Revenue Recognition - Impact on Balance Sheets (Details) - USD ($) $ / shares in Units, $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 812,362	$ 800,849	$ 869,150	$ 700,900
Short-term investment securities	101,714	100,937		100,155
Accounts receivable, net	58,547	49,323		41,136
Aircraft maintenance deposits, net	107,252	175,615		87,035
Income tax receivable	70,672	69,844		0
Prepaid expenses and other current assets	79,788	85,542		48,321
Total current assets	1,230,335	1,282,110		977,547
Property and equipment:
Flight equipment	2,911,378	2,291,110		1,461,525
Ground property and equipment	168,039	155,166		126,206
Less accumulated depreciation	(261,314)	(207,808)		(122,509)
Total property and equipment	2,818,103	2,238,468		1,465,222
Deposits on flight equipment purchase contracts	240,224	253,687		325,688
Long-term aircraft maintenance deposits	141,183	150,617		199,415
Deferred heavy maintenance, net	172,799	99,915		75,534
Other long-term assets	79,081	121,003		110,223
Total assets	4,681,725	4,145,800		3,153,629
Current liabilities:
Accounts payable	50,310	22,822		15,193
Air traffic liability	357,645	263,711		220,184
Current maturities of long-term debt	145,900	115,430		84,354
Other current liabilities	346,407	262,370		226,011
Total current liabilities	900,227	664,333		545,742
Long-term debt, less current maturities	1,731,766	1,387,498		897,359
Deferred income taxes	295,601	308,814		302,700
Deferred gains and other long-term liabilities	20,630	22,581		22,644
Shareholders’ equity:
Common stock	7	7		7
Additional paid-in-capital	365,536	360,153		551,004
Treasury stock, at cost	(66,840)	(65,854)		(218,692)
Retained earnings	1,436,064	1,469,732		1,054,210
Accumulated other comprehensive income (loss)	(1,266)	(1,464)		(1,345)
Total shareholders’ equity	1,733,501	1,762,574		1,385,184
Total liabilities and shareholders’ equity	$ 4,681,725	$ 4,145,800		$ 3,153,629
Common stock, par value (in dollars per share)		$ 0.0001		$ 0.0001
Common stock, shares authorized (in shares)		240,000,000		240,000,000
Common stock, shares issued (in shares)		69,770,795		73,549,872
Common stock, shares outstanding (in shares)		68,196,964		69,326,202
Treasury stock (in shares)		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
Deposits on flight equipment purchase contracts		253,687		325,688
Long-term aircraft maintenance deposits		150,617		199,415
Deferred heavy maintenance, net		99,915		75,534
Other long-term assets		121,003		110,223
Total assets		4,143,950		3,151,927
Current liabilities:
Accounts payable		22,822		15,193
Air traffic liability		246,404		206,392
Current maturities of long-term debt		115,430		84,354
Other current liabilities		262,370		226,011
Total current liabilities		647,026		531,950
Long-term debt, less current maturities		1,387,498		897,359
Deferred income taxes		313,140		308,143
Deferred gains and other long-term liabilities		19,205		19,868
Shareholders’ equity:
Common stock		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
Total liabilities and shareholders’ equity		4,143,950		3,151,927
Accounting Standards Update 2014-09 [Member] | 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
Deposits on flight equipment purchase contracts		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, 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)
Total liabilities and shareholders’ equity		$ 1,850		$ 1,702