Condensed Consolidating Financial Information - Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 592,520		$ 373,056
Short-term investments	168,410		245,599
Accounts receivable, net	28,467		97,380
Income taxes receivable	92,365		64,192
Spare parts and supplies, net	35,660		37,630
Prepaid expenses and other	45,431		56,849
Total	962,853		874,706
Property and equipment at cost	3,091,583		3,079,316
Less accumulated depreciation and amortization	(823,757)		(762,544)
Property and equipment, net	2,267,826		2,316,772
Operating lease right-of-use assets	592,933		632,545
Long-term prepayments and other	159,383		182,438
Goodwill and other intangible assets, net	13,500		120,163
Intercompany receivable	0		0
Investment in consolidated subsidiaries	0		0
Total Assets	3,996,495		4,126,624
Current liabilities:
Accounts payable	101,641		148,748
Air traffic liability and current frequent flyer deferred revenue	553,554		606,684
Other accrued liabilities	221,881		161,430
Current maturities of long-term debt, less discount	60,079		53,273
Current maturities of finance lease obligations	21,667		21,857
Current maturities of operating leases	78,655		83,224
Total	1,037,477		1,075,216
Long-term debt	792,766		547,254
Intercompany payable	0		0
Other liabilities and deferred credits:
Noncurrent finance lease obligations	131,631		141,861
Noncurrent operating leases	476,401		514,685
Accumulated pension and other post-retirement benefit obligations	200,411		203,596
Other liabilities and deferred credits	80,350		97,434
Noncurrent frequent flyer deferred revenue	179,740		175,218
Deferred tax liabilities, net	271,786		289,564
Total	1,340,319		1,422,358
Shareholders’ equity	825,933	$ 924,366	1,081,796	$ 1,006,690	$ 975,060	$ 947,994
Total Liabilities and Shareholders’ Equity	3,996,495		4,126,624
Eliminations
Current assets:
Cash and cash equivalents	0		0
Short-term investments	0		0
Accounts receivable, net	(122)		(949)
Income taxes receivable	0		0
Spare parts and supplies, net	0		0
Prepaid expenses and other	0		0
Total	(122)		(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	(561,095)		(550,075)
Investment in consolidated subsidiaries	(1,376,415)		(1,620,453)
Total Assets	(1,937,632)		(2,171,477)
Current liabilities:
Accounts payable	(122)		(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	(122)		(949)
Long-term debt	0		0
Intercompany payable	(561,095)		(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	(1,376,415)		(1,620,453)
Total Liabilities and Shareholders’ Equity	(1,937,632)		(2,171,477)
Parent Issuer / Guarantor | Reportable Legal Entities
Current assets:
Cash and cash equivalents	274		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	91		90
Total	365		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	341		0
Goodwill and other intangible assets, net	0		0
Intercompany receivable	0		0
Investment in consolidated subsidiaries	1,375,911		1,619,949
Total Assets	1,376,617		1,621,267
Current liabilities:
Accounts payable	871		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	871		529
Long-term debt	0		0
Intercompany payable	549,813		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	825,933		1,081,796
Total Liabilities and Shareholders’ Equity	1,376,617		1,621,267
Subsidiary Issuer / Guarantor | Reportable Legal Entities
Current assets:
Cash and cash equivalents	582,748		362,933
Short-term investments	168,410		245,599
Accounts receivable, net	28,289		95,141
Income taxes receivable	92,365		64,192
Spare parts and supplies, net	35,660		37,630
Prepaid expenses and other	45,266		56,743
Total	952,738		862,238
Property and equipment at cost	3,023,305		2,987,222
Less accumulated depreciation and amortization	(797,193)		(739,930)
Property and equipment, net	2,226,112		2,247,292
Operating lease right-of-use assets	592,933		632,545
Long-term prepayments and other	158,637		182,051
Goodwill and other intangible assets, net	13,000		119,663
Intercompany receivable	561,095		550,075
Investment in consolidated subsidiaries	0		0
Total Assets	4,504,515		4,593,864
Current liabilities:
Accounts payable	98,869		139,764
Air traffic liability and current frequent flyer deferred revenue	547,676		600,851
Other accrued liabilities	221,723		161,125
Current maturities of long-term debt, less discount	60,079		53,273
Current maturities of finance lease obligations	21,667		21,857
Current maturities of operating leases	78,655		83,224
Total	1,028,669		1,060,094
Long-term debt	792,766		547,254
Intercompany payable	0		0
Other liabilities and deferred credits:
Noncurrent finance lease obligations	131,631		141,861
Noncurrent operating leases	476,401		514,685
Accumulated pension and other post-retirement benefit obligations	200,411		203,596
Other liabilities and deferred credits	79,247		96,338
Noncurrent frequent flyer deferred revenue	179,740		175,218
Deferred tax liabilities, net	271,786		289,564
Total	1,339,216		1,421,262
Shareholders’ equity	1,343,864		1,565,254
Total Liabilities and Shareholders’ Equity	4,504,515		4,593,864
Non-Guarantor Subsidiaries | Reportable Legal Entities
Current assets:
Cash and cash equivalents	9,498		8,895
Short-term investments	0		0
Accounts receivable, net	300		3,188
Income taxes receivable	0		0
Spare parts and supplies, net	0		0
Prepaid expenses and other	74		16
Total	9,872		12,099
Property and equipment at cost	68,278		92,094
Less accumulated depreciation and amortization	(26,564)		(22,614)
Property and equipment, net	41,714		69,480
Operating lease right-of-use assets	0		0
Long-term prepayments and other	405		387
Goodwill and other intangible assets, net	500		500
Intercompany receivable	0		0
Investment in consolidated subsidiaries	504		504
Total Assets	52,995		82,970
Current liabilities:
Accounts payable	2,023		9,404
Air traffic liability and current frequent flyer deferred revenue	5,878		5,833
Other accrued liabilities	158		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	8,059		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	1,103		1,096
Noncurrent frequent flyer deferred revenue	0		0
Deferred tax liabilities, net	0		0
Total	1,103		1,096
Shareholders’ equity	32,551		55,199
Total Liabilities and Shareholders’ Equity	$ 52,995		$ 82,970