Condensed Consolidating Financial Information - Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 600,609	$ 373,056
Short-term investments	213,974	245,599
Accounts receivable, net	30,585	97,380
Income taxes receivable	99,665	64,192
Spare parts and supplies, net	38,481	37,630
Prepaid expenses and other	46,133	56,849
Total	1,029,447	874,706
Property and equipment at cost	3,095,693	3,079,316
Less accumulated depreciation and amortization	(796,958)	(762,544)
Property and equipment, net	2,298,735	2,316,772
Operating lease right-of-use assets	611,693	632,545
Long-term prepayments and other	183,355	182,438
Goodwill and other intangible assets, net	13,500	120,163
Intercompany receivable	0	0
Investment in consolidated subsidiaries	0	0
Total Assets	4,136,730	4,126,624
Current liabilities:
Accounts payable	152,310	148,748
Air traffic liability and current frequent flyer deferred revenue	623,741	606,684
Other accrued liabilities	138,355	161,430
Current maturities of long-term debt, less discount	59,794	53,273
Current maturities of finance lease obligations	22,045	21,857
Current maturities of operating leases	79,718	83,224
Total	1,075,963	1,075,216
Long-term debt	757,221	547,254
Intercompany payable	0	0
Other liabilities and deferred credits:
Noncurrent finance lease obligations	137,059	141,861
Noncurrent operating leases	495,500	514,685
Accumulated pension and other post-retirement benefit obligations	199,964	203,596
Other liabilities and deferred credits	79,911	97,434
Noncurrent frequent flyer deferred revenue	172,281	175,218
Deferred tax liabilities, net	294,465	289,564
Total	1,379,180	1,422,358
Shareholders’ equity	924,366	1,081,796	$ 975,060	$ 947,994
Total Liabilities and Shareholders’ Equity	4,136,730	4,126,624
Eliminations
Current assets:
Cash and cash equivalents	0	0
Short-term investments	0	0
Accounts receivable, net	(747)	(949)
Income taxes receivable	0	0
Spare parts and supplies, net	0	0
Prepaid expenses and other	0	0
Total	(747)	(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	(573,580)	(550,075)
Investment in consolidated subsidiaries	(1,487,136)	(1,620,453)
Total Assets	(2,061,463)	(2,171,477)
Current liabilities:
Accounts payable	(747)	(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	(747)	(949)
Long-term debt	0	0
Intercompany payable	(573,580)	(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	(1,487,136)	(1,620,453)
Total Liabilities and Shareholders’ Equity	(2,061,463)	(2,171,477)
Parent Issuer / Guarantor | Reportable Legal Entities
Current assets:
Cash and cash equivalents	354	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	132	90
Total	486	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	1,486,632	1,619,949
Total Assets	1,487,168	1,621,267
Current liabilities:
Accounts payable	504	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	504	529
Long-term debt	0	0
Intercompany payable	562,298	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	924,366	1,081,796
Total Liabilities and Shareholders’ Equity	1,487,168	1,621,267
Subsidiary Issuer / Guarantor | Reportable Legal Entities
Current assets:
Cash and cash equivalents	591,450	362,933
Short-term investments	213,974	245,599
Accounts receivable, net	28,991	95,141
Income taxes receivable	99,665	64,192
Spare parts and supplies, net	38,481	37,630
Prepaid expenses and other	45,779	56,743
Total	1,018,340	862,238
Property and equipment at cost	2,997,359	2,987,222
Less accumulated depreciation and amortization	(772,372)	(739,930)
Property and equipment, net	2,224,987	2,247,292
Operating lease right-of-use assets	611,693	632,545
Long-term prepayments and other	182,895	182,051
Goodwill and other intangible assets, net	13,000	119,663
Intercompany receivable	573,580	550,075
Investment in consolidated subsidiaries	0	0
Total Assets	4,624,495	4,593,864
Current liabilities:
Accounts payable	147,510	139,764
Air traffic liability and current frequent flyer deferred revenue	618,126	600,851
Other accrued liabilities	138,205	161,125
Current maturities of long-term debt, less discount	59,794	53,273
Current maturities of finance lease obligations	22,045	21,857
Current maturities of operating leases	79,718	83,224
Total	1,065,398	1,060,094
Long-term debt	757,221	547,254
Intercompany payable	0	0
Other liabilities and deferred credits:
Noncurrent finance lease obligations	137,059	141,861
Noncurrent operating leases	495,500	514,685
Accumulated pension and other post-retirement benefit obligations	199,964	203,596
Other liabilities and deferred credits	78,808	96,338
Noncurrent frequent flyer deferred revenue	172,281	175,218
Deferred tax liabilities, net	294,465	289,564
Total	1,378,077	1,421,262
Shareholders’ equity	1,423,799	1,565,254
Total Liabilities and Shareholders’ Equity	4,624,495	4,593,864
Non-Guarantor Subsidiaries | Reportable Legal Entities
Current assets:
Cash and cash equivalents	8,805	8,895
Short-term investments	0	0
Accounts receivable, net	2,341	3,188
Income taxes receivable	0	0
Spare parts and supplies, net	0	0
Prepaid expenses and other	222	16
Total	11,368	12,099
Property and equipment at cost	98,334	92,094
Less accumulated depreciation and amortization	(24,586)	(22,614)
Property and equipment, net	73,748	69,480
Operating lease right-of-use assets	0	0
Long-term prepayments and other	410	387
Goodwill and other intangible assets, net	500	500
Intercompany receivable	0	0
Investment in consolidated subsidiaries	504	504
Total Assets	86,530	82,970
Current liabilities:
Accounts payable	5,043	9,404
Air traffic liability and current frequent flyer deferred revenue	5,615	5,833
Other accrued liabilities	150	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	10,808	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	63,337	55,199
Total Liabilities and Shareholders’ Equity	$ 86,530	$ 82,970