472	96,895
Special charges					0	7,853	0	4,776	88,921	12,629	37,189
Loss on disposal of assets					1,054	516	1,493	1,105	9,580	4,168	4,187
Other operating					79,267	87,965	102,885	77,703	379,536	347,820	267,191
Total operating expenses					574,886	583,353	568,873	531,615	2,972,120	2,258,727	1,878,563
Operating income	136,065	145,125	108,521	(38,797)	91,296	103,874	131,312	58,342	350,914	384,825	441,458
Other (income) expense:
Interest expense					16,065	15,018	13,746	12,473	83,777	57,302	41,654
Capitalized interest					(3,668)	(3,203)	(3,342)	(3,580)	(9,841)	(13,793)	(12,705)
Interest income					(2,990)	(2,605)	(1,828)	(1,313)	(19,107)	(8,736)	(5,276)
Other expense					145	114	104	3	752	366	528
Total other (income) expense					9,552	9,324	8,680	7,583	145,938	35,139	24,201
Income before income taxes					81,744	94,550	122,632	50,759	204,976	349,686	417,257
Provision (benefit) for income taxes					(165,231)	34,506	45,391	19,498	49,227	(65,836)	153,774
Net income	$ 91,937	$ 97,480	$ 11,254	$ (44,922)	$ 246,975	$ 60,044	$ 77,241	$ 31,261	$ 155,749	$ 415,522	$ 263,483
Basic earnings per share (in dollars per share)	$ 1.35	$ 1.43	$ 0.16	$ (0.66)	$ 3.59	$ 0.87	$ 1.11	$ 0.45	$ 2.28	$ 6	$ 3.75
Diluted earnings per share (in dollars per share)	$ 1.34	$ 1.42	$ 0.16	$ (0.66)	$ 3.58	$ 0.86	$ 1.11	$ 0.45	$ 2.28	$ 5.99	$ 3.74
Common stock, shares authorized (in shares)	240,000,000				240,000,000				240,000,000	240,000,000	240,000,000
Common stock, shares issued (in shares)	69,871,535				69,770,795				69,871,535	69,770,795	73,549,872
Common stock, shares outstanding (in shares)	68,269,567				68,196,964				68,269,567	68,196,964	69,326,202
Treasury stock (in shares)	1,601,968				1,573,831				1,601,968	1,573,831	4,223,670
As Reported [Member]
Operating revenues:
Total operating revenues										$ 2,647,666	$ 2,321,956
Operating expenses:
Aircraft fuel										615,581	447,553
Salaries, wages and benefits										527,959	472,471
Aircraft rent										205,852	201,675
Landing fees and other rents										180,655	151,679
Depreciation and amortization										140,152	101,136
Maintenance, materials and repairs										110,439	98,587
Distribution										113,620	96,627
Special charges										12,629	37,189
Loss on disposal of assets										4,168	4,187
Other operating										347,820	267,191
Total operating expenses										2,258,875	1,878,295
Operating income										388,791	443,661
Other (income) expense:
Interest expense										57,302	41,654
Capitalized interest										(13,793)	(12,705)
Interest income										(8,736)	(5,276)
Other expense										366	528
Total other (income) expense										35,139	24,201
Income before income taxes										353,652	419,460
Provision (benefit) for income taxes										(66,954)	154,581
Net income										$ 420,606	$ 264,879
Basic earnings per share (in dollars per share)										$ 6.08	$ 3.77
Diluted earnings per share (in dollars per share)										$ 6.06	$ 3.76
ASU 2014-09 | Restatement Adjustment [Member]
Operating revenues:
Total operating revenues					$ (825)	$ (4)	$ (1,496)	$ (1,789)		$ (4,114)	$ (1,935)
Operating expenses:
Aircraft fuel					0	0	0	0		0	0
Salaries, wages and benefits					0	0	0	0		0	0
Aircraft rent					0	0	0	0		0	0
Landing fees and other rents					0	0	0	0		0	0
Depreciation and amortization					0	0	0	0		0	0
Maintenance, materials and repairs					0	0	0	0		0	0
Distribution					425	226	(73)	(726)		(148)	268
Special charges					0	0	0	0		0	0
Loss on disposal of assets					0	0	0	0		0	0
Other operating					0	0	0	0		0	0
Total operating expenses					425	226	(73)	(726)		(148)	268
Operating income					(1,250)	(230)	(1,423)	(1,063)		(3,966)	(2,203)
Other (income) expense:
Interest expense					0	0	0	0		0	0
Capitalized interest					0	0	0	0		0	0
Interest income					0	0	0	0		0	0
Other expense					0	0	0	0		0	0
Total other (income) expense					0	0	0	0		0	0
Income before income taxes					(1,250)	(230)	(1,423)	(1,063)		(3,966)	(2,203)
Provision (benefit) for income taxes					2,113	(84)	(522)	(389)		1,118	(807)
Net income					$ (3,363)	$ (146)	$ (901)	$ (674)		$ (5,084)	$ (1,396)
Basic earnings per share (in dollars per share)					$ (0.05)	$ 0	$ (0.01)	$ (0.01)		$ (0.07)	$ (0.02)
Diluted earnings per share (in dollars per share)					$ (0.05)	$ 0	$ (0.01)	$ (0.01)		$ (0.07)	$ (0.02)
Passenger
Operating revenues:
Total operating revenues	$ 846,568	$ 887,956	$ 836,350	$ 689,141	$ 650,647	$ 669,072	$ 680,880	$ 572,287	$ 3,260,015	$ 2,572,887	$ 2,257,801
Passenger | As Reported [Member]
Operating revenues:
Total operating revenues										1,366,034	1,200,621
Passenger | ASU 2014-09 | Restatement Adjustment [Member]
Operating revenues:
Total operating revenues					312,504	312,865	308,959	272,525		1,206,853	1,057,180
Other
Operating revenues:
Total operating revenues	$ 16,227	$ 16,374	$ 15,421	$ 14,997	15,535	18,155	19,305	17,670	$ 63,019	70,665	62,220
Other | As Reported [Member]
Operating revenues:
Total operating revenues										1,281,632	1,121,335
Other | ASU 2014-09 | Restatement Adjustment [Member]
Operating revenues:
Total operating revenues					$ (313,329)	$ (312,869)	$ (310,455)	$ (274,314)		$ (1,210,967)	$ (1,059,115)

