Condensed Consolidating Financial Information - Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 537,002			$ 373,056
Short-term investments	442,106			245,599
Accounts receivable, net	30,106			97,380
Income taxes receivable	67,758			64,192
Spare parts and supplies, net	36,621			37,630
Prepaid expenses and other	54,918			56,849
Total	1,168,511			874,706
Property and equipment at cost	2,980,144			3,079,316
Less accumulated depreciation and amortization	(858,926)			(762,544)
Property and equipment, net	2,121,218			2,316,772
Operating lease right-of-use assets	647,288			632,545
Long-term prepayments and other	146,619			182,438
Goodwill and other intangible assets, net	13,500			120,163
Intercompany receivable	0			0
Investment in consolidated subsidiaries	0			0
Total Assets	4,097,136			4,126,624
Current liabilities:
Accounts payable	96,160			148,748
Air traffic liability and current frequent flyer deferred revenue	515,424			606,684
Other accrued liabilities	132,734			161,430
Current maturities of long-term debt, less discount	114,810			53,273
Current maturities of finance lease obligations	21,618			21,857
Current maturities of operating leases	81,881			83,224
Total	962,627			1,075,216
Long-term debt	1,035,971			547,254
Intercompany payable	0			0
Other liabilities and deferred credits:
Noncurrent finance lease obligations	126,159			141,861
Noncurrent operating leases	524,172			514,685
Accumulated pension and other post-retirement benefit obligations	223,907			203,596
Other liabilities and deferred credits	78,849			97,434
Noncurrent frequent flyer deferred revenue	186,618			175,218
Deferred tax liabilities, net	241,618			289,564
Total	1,381,323			1,422,358
Shareholders’ equity	717,215	$ 825,933	$ 924,366	1,081,796	$ 1,066,847	$ 1,006,690	$ 975,060	$ 947,994
Total Liabilities and Shareholders’ Equity	4,097,136			4,126,624
Eliminations
Current assets:
Cash and cash equivalents	0			0
Short-term investments	0			0
Accounts receivable, net	(671)			(949)
Income taxes receivable	0			0
Spare parts and supplies, net	0			0
Prepaid expenses and other	0			0
Total	(671)			(949)
Property and equipment at cost	0			0
Less accumulated depreciation and amortization	0			0
Property and equipment, net	0			0
Operating lease right-of-use assets	0			0
Long-term prepayments and other	0			0
Goodwill and other intangible assets, net	0			0
Intercompany receivable	(569,107)			(550,075)
Investment in consolidated subsidiaries	(1,266,865)			(1,620,453)
Total Assets	(1,836,643)			(2,171,477)
Current liabilities:
Accounts payable	(671)			(949)
Air traffic liability and current frequent flyer deferred revenue	0			0
Other accrued liabilities	0			0
Current maturities of long-term debt, less discount	0			0
Current maturities of finance lease obligations	0			0
Current maturities of operating leases	0			0
Total	(671)			(949)
Long-term debt	0			0
Intercompany payable	(569,107)			(550,075)
Other liabilities and deferred credits:
Noncurrent finance lease obligations	0			0
Noncurrent operating leases	0			0
Accumulated pension and other post-retirement benefit obligations	0			0
Other liabilities and deferred credits	0			0
Noncurrent frequent flyer deferred revenue	0			0
Deferred tax liabilities, net	0			0
Total	0			0
Shareholders’ equity	(1,266,865)			(1,620,453)
Total Liabilities and Shareholders’ Equity	(1,836,643)			(2,171,477)
Parent Issuer / Guarantor | Reportable legal entities
Current assets:
Cash and cash equivalents	9,452			1,228
Short-term investments	0			0
Accounts receivable, net	0			0
Income taxes receivable	0			0
Spare parts and supplies, net	0			0
Prepaid expenses and other	124			90
Total	9,576			1,318
Property and equipment at cost	0			0
Less accumulated depreciation and amortization	0			0
Property and equipment, net	0			0
Operating lease right-of-use assets	0			0
Long-term prepayments and other	50			0
Goodwill and other intangible assets, net	0			0
Intercompany receivable	0			0
Investment in consolidated subsidiaries	1,266,361			1,619,949
Total Assets	1,275,987			1,621,267
Current liabilities:
Accounts payable	947			529
