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