Revenue Recognition - Impact _2

Revenue Recognition - Impact on Balance Sheets (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 1,004,733	$ 800,849	$ 700,900	$ 803,632
Short-term investment securities	102,789	100,937	100,155
Accounts receivable, net	47,660	49,323	41,136
Aircraft maintenance deposits, net	106,901	175,615	87,035
Income tax receivable	0	69,844	0
Prepaid expenses and other current assets	83,383	85,542	48,321
Total current assets	1,345,466	1,282,110	977,547
Property and equipment:
Flight equipment	3,257,215	2,291,110	1,461,525
Ground property and equipment	191,661	155,166	126,206
Less accumulated depreciation	(332,864)	(207,808)	(122,509)
Total property and equipment	3,116,012	2,238,468	1,465,222
Pre-delivery deposits on flight equipment	236,775	253,687	325,688
Long-term aircraft maintenance deposits	138,738	150,617	199,415
Deferred heavy maintenance, net	249,010	99,915	75,534
Other long-term assets	79,456	121,003	110,223
Total assets	5,165,457	4,145,800	3,153,629
Current liabilities:
Accounts payable	39,320	22,822	15,193
Air traffic liability	291,981	263,711	220,184
Current maturities of long-term debt	162,800	115,430	84,354
Other current liabilities	339,677	262,370	226,011
Total current liabilities	834,535	664,333	545,742
Long-term debt and capital leases, less current maturities	2,024,000	1,387,498	897,359
Deferred income taxes	355,141	308,814	302,700
Deferred gains and other long-term liabilities	22,503	22,581	22,644
Shareholders’ equity:
Common stock	7	7	7
Additional paid-in-capital	371,225	360,153	551,004
Treasury stock, at cost	(67,016)	(65,854)	(218,692)
Retained earnings	1,625,481	1,469,732	1,054,210
Accumulated other comprehensive income (loss)	(1,193)	(1,464)	(1,345)
Total shareholders’ equity	1,928,504	1,762,574	1,385,184	1,217,283
Total liabilities and shareholders’ equity	$ 5,165,457	$ 4,145,800	$ 3,153,629
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001	$ 0.0001
Common stock, shares authorized (in shares)	240,000,000	240,000,000	240,000,000
Common stock, shares issued (in shares)	69,871,535	69,770,795	73,549,872
Common stock, shares outstanding (in shares)	68,269,567	68,196,964	69,326,202
Treasury stock (in shares)	1,601,968	1,573,831	4,223,670
As Reported [Member]
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
Pre-delivery deposits on flight equipment		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 and capital leases, 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		7	7
Additional paid-in-capital		360,153	551,004
Treasury stock, at cost		(65,854)	(218,692)
Retained earnings		1,484,239	1,063,633
Accumulated other comprehensive income (loss)		(1,464)	(1,345)
Total shareholders’ equity		1,777,081	1,394,607	1,225,310
Total liabilities and shareholders’ equity		4,143,950	3,151,927
ASU 2014-09 | Restatement Adjustment [Member]
Current assets:
Cash and cash equivalents		0	0
Short-term investment securities		0	0
Accounts receivable, net		0	0
Aircraft maintenance deposits, net		0	0
Income tax receivable		0	0
Prepaid expenses and other current assets		1,850	1,702
Total current assets		1,850	1,702
Property and equipment:
Flight equipment		0	0
Ground property and equipment		0	0
Less accumulated depreciation		0	0
Total property and equipment		0	0
Pre-delivery deposits on flight equipment		0	0
Long-term aircraft maintenance deposits		0	0
Deferred heavy maintenance, net		0	0
Other long-term assets		0	0
Total assets		1,850	1,702
Current liabilities:
Accounts payable		0	0
Air traffic liability		17,307	13,792
Current maturities of long-term debt		0	0
Other current liabilities		0	0
Total current liabilities		17,307	13,792
Long-term debt and capital leases, less current maturities		0	0
Deferred income taxes		(4,326)	(5,443)
Deferred gains and other long-term liabilities		3,376	2,776
Shareholders’ equity:
Common stock		0	0
Additional paid-in-capital		0	0
Treasury stock, at cost		0	0
Retained earnings		(14,507)	(9,423)
Accumulated other comprehensive income (loss)		0	0
Total shareholders’ equity		(14,507)	(9,423)	$ (8,027)
Total liabilities and shareholders’ equity		$ 1,850	$ 1,702

Revenue Disaggregation (Details

Revenue Disaggregation (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 862,795	$ 904,330	$ 851,771	$ 704,138	$ 666,182	$ 687,227	$ 700,185	$ 589,957	$ 3,323,034	$ 2,643,552	$ 2,320,021
As Reported [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues										2,647,666	2,321,956
ASU 2014-09 | Restatement Adjustment [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues					(825)	(4)	(1,496)	(1,789)		(4,114)	(1,935)
Fare
Disaggregation of Revenue [Line Items]
Total operating revenues	445,203	476,660	439,549	342,695	337,324	355,593	371,443	299,035		1,363,395	1,198,107
Fare | As Reported [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues										1,366,034	1,200,621
Fare | ASU 2014-09 | Restatement Adjustment [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues										(2,639)	(2,514)
Non Fare
Disaggregation of Revenue [Line Items]
Total operating revenues	401,365	411,296	396,801	346,446	313,323	313,479	309,437	273,252	1,555,908	1,209,492	1,059,694
Non Fare | As Reported [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues										0	0
Non Fare | ASU 2014-09 | Restatement Adjustment [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues										1,209,492	1,059,694
Passenger
Disaggregation of Revenue [Line Items]
Total operating revenues	846,568	887,956	836,350	689,141	650,647	669,072	680,880	572,287	3,260,015	2,572,887	2,257,801
Passenger | As Reported [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues										1,366,034	1,200,621
Passenger | ASU 2014-09 | Restatement Adjustment [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues					312,504	312,865	308,959	272,525		1,206,853	1,057,180
Other
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 16,227	$ 16,374	$ 15,421	$ 14,997	15,535	18,155	19,305	17,670	$ 63,019	70,665	62,220
Other | As Reported [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues										1,281,632	1,121,335
Other | ASU 2014-09 | Restatement Adjustment [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues					$ (313,329)	$ (312,869)	$ (310,455)	$ (274,314)		$ (1,210,967)	$ (1,059,115)

Special Charges (Details)