Air traffic liability and current frequent flyer deferred revenue	0			0
Other accrued liabilities	0			0
Current maturities of long-term debt, less discount	0			0
Current maturities of finance lease obligations	0			0
Current maturities of operating leases	0			0
Total	947			529
Long-term debt	0			0
Intercompany payable	557,825			538,942
Other liabilities and deferred credits:
Noncurrent finance lease obligations	0			0
Noncurrent operating leases	0			0
Accumulated pension and other post-retirement benefit obligations	0			0
Other liabilities and deferred credits	0			0
Noncurrent frequent flyer deferred revenue	0			0
Deferred tax liabilities, net	0			0
Total	0			0
Shareholders’ equity	717,215			1,081,796
Total Liabilities and Shareholders’ Equity	1,275,987			1,621,267
Subsidiary Issuer / Guarantor | Reportable legal entities
Current assets:
Cash and cash equivalents	518,402			362,933
Short-term investments	442,106			245,599
Accounts receivable, net	30,533			95,141
Income taxes receivable	67,758			64,192
Spare parts and supplies, net	36,621			37,630
Prepaid expenses and other	54,748			56,743
Total	1,150,168			862,238
Property and equipment at cost	2,912,019			2,987,222
Less accumulated depreciation and amortization	(831,355)			(739,930)
Property and equipment, net	2,080,664			2,247,292
Operating lease right-of-use assets	647,288			632,545
Long-term prepayments and other	146,101			182,051
Goodwill and other intangible assets, net	13,000			119,663
Intercompany receivable	569,107			550,075
Investment in consolidated subsidiaries	0			0
Total Assets	4,606,328			4,593,864
Current liabilities:
Accounts payable	93,885			139,764
Air traffic liability and current frequent flyer deferred revenue	509,326			600,851
Other accrued liabilities	132,545			161,125
Current maturities of long-term debt, less discount	114,810			53,273
Current maturities of finance lease obligations	21,618			21,857
Current maturities of operating leases	81,881			83,224
Total	954,065			1,060,094
Long-term debt	1,035,971			547,254
Intercompany payable	0			0
Other liabilities and deferred credits:
Noncurrent finance lease obligations	126,159			141,861
Noncurrent operating leases	524,172			514,685
Accumulated pension and other post-retirement benefit obligations	223,907			203,596
Other liabilities and deferred credits	77,748			96,338
Noncurrent frequent flyer deferred revenue	186,618			175,218
Deferred tax liabilities, net	241,618			289,564
Total	1,380,222			1,421,262
Shareholders’ equity	1,236,070			1,565,254
Total Liabilities and Shareholders’ Equity	4,606,328			4,593,864
Non-Guarantor Subsidiaries | Reportable legal entities
Current assets:
Cash and cash equivalents	9,148			8,895
Short-term investments	0			0
Accounts receivable, net	244			3,188
Income taxes receivable	0			0
Spare parts and supplies, net	0			0
Prepaid expenses and other	46			16
Total	9,438			12,099
Property and equipment at cost	68,125			92,094
Less accumulated depreciation and amortization	(27,571)			(22,614)
Property and equipment, net	40,554			69,480
Operating lease right-of-use assets	0			0
Long-term prepayments and other	468			387
Goodwill and other intangible assets, net	500			500
Intercompany receivable	0			0
Investment in consolidated subsidiaries	504			504
Total Assets	51,464			82,970
Current liabilities:
Accounts payable	1,999			9,404
Air traffic liability and current frequent flyer deferred revenue	6,098			5,833
Other accrued liabilities	189			305
Current maturities of long-term debt, less discount	0			0
Current maturities of finance lease obligations	0			0
Current maturities of operating leases	0			0
Total	8,286			15,542
Long-term debt	0			0
Intercompany payable	11,282			11,133
Other liabilities and deferred credits:
Noncurrent finance lease obligations	0			0
Noncurrent operating leases	0			0
Accumulated pension and other post-retirement benefit obligations	0			0
Other liabilities and deferred credits	1,101			1,096
Noncurrent frequent flyer deferred revenue	0			0
Deferred tax liabilities, net	0			0
Total	1,101			1,096
Shareholders’ equity	30,795			55,199
Total Liabilities and Shareholders’ Equity	$ 51,464			$ 82,970