Condensed Consolidated Financial Statements (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017		Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts receivable	$ 62,233			$ 62,233		$ 42,731	[1]
Income Statement:
Total operating revenues	829,275	$ 812,673	[1]	2,418,190	$ 2,351,351
Salaries, wages and benefits	301,378	304,014	[1]	901,775	896,476
Other operating expenses	68,859	65,432	[1]	206,511	190,235
Total operating expenses	691,350	700,304	[1]	2,065,412	2,056,092
OPERATING INCOME	137,925	112,369	[1]	352,778	295,259
ASSETS
Other long-term assets	68,164			68,164		65,341	[1]
LIABILITIES:
Other long-term liabilities	62,678			62,678		58,662	[1]
Previously Reported | ASU 2014-09
Income Statement:
Total operating revenues		831,936			2,407,110
Salaries, wages and benefits		303,997			899,966
Other operating expenses		78,948			229,211
Total operating expenses		719,567			2,111,851
OPERATING INCOME		112,369			295,259
ASSETS
Other long-term assets						49,220
LIABILITIES:
Other long-term liabilities						42,541
Adjustments | ASU 2014-09
Income Statement:
Total operating revenues		(19,263)			(55,759)
Salaries, wages and benefits		17			(3,490)
Other operating expenses		(13,516)			(38,976)
Total operating expenses		(19,263)			(55,759)
ASSETS
Other long-term assets						16,121
LIABILITIES:
Other long-term liabilities						16,121
Flying agreements
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts receivable	56,400			56,400		$ 33,900
Income Statement:
Total operating revenues	816,057	800,965	[1]	2,377,659	2,317,218
Flying agreements | Previously Reported | ASU 2014-09
Income Statement:
Total operating revenues		812,295			2,349,047
Flying agreements | Adjustments | ASU 2014-09
Income Statement:
Total operating revenues		(11,330)			(31,829)
Airport customer service and other
Income Statement:
Total operating revenues	13,218	11,708	[1]	40,531	34,133
Airport customer service and other | Previously Reported | ASU 2014-09
Income Statement:
Total operating revenues		19,641			58,063
Airport customer service and other | Adjustments | ASU 2014-09
Income Statement:
Total operating revenues		(7,933)			(23,930)
Aircraft fuel
Income Statement:
Other operating expenses		22,791			61,295
Total operating expenses	25,655	30,209	[1]	80,852	91,106
Aircraft fuel | Previously Reported | ASU 2014-09
Income Statement:
Other operating expenses		42,071			113,564
Aircraft fuel | Adjustments | ASU 2014-09
Income Statement:
Other operating expenses		(19,280)			(52,269)
Airport related expenses
Income Statement:
Other operating expenses		30,209			91,106
Total operating expenses	$ 30,258	22,791	[1]	87,208	61,295
Airport related expenses | Previously Reported | ASU 2014-09
Income Statement:
Other operating expenses		16,693			52,130
Airport related expenses | Adjustments | ASU 2014-09
Income Statement:
Other operating expenses		$ 13,516			38,976
Up-front contract reimbursements
Income Statement:
Total operating revenues				$ 1,400	$ 1,100

[1] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606). See Note 1 to the financial statements contained in Part I, Item 1 of this report for additional information.