Special Charges (Details) $ in Thousands	3 Months Ended						12 Months Ended
	Jun. 30, 2018USD ($)	Mar. 31, 2018USD ($)aircraft	Dec. 31, 2017USD ($)	Sep. 30, 2017USD ($)	Jun. 30, 2017USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2018USD ($)aircraft	Dec. 31, 2017USD ($)aircraft	Dec. 31, 2016USD ($)aircraft
Property, Plant and Equipment [Line Items]
Purchase of property and equipment							$ 606,971	$ 628,881	$ 539,209
Special charges			$ 0	$ 7,853	$ 0	$ 4,776	88,921	12,629	37,189
Special charges, non-operating							$ 90,357	$ 0	$ 0
Aircraft
Property, Plant and Equipment [Line Items]
Number of previously leased aircraft purchased off lease | aircraft							14	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
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment Purchased, Consideration Transferred	$ 285,000	$ 285,000
Airbus A319 | Aircraft
Property, Plant and Equipment [Line Items]
Number of previously leased aircraft purchased off lease | 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 off lease | aircraft							1
Spare engines | Spare engines
Property, Plant and Equipment [Line Items]
Number of previously leased aircraft purchased off lease | aircraft								1
Purchase of property and equipment								$ 3,800
Value of noncash consideration								4,300
Fair value of assets acquired								3,100
Special charges								4,800
Property, Plant and Equipment Purchased, Consideration Transferred								8,100
Spare engines | Spare engines | Other Non-Cash Items
Property, Plant and Equipment [Line Items]
Value of noncash consideration								$ 200
Capital Lease Obligations | Airbus A319
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Number Of Aircraft Purchased | aircraft		14
Pilots
Property, Plant and Equipment [Line Items]
Ratification Incentive							$ 80,200
Collective Bargaining Agreement, Contractual Provision Adjustments							8,500
Special charges							$ 88,700

Letters of Credit (Details)

Letters of Credit (Details) - Unsecured, standby letter of credit facility - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Letters of credit, limit, amount	$ 35	$ 35
Letter of credit facility, amount outstanding	$ 18.1	$ 17.5

Credit Card Processing Arrang_2

Credit Card Processing Arrangements (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Credit Card Processing Arrangements [Abstract]
Cash holdbacks	$ 0	$ 0
Maximum potential exposure to cash holdbacks by the credit card processors	$ 321,000,000	$ 286,300,000

Short-term Investment Securit_2

Short-term Investment Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Securities, Available-for-sale [Line Items]
Short-term investment securities	$ 102,789	$ 100,937	$ 100,155
Unrealized loss on investment securities, net of deferred taxes	30	(82)	(23)
Deferred taxes on unrealized loss on investment securities	44	(41)	(13)
Accumulated other comprehensive income (loss)	$ (1,193)	$ (1,464)	$ (1,345)
Available-for-sale Securities
Debt Securities, Available-for-sale [Line Items]
Weighted-average fixed rate	1.60%	1.40%
Accumulated other comprehensive income (loss)	$ (74)	$ (105)

Accrued Liabilities (Details)

Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Payables and Accruals [Abstract]
Salaries and wages	$ 82,900	$ 54,338
Federal excise and other passenger taxes and fees payable	60,604	42,036
Aircraft maintenance	59,805	33,033
Airport obligations	52,029	56,299
Fuel	25,368	25,171
Interest payable	18,086	11,384
Aircraft and facility lease obligations	15,149	16,992
Other	25,736	23,117
Other current liabilities	$ 339,677	$ 262,370	$ 226,011

Common Stock and Preferred St_2

Common Stock and Preferred Stock (Details) - $ / shares	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Common stock, shares authorized (in shares)	240,000,000	240,000,000	240,000,000
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000
Preferred stock, par value (in dollars per share)	$ 0.0001
Common stock, shares outstanding (in shares)	68,269,567	68,196,964	69,326,202
Preferred stock, shares outstanding	0	0
Voting Common Stock
Class of Stock [Line Items]
Common stock, shares authorized (in shares)	240,000,000
Common stock, par value (in dollars per share)	$ 0.0001
Non-Voting Common Stock
Class of Stock [Line Items]
Common stock, shares authorized (in shares)	50,000,000
Common stock, par value (in dollars per share)	$ 0.0001
Common stock, shares outstanding (in shares)	0	0

Stock-Based Compensation - Narr

Stock-Based Compensation - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense	$ 11,000	$ 8,500	$ 7,100
Stock-based compensation	11,021	8,522	7,105
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense	$ (2,600)	$ (1,600)	(2,600)
Shares repurchased (in shares)	28,000	1,200,000
Repurchase of common stock	$ 1,162	$ 46,580	$ 102,510
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)	2,110,598	2,377,650

Stock-Based Compensation - Rest

Stock-Based Compensation - Restricted Stock Activity (Details) - Restricted Stock - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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)	266,522
Granted (in shares)	185,054	103,030
Vested (in shares)	(141,089)
Forfeited (in shares)	(12,690)
Outstanding (in shares)	297,797	266,522
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)	$ 50.48
Granted, weighted-average grant date fair value (in dollars per share)	46.90	$ 51.68	$ 42.91
Vested, weighted-average grant date fair value (in dollars per share)	51.09
Forfeited, weighted-average grant date fair value (in dollars per share)	50.87
Outstanding, weighted-average grant date fair value (in dollars per share)	$ 47.95	$ 50.48
Total compensation cost not yet recognized	$ 9.1	$ 7.5
Total compensation cost not yet recognized, period for recognition	2 years 6 months	2 years
Total fair value of shares vested	$ 6.5	$ 4.3	$ 6.6
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 - Simu

Stock-Based Compensation - Simulation Model Input Variables (Details) - Market Condition Performance Share Awards	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility factor	0.39%	0.40%
Risk free interest rate (percent)	2.11%	1.47%
Expected term (in years)	2 years 11 months 15 days	2 years 11 months 4 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			24 Months Ended	48 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2021	Dec. 31, 2021
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Allocated share-based compensation expense	$ 11	$ 8.5	$ 7.1
Performance Shares Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award requisite service period	3 years
Period for share issuance after vesting	60 days
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Total compensation cost not yet recognized	$ 3.4	$ 3.7
Total compensation cost not yet recognized, period for recognition	1 year 9 months 18 days	1 year 6 months 7 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)	120,359				120,359
Granted (in shares)	52,074
Vested (in shares)	(83,977)
Forfeited (in shares)	(5,549)
Outstanding (in shares)	82,907	120,359
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)	$ 52.84				$ 52.84
Granted, weighted-average grant date fair value (in dollars per share)	52.07
Vested, weighted-average grant date fair value (in dollars per share)	50.88
Forfeited, weighted-average grant date fair value (in dollars per share)	59.01
Outstanding, weighted-average grant date fair value (in dollars per share)	$ 53.92	$ 52.84
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)	35,507				35,507
Granted (in shares)	26,037
Vested (in shares)	(18,717)
Forfeited (in shares)	(1,373)
Outstanding (in shares)	41,454	35,507
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)	$ 45.72				$ 45.72
Granted, weighted-average grant date fair value (in dollars per share)	46.21
Vested, weighted-average grant date fair value (in dollars per share)	39.18
Forfeited, weighted-average grant date fair value (in dollars per share)	51.04
Outstanding, weighted-average grant date fair value (in dollars per share)	$ 49.81	$ 45.72
Stock Appreciation Rights (SARs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
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]
Award performance period	2 years
Allocated share-based compensation expense	$ 1.2
Stock Appreciation Rights (SARs) [Member] | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Potential vesting percentage	0.00%
Stock Appreciation Rights (SARs) [Member] | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Potential vesting percentage	370.00%
Scenario, Forecast | Stock Appreciation Rights (SARs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award requisite service period				2 years	4 years

Earnings per Share (Details)

Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net income	$ 91,937	$ 97,480	$ 11,254	$ (44,922)	$ 246,975	$ 60,044	$ 77,241	$ 31,261	$ 155,749	$ 415,522	$ 263,483
Denominator:
Weighted-average shares outstanding, basic (in shares)									68,249	69,221	70,344
Effect of dilutive stock awards (in shares)									182	156	164
Adjusted weighted-average shares outstanding, diluted (in shares)									68,431	69,377	70,508
Earnings per Share:
Basic earnings per common share (in dollars per share)	$ 1.35	$ 1.43	$ 0.16	$ (0.66)	$ 3.59	$ 0.87	$ 1.11	$ 0.45	$ 2.28	$ 6	$ 3.75
Diluted earnings per common share (in dollars per share)	$ 1.34	$ 1.42	$ 0.16	$ (0.66)	$ 3.58	$ 0.86	$ 1.11	$ 0.45	$ 2.28	$ 5.99	$ 3.74
Antidilutive awards excluded from computation of earnings per common share (in shares)									145	85	66
Non-recurring income tax benefit due to the Tax Cuts and Jobs Act of 2017					$ 196,700
Tax Cuts And Jobs Act Of 2017, Income Tax Expense (Benefit), Per Basic Share					$ (2.86)					$ (2.84)
Non-recurring income tax benefit due to the Tax Cuts and Jobs Act of 2017 per diluted share (in dollars per share)					$ 2.85					$ 2.84

Debt and Other Obligations - Na

Debt and Other Obligations - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Corporate credit cards
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 33.6	$ 33.6
Line of credit, current	3.5	1.7
Lines of Credit with Counterparties for Jet Fuel and Derivatives [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	41.5	51.5
Aircraft fuel
Line of Credit Facility [Line Items]
Line of credit, current	$ 23	$ 24.2

Debt and Other Obligations - Lo

Debt and Other Obligations - Long-term Debt (Details) $ in Thousands	1 Months Ended		12 Months Ended
	May 31, 2018USD ($)aircraftagreement	Nov. 30, 2017USD ($)aircrafttrust	Dec. 31, 2018USD ($)aircraft	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Debt Instrument [Line Items]
Long-term Debt, Gross			$ 2,234,500	$ 1,539,600
Repayments of Long-term Debt			137,300	102,300
Long-term Debt, Current Maturities			$ 162,800	115,430	$ 84,354
Future Aircraft To Be Received | aircraft			50
Less unamortized discount, net			$ 47,700	36,700
Total			$ 2,024,000	1,387,498	$ 897,359
Enhanced Equipment Trust Certificate
Debt Instrument [Line Items]
Number of trusts | trust		3
Face amount, commitment for future issuance		$ 420,500
Enhanced Equipment Trust Certificate | 2017-1 EETC Class AA
Debt Instrument [Line Items]
State interest rate percentage			3.375%
Enhanced Equipment Trust Certificate | 2017-1 EETC Class A
Debt Instrument [Line Items]
State interest rate percentage			3.65%
Enhanced Equipment Trust Certificate | 2017-1 EETC Class B
Debt Instrument [Line Items]
State interest rate percentage			3.80%
2015-1 EETC Class B
Debt Instrument [Line Items]
Long-term Debt, Gross			$ 80,000	$ 92,000
Weighted-average interest rates			4.45%	4.45%
2015-1 EETC Class A
Debt Instrument [Line Items]
Long-term Debt, Gross			$ 378,600	$ 408,600
Weighted-average interest rates			4.10%	4.10%
Junior loans
Debt Instrument [Line Items]
Long-term Debt, Gross			$ 31,100	$ 39,300
Weighted-average interest rates			6.90%	6.90%
Senior loans
Debt Instrument [Line Items]
Long-term Debt, Gross			$ 382,400	$ 417,900
Weighted-average interest rates			4.10%	4.10%
2017-1 EETC Class B
Debt Instrument [Line Items]
Long-term Debt, Gross			$ 83,700	$ 13,800
Weighted-average interest rates			3.80%	3.80%
2017-1 EETC Class A
Debt Instrument [Line Items]
Long-term Debt, Gross			$ 80,800	$ 12,500
Weighted-average interest rates			3.65%	3.65%
2017-1 EETC Class AA
Debt Instrument [Line Items]
Long-term Debt, Gross			$ 242,500	$ 37,500
Weighted-average interest rates			3.38%	3.38%
Fixed-rate term loans
Debt Instrument [Line Items]
Long-term Debt, Gross			$ 625,100	$ 518,000
Debt instrument, face amount			$ 139,500
Weighted-average interest rates			3.88%	3.83%
Equipment Notes, Series C 2017-1
Debt Instrument [Line Items]
Long-term Debt, Gross			$ 85,500	$ 0
Number Of Amendments To Participation Agreements Entered Into During the Period | agreement	9
Weighted-average interest rates			5.11%
Equipment Notes, Series C 2017-1 | Enhanced Equipment Trust Certificate
Debt Instrument [Line Items]
State interest rate percentage			5.11%
Debt instrument, face amount	$ 65,700		$ 19,800
Debt Instrument, Face Amount, Commitment	$ 85,500
Revolving Credit Facility
Debt Instrument [Line Items]
Long-term Debt, Gross				0
Long-term line of credit			$ 135,300
Weighted-average interest rates			3.72%
Equipment Notes, Series C 2015-1
Debt Instrument [Line Items]
Long-term Debt, Gross			$ 109,500	0
Number Of Amendments To Participation Agreements Entered Into During the Period | agreement	15
Weighted-average interest rates			4.93%
Equipment Notes, Series C 2015-1 | Enhanced Equipment Trust Certificate
Debt Instrument [Line Items]
State interest rate percentage			4.93%
Debt instrument, face amount	$ 115,200
Minimum | Fixed-rate term loans
Debt Instrument [Line Items]
Long-term Debt, Term			10 years
Minimum | Fixed-rate term loans | Fixed-rate term loans
Debt Instrument [Line Items]
State interest rate percentage			4.06%
Maximum | Fixed-rate term loans
Debt Instrument [Line Items]
Long-term Debt, Term			12 years
Maximum | Fixed-rate term loans | Fixed-rate term loans
Debt Instrument [Line Items]
State interest rate percentage			4.10%
A321 | Equipment Notes, 2017-1 [Member] | Enhanced Equipment Trust Certificate
Debt Instrument [Line Items]
Number of delivered aircraft with secured debt financing commitments | aircraft			5
A320 | Equipment Notes, 2017-1 [Member] | Enhanced Equipment Trust Certificate
Debt Instrument [Line Items]
Number of delivered aircraft with secured debt financing commitments | aircraft			7
Airbus
Debt Instrument [Line Items]
Number of delivered aircraft with secured debt financing commitments | aircraft			60
Airbus | A321 | Enhanced Equipment Trust Certificate
Debt Instrument [Line Items]
Number of aircraft with secured debt financing commitments scheduled for delivery | aircraft		5
Airbus | A320
Debt Instrument [Line Items]
Future Aircraft To Be Received | aircraft			2
Airbus | A320 | Enhanced Equipment Trust Certificate
Debt Instrument [Line Items]
Number of aircraft with secured debt financing commitments scheduled for delivery | aircraft		7
Airbus | A320 | Fixed-rate term loans
Debt Instrument [Line Items]
Number of delivered aircraft with secured debt financing commitments | aircraft			4
Airbus | A320NEO
Debt Instrument [Line Items]
Future Aircraft To Be Received | aircraft			43
October 2015 to January 2017 | A321 | Equipment notes, 2015-1 [Member] | Enhanced Equipment Trust Certificate
Debt Instrument [Line Items]
Number of delivered aircraft with secured debt financing commitments | aircraft	12
February 2018 To March 2018 | A321 | Equipment Notes, 2017-1 [Member] | Enhanced Equipment Trust Certificate
Debt Instrument [Line Items]
Number of delivered aircraft with secured debt financing commitments | aircraft	5
February 2018 To March 2018 | Airbus | A321
Debt Instrument [Line Items]
Number of delivered aircraft with secured debt financing commitments | aircraft		5
December 2017 to April 2018 | A320 | Equipment Notes, 2017-1 [Member] | Enhanced Equipment Trust Certificate
Debt Instrument [Line Items]
Number of delivered aircraft with secured debt financing commitments | aircraft	4
October 2018 to August 2018 | A320 | Equipment Notes, 2017-1 [Member] | Enhanced Equipment Trust Certificate
Debt Instrument [Line Items]
Number of delivered aircraft with secured debt financing commitments | aircraft			3
March 2016 to June 2016 | A320 | Equipment notes, 2015-1 [Member] | Enhanced Equipment Trust Certificate
Debt Instrument [Line Items]
Number of delivered aircraft with secured debt financing commitments | aircraft	3
December 2017 To January 2018 | Airbus | A320
Debt Instrument [Line Items]
Number of delivered aircraft with secured debt financing commitments | aircraft		3
April 2018 To October 2018 | Airbus | A320
Debt Instrument [Line Items]
Number of delivered aircraft with secured debt financing commitments | aircraft		4
Lines of Credit with Counterparties for Jet Fuel and Derivatives [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity			$ 41,500	$ 51,500
Revolving Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity			$ 160,000

Debt and Other Obligations - Fu

Debt and Other Obligations - Future Maturities (Details) $ in Millions	Dec. 31, 2018USD ($)
Debt Disclosure [Abstract]
2,019	$ 171.3
2,020	303.6
2,021	162.1
2,022	159.5
2,023	298
2024 and beyond	1,140
Long-term debt	$ 2,234.5

Debt and Other Obligations - In

Debt and Other Obligations - Interest Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Commitment fees	$ 262	$ 124
Amortization of deferred financing costs	7,069	5,280
Total	83,777	57,302
Equipment Notes, Series C 2017-1
Debt Instrument [Line Items]
Interest expense, debt	2,478	0
2017-1 EETC Class B
Debt Instrument [Line Items]
Interest expense, debt	2,420	22
2017-1 EETC Class A
Debt Instrument [Line Items]
Interest expense, debt	2,172	19
Equipment Notes, Series C 2015-1
Debt Instrument [Line Items]
Interest expense, debt	3,575	0
2017-1 EETC Class AA
Debt Instrument [Line Items]
Interest expense, debt	6,026	54
2015-1 EETC Class B
Debt Instrument [Line Items]
Interest expense, debt	3,781	4,446
2015-1 EETC Class A
Debt Instrument [Line Items]
Interest expense, debt	16,138	17,230
Fixed-rate term loans
Debt Instrument [Line Items]
Interest expense, debt	19,615	8,610
Junior loans
Debt Instrument [Line Items]
Interest expense, debt	2,475	3,035
Senior loans
Debt Instrument [Line Items]
Interest expense, debt	16,869	18,328
Revolving Credit Facility
Debt Instrument [Line Items]
Interest expense, debt	793	0
Capital Lease Obligations
Debt Instrument [Line Items]
Interest expense, debt	$ 104	$ 154

Leases and Prepaid Maintenanc_3

Leases and Prepaid Maintenance Deposits - Operating Leases (Narrative) (Details) $ in Thousands	3 Months Ended				12 Months Ended
	Dec. 31, 2017USD ($)	Sep. 30, 2017USD ($)	Jun. 30, 2017USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2018USD ($)aircraft_engineaircraft	Dec. 31, 2017USD ($)aircraft	Dec. 31, 2016USD ($)aircraft
Operating Leased Assets [Line Items]
Total rental expense					$ 312,000	$ 309,800	$ 283,900
Rental expense charged to operations for aircraft and engine operating leases	$ 42,820	$ 53,396	$ 52,566	$ 57,070	177,641	205,852	201,675
Maintenance reserves, supplemental rent					1,300	400	2,200
Aircraft maintenance deposits					245,600
Aircraft maintenance deposits, net	175,615				106,901	175,615	87,035
Long-term aircraft maintenance deposits	$ 150,617				$ 138,738	150,617	199,415
Number of Spare Engines Capitalized | aircraft					8
Number of leased engines delivered | aircraft_engine					1
Property, Plant And Equipment, Number Of Engines Sold | aircraft_engine					6
Spare engines
Operating Leased Assets [Line Items]
Number of leased assets financed under operating leases | aircraft_engine					12
A320 Family
Operating Leased Assets [Line Items]
Number of Aircraft Held | aircraft					128
A320 Family | Aircraft
Operating Leased Assets [Line Items]
Number of leased assets financed under operating leases | aircraft					46
Fixed Maintenance Reserve Payments
Operating Leased Assets [Line Items]
Committed expenditures, 2019					$ 5,800
Committed expenditures, 2020					5,600
Committed expenditures, 2021					5,700
Committed expenditures, 2022					4,900
Committed expenditures, 2023					4,100
Committed expenditures, 2024 and beyond					8,800
Aircraft Supplemental Rent
Operating Leased Assets [Line Items]
Supplemental rent expense					$ 3,400	$ 8,400	$ 9,000
Minimum
Operating Leased Assets [Line Items]
Lease term					3 years
Maximum
Operating Leased Assets [Line Items]
Lease term					5 years
Aircraft
Operating Leased Assets [Line Items]
Number of previously leased aircraft purchased off lease | aircraft					14	1
Number of aircraft and spare engines financed under operating leases with a variable rate | aircraft					1
Number of aircrafts delivered under secured debt arrangements | aircraft					14
Aircraft | A320 Family
Operating Leased Assets [Line Items]
Number of aircraft capitalized | aircraft					82
Number of previously leased aircraft purchased off lease | aircraft					22
Aircraft | Airbus A319
Operating Leased Assets [Line Items]
Number of previously leased aircraft purchased off lease | aircraft							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					10
Spare engines | Minimum
Operating Leased Assets [Line Items]
Operating leases, term					2 years
Spare engines | Maximum
Operating Leased Assets [Line Items]
Operating leases, term					14 years
Third Party Lessor
Operating Leased Assets [Line Items]
Number of direct lease aircraft from a third-party lessor | aircraft					2
Spare engines
Operating Leased Assets [Line Items]
Number of previously leased aircraft purchased off lease | aircraft					1
Proceeds from Sale of Other Property, Plant, and Equipment					$ 11,400
Gain (Loss) on Disposition of Assets					$ (5,200)
Spare engines | Spare engines
Operating Leased Assets [Line Items]
Number of previously leased aircraft purchased off lease | aircraft						1

Leases and Prepaid Maintenanc_4

Leases and Prepaid Maintenance Deposits - Future Minimum Lease Payments Under Noncancelable Operating Leases (Details) $ in Thousands	Dec. 31, 2018USD ($)
Total Operating and Capital Lease Obligations
2,019	$ 229,693
2,020	200,476
2,021	185,606
2,022	170,011
2,023	147,847
2024 and thereafter	439,802
Total minimum lease payments	1,373,435
Office Equipment
Capital Leases
2,019	810
2,020	408
2,021	186
2,022	158
2,023	145
2024 and thereafter	0
Total minimum lease payments	1,707
Less amount representing interest	152
Present value of minimum lease payments	1,555
Less current portion	721
Long-term portion	834
Aircraft and Spare Engine Leases
Operating Leases
2,019	176,256
2,020	170,106
2,021	168,416
2,022	155,135
2,023	133,915
2024 and thereafter	350,266
Total minimum lease payments	1,154,094
Property Facility Leases
Operating Leases
2,019	52,627
2,020	29,962
2,021	17,004
2,022	14,718
2,023	13,787
2024 and thereafter	89,536
Total minimum lease payments	$ 217,634

Financial Instruments and Ris_2

Financial Instruments and Risk Management - Narrative (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2015derivative_instrument	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Derivative [Line Items]
Interest rate derivative losses reclassified into earnings, net of tax		$ 241,000	$ (37,000)	$ 224,000
Loss reclassified from AOCI into income, tax		75,000	372,000	130,000
Accumulated other comprehensive loss		1,193,000	1,464,000	$ 1,345,000
Interest rate swap
Derivative [Line Items]
Number of interest rate derivatives settled | derivative_instrument	6
Accumulated other comprehensive loss		1,100,000	1,400,000
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		316,000	335,000
Interest rate derivative losses reclassified into earnings, net of tax		$ 241,000	$ (37,000)

Defined Contribution 401(k) P_2

Defined Contribution 401(k) Plan (Details) $ in Millions	Mar. 01, 2018	Apr. 30, 2016	Dec. 31, 2018USD ($)defined_contribution_plan	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
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 | $			$ 36.7	$ 19.6	$ 16.2
Employee Retirement Savings Plan | Flight Attendants, Dispatchers and Mechanics
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, Dispatchers and Mechanics
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%
Defined Contribution Plan, Employer Contribution, Percent Of Employees' Gross Pay, Initial Percentage	11.00%
Defined Contribution Plan, Employer Contribution, Percent Of Employees' Gross Pay, Annual Percentage Increase	1.00%
Defined Contribution Plan, Employer Contribution, Percent Of Employees' Gross Pay, Final Percentage	15.00%
Pilots’ Retirement Savings Plan | Maximum
Defined Contribution Plan Disclosure [Line Items]
Defined contribution 401(k) plan, maximum employer contribution percent of the employee’s annual compensation			9.00%
Puerto Rico Retirement Savings Plan | Pilots
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, employer matching contribution, percent of match			100.00%
Defined Contribution Plan, Employer Contribution, Percent Of Employees' Gross Pay, Initial Percentage	11.00%
Defined Contribution Plan, Employer Contribution, Percent Of Employees' Gross Pay, Annual Percentage Increase	1.00%
Defined Contribution Plan, Employer Contribution, Percent Of Employees' Gross Pay, Final Percentage	15.00%
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	3 Months Ended				12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current:
Federal					$ (2,178)	$ (68,601)	$ 60,079
State and local					410	515	6,322
Foreign					4,692	2,742	2,034
Total current expense (benefit)					2,924	(65,344)	68,435
Deferred:
Federal					42,246	(9,349)	81,682
State and local					4,057	8,857	3,657
Total deferred expense (benefit)					46,303	(492)	85,339
Total income tax expense (benefit)	$ (165,231)	$ 34,506	$ 45,391	$ 19,498	$ 49,227	$ (65,836)	$ 153,774

Income Taxes - Income Tax Recon

Income Taxes - Income Tax Reconciliation (Details)	Dec. 22, 2017	Dec. 21, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Income Tax Expense
Expected provision at federal statutory tax rate	21.00%	35.00%	21.00%	35.00%	35.00%
State tax expense, net of federal benefit			1.70%	1.70%	1.60%
Revaluation of deferred taxes			0.00%	(56.30%)	0.00%
Other			1.30%	0.70%	0.30%
Total income tax expense (benefit)			24.00%	(18.90%)	36.90%

Income Taxes - Deferred Taxes (

Income Taxes - Deferred Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Deferred Tax Assets, Tax Credit Carryforwards	$ 10,004	$ 5,980
Deferred tax assets:
Income tax credits	155,670	871
Deferred revenue	6,824	5,957
Nondeductible accruals	14,691	10,107
Deferred manufacturing credits	0	258
Accrued maintenance	2,168	1,991
Equity compensation	2,592	2,392
Other	5,262	4,684
Valuation allowance	(254)	(454)
Deferred tax assets	196,957	31,786
Deferred tax liabilities:
Deferred gain (loss) on leases, net	1,672	1,605
Accrued rent	6,068	12,055
Prepaid expenses	793	754
Property, plant and equipment	481,847	298,703
Deferred financing costs	189	225
Accrued aircraft and engine maintenance	61,529	27,258
Deferred tax liabilities	552,098	340,600
Net deferred tax assets (liabilities)	$ (355,141)	$ (308,814)

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($) $ in Thousands	Dec. 22, 2017	Dec. 21, 2017	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Summary Of Income Taxes [Line Items]
Expected provision at federal statutory tax rate	21.00%	35.00%					21.00%	35.00%	35.00%
Reduction in net deferred tax liabilities			$ 196,000
Alternative minimum tax							$ 1,100
Valuation allowance			454				254	$ 454
Foreign tax credits							8,600
State net operating losses							1,400
Excess tax (deficiency) benefit from equity-based compensation							0	0	$ (470)
Income tax deficiency			$ (165,231)	$ 34,506	$ 45,391	$ 19,498	49,227	(65,836)	153,774
Domestic Tax Authority
Summary Of Income Taxes [Line Items]
Operating Loss Carryforwards							703,000
State and Local Jurisdiction
Summary Of Income Taxes [Line Items]
Operating Loss Carryforwards							114,500
Accounting Standards Update 2016-09
Summary Of Income Taxes [Line Items]
Income tax deficiency							$ 400	$ 500	$ (500)

Commitments and Contingencies -

Commitments and Contingencies - Schedule of Future Aircraft Operating Leases (Details)	1 Months Ended	3 Months Ended
	Jul. 31, 2015aircraftseat	Mar. 31, 2018aircraft	Dec. 31, 2018aircraft_engineaircraft	Sep. 30, 2018aircraft
Long-term Purchase Commitment [Line Items]
2,019			16
2,020			16
2,021			18
Total future aircraft to be received			50
Airbus
Long-term Purchase Commitment [Line Items]
Number of delivered aircraft with secured debt financing commitments			60
Airbus | A320
Long-term Purchase Commitment [Line Items]
2,019			2
2,020
2,021
Total future aircraft to be received			2
Number of aircrafts with increased seating capacity	40
Airbus | A320NEO
Long-term Purchase Commitment [Line Items]
2,019			9
2,020			16
2,021			18
Total future aircraft to be received			43
Third Party Lessor | A320NEO
Long-term Purchase Commitment [Line Items]
2,019			5
2,020
Total future aircraft to be received			5
Aircraft order contract renegotiation | Airbus | Airbus A320
Long-term Purchase Commitment [Line Items]
Number of Aircraft Order Conversions			5
Number Of Aircraft Order Conversions, Delivered			3
2018 | Airbus
Long-term Purchase Commitment [Line Items]
Number of aircraft with secured debt financing commitments scheduled for delivery			2
2018-2021 [Member] | Airbus
Long-term Purchase Commitment [Line Items]
Number of aircraft without secured financing commitments scheduled for delivery			43
2019 | Airbus | A320NEO
Long-term Purchase Commitment [Line Items]
Future Aircraft To Be Received, Prior to Amended Agreement			14
Capital Lease Obligations | Airbus A319
Long-term Purchase Commitment [Line Items]
Property, Plant and Equipment, Number Of Aircraft Purchased		14
V2500 SelectOne Engine
Long-term Purchase Commitment [Line Items]
Number of spare aircraft engines ordered | aircraft_engine			2
PurePower PW1100G-JM Engine
Long-term Purchase Commitment [Line Items]
Number of spare aircraft engines ordered | aircraft_engine			8
Equipment Notes, 2017-1 [Member] | 2018 | A320
Long-term Purchase Commitment [Line Items]
Number of aircraft with secured debt financing commitments scheduled for delivery			2
Minimum | Airbus | A320
Long-term Purchase Commitment [Line Items]
Number of seats | seat	178
Maximum | Airbus | A320
Long-term Purchase Commitment [Line Items]
Number of seats | seat	182

Commitments and Contingencies_2

Commitments and Contingencies - Aircraft-Related Commitments and Financing Arrangements (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2018	Nov. 30, 2017
Unrecorded Unconditional Purchase Obligation [Line Items]
Capital Leases, Lessee Balance Sheet, Assets by Major Class, Flight Equipment, Gross		$ 236.7
Capital Lease Obligations		143.8
Interest Commitments
Interest commitments, 2019			$ 171.3
Secured Debt
Interest Commitments
Interest commitments, 2019			83.3
Interest commitments, 2020			76.2
Interest commitments, 2021			69.2
Interest commitments, 2022			62.4
Interest commitments, 2023			52.3
Interest commitments, 2024 and beyond			150.3
Principal and Interest Commitments
Principal and interest commitments, 2019			5.7
Principal and interest commitments, 2020			9.4
Principal and interest commitments, 2021			8.1
Principal and interest commitments, 2022			7.1
Principal and interest commitments, 2023 and beyond			52.4
Enhanced Equipment Trust Certificate
Committed Expenditures
Face amount, commitment for future issuance				$ 420.5
Aircraft and Related Flight Equipment
Committed Expenditures
Committed expenditures, 2019			583.8
Committed expenditures, 2020			811.4
Committed expenditures, 2021			775.6
Committed expenditures, 2022			17.7
Committed expenditures, 2023			8.4
Committed expenditures, 2024 and beyond			0
Face amount, commitment for future issuance			70
Non-aircraft Related Commitments
Committed Expenditures
Committed expenditures, 2019			14.4
Committed expenditures, 2020			14.3
Committed expenditures, 2021			11.3
Committed expenditures, 2022			11.2
Committed expenditures, 2023			11.3
Committed expenditures, 2024 and beyond			56
A320NEO | Third Party Lessor
Aircraft Rent Commitments [Abstract]
Aircraft rent commitments, 2019			18.2
Aircraft rent commitments, 2020			20.2
Aircraft rent commitments, 2021			20.2
Aircraft rent commitments, 2022			20.2
Aircraft rent commitments, 2023			20.2
Aircraft rent commitments, 2024 and thereafter			$ 143.3
Airbus A319
Unrecorded Unconditional Purchase Obligation [Line Items]
Property, Plant and Equipment Purchased, Consideration Transferred	$ 285	$ 285

Commitments and Contingencies_3

Commitments and Contingencies - Employees (Details) $ in Millions	1 Months Ended				12 Months Ended
	Oct. 31, 2018	Feb. 28, 2018	May 31, 2016	Mar. 31, 2016	Dec. 31, 2018USD ($)employee_group	Dec. 31, 2017USD ($)employee_group	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Health Insurance Product Line
Concentration Risk [Line Items]
Accrued health care claims					$ 4.4	$ 3.9
Air Line Pilots Association, International (ALPA)
Concentration Risk [Line Items]
Collective Bargaining Arrangement, Term		5 years
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
Professional Airline Flight Control Association (PAFCA)
Concentration Risk [Line Items]
Collective Bargaining Arrangement, Term	5 years
Unionized employees concentration risk | Number of employees (percent)
Concentration Risk [Line Items]
Union-represented employee groups | employee_group					5	4
Concentration of risk					80.00%	75.00%
Unionized employees concentration risk | Number of employees (percent) | Air Line Pilots Association, International (ALPA)
Concentration Risk [Line Items]
Concentration of risk					27.00%
Unionized employees concentration risk | Number of employees (percent) | Association of Flight Attendants (AFA-CWA)
Concentration Risk [Line Items]
Concentration of risk					45.00%
Unionized employees concentration risk | Number of employees (percent) | Professional Airline Flight Control Association (PAFCA)
Concentration Risk [Line Items]
Concentration of risk					1.00%
Unionized employees concentration risk | Number of employees (percent) | International Association of Machinists and Aerospace Workers (IAMAW)
Concentration Risk [Line Items]
Concentration of risk					4.00%
Unionized employees concentration risk | Number of employees (percent) | Transport Workers Union of America (TWU)
Concentration Risk [Line Items]
Concentration of risk					3.00%

Fair Value Measurements - Long-

Fair Value Measurements - Long-term Debt (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	$ 2,234.5	$ 1,539.6
Estimate of Fair Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	2,169.6	1,583.2
Equipment Notes, Series C 2015-1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	109.5	0
Equipment Notes, Series C 2015-1 | Estimate of Fair Value Measurement [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	107.9	0
2015-1 EETC Class B
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	80	92
2015-1 EETC Class B | Estimate of Fair Value Measurement [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	78.1	94.2
2015-1 EETC Class A
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	378.6	408.6
2015-1 EETC Class A | Estimate of Fair Value Measurement [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	374.8	420.9
Fixed-rate term loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	625.1	518
Fixed-rate term loans | Estimate of Fair Value Measurement [Member] | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	600.1	528.6
Junior loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	31.1	39.3
Junior loans | Estimate of Fair Value Measurement [Member] | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	31.1	40.4
Senior loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	382.4	417.9
Senior loans | Estimate of Fair Value Measurement [Member] | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	373.6	435.3
Revolving Credit Facility
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value		0
Long-term line of credit	135.3
Revolving Credit Facility | Estimate of Fair Value Measurement [Member] | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	135.3	0
Equipment Notes, Series C 2017-1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	85.5	0
Equipment Notes, Series C 2017-1 | Estimate of Fair Value Measurement [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	84.2	0
2017-1 EETC Class B
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	83.7	13.8
2017-1 EETC Class B | Estimate of Fair Value Measurement [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	79.1	13.8
2017-1 EETC Class A
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	80.8	12.5
2017-1 EETC Class A | Estimate of Fair Value Measurement [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	76.6	12.6
2017-1 EETC Class AA
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	242.5	37.5
2017-1 EETC Class AA | Estimate of Fair Value Measurement [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	$ 228.8	$ 37.4

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, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 1,004.7	$ 800.8
Short-term investment securities	102.8	100.9
Total assets	1,107.5	901.7
Total liabilities	0	0
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	1,004.7	800.8
Short-term investment securities	102.8	100.9
Total assets	1,107.5	901.7
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 Relate_3

Operating Segments and Related Disclosures - Geographic Revenues (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Total operating revenues	$ 862,795	$ 904,330	$ 851,771	$ 704,138	$ 666,182	$ 687,227	$ 700,185	$ 589,957	$ 3,323,034	$ 2,643,552	$ 2,320,021
DOT—Domestic
Revenues from External Customers and Long-Lived Assets [Line Items]
Total operating revenues									2,990,700	2,432,100	2,134,400
DOT—Latin America and Caribbean
Revenues from External Customers and Long-Lived Assets [Line Items]
Total operating revenues									$ 332,300	$ 211,500	$ 185,600

Operating Segments and Relate_4

Operating Segments and Related Disclosures - Foreign Revenues (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Total passenger revenue | Any individual foreign country
Concentration Risk [Line Items]
Concentration of risk (not greater than)	4.00%	4.00%	4.00%

Quarterly Financial Data (Una_3

Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Total operating revenues	$ 862,795	$ 904,330	$ 851,771	$ 704,138	$ 666,182	$ 687,227	$ 700,185	$ 589,957	$ 3,323,034	$ 2,643,552	$ 2,320,021
Operating income (loss)	136,065	145,125	108,521	(38,797)	91,296	103,874	131,312	58,342	350,914	384,825	441,458
Net income	$ 91,937	$ 97,480	$ 11,254	$ (44,922)	$ 246,975	$ 60,044	$ 77,241	$ 31,261	$ 155,749	$ 415,522	$ 263,483
Basic earnings per share (in dollars per share)	$ 1.35	$ 1.43	$ 0.16	$ (0.66)	$ 3.59	$ 0.87	$ 1.11	$ 0.45	$ 2.28	$ 6	$ 3.75
Diluted earnings per share (in dollars per share)	$ 1.34	$ 1.42	$ 0.16	$ (0.66)	$ 3.58	$ 0.86	$ 1.11	$ 0.45	$ 2.28	5.99	$ 3.74
Non-recurring income tax benefit due to the Tax Cuts and Jobs Act of 2017					$ 196,700
Non-recurring income tax benefit due to the Tax Cuts and Jobs Act of 2017 per basic share (in dollars per share)					$ 2.86					2.84
Non-recurring income tax benefit due to the Tax Cuts and Jobs Act of 2017 per diluted share (in dollars per share)					$ 2.85					$ 2.84