Condensed Consolidating Financial Information - Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 405,713	$ 281,502	$ 345,174	$ 264,087
Restricted cash	5,000	5,000
Short-term investments	287,995	278,545
Accounts receivable, net	85,388	81,723
Spare parts and supplies, net	18,238	19,164
Prepaid expenses and other	57,918	75,050
Total	860,252	740,984
Property and equipment at cost	2,075,148	1,985,252
Less accumulated depreciation and amortization	(494,121)	(432,510)
Property and equipment, net	1,581,027	1,552,742
Long-term prepayments and other	67,468	70,873
Goodwill and other intangible assets, net	125,389	125,323
Total Assets	2,634,136	2,489,922
Current liabilities:
Accounts payable	107,456	101,310
Air traffic liability	515,388	430,766
Other accrued liabilities	150,848	160,258
Current maturities of long-term debt, less discount, and capital lease obligations	58,349	74,441
Total	832,041	766,775
Long-term debt and capital lease obligations	506,328	677,915
Other liabilities and deferred credits:
Accumulated pension and other postretirement benefit obligations	363,732	372,700
Other liabilities and deferred credits	100,674	89,845
Deferred tax liabilities, net	165,677	136,625
Total	630,083	599,170
Shareholders’ equity	665,684	446,062
Total Liabilities and Shareholders’ Equity	2,634,136	2,489,922
Eliminations
Current assets:
Cash and cash equivalents	0		0
Accounts receivable, net	(181)	(211)
Total	(181)	(211)
Deferred tax assets, net	(28,141)	(26,059)
Intercompany receivable	(277,511)	(242,248)
Investment in consolidated subsidiaries	(840,109)	(596,570)
Total Assets	(1,145,942)	(865,088)
Current liabilities:
Accounts payable	(181)	(211)
Total	(181)	(211)
Intercompany payable	(277,511)	(242,248)
Other liabilities and deferred credits:
Deferred tax liabilities, net	(28,141)	(26,059)
Total	(28,141)	(26,059)
Shareholders’ equity	(840,109)	(596,570)
Total Liabilities and Shareholders’ Equity	(1,145,942)	(865,088)
Parent Issuer / Guarantor
Current assets:
Cash and cash equivalents	67,381	69,420	65,949	79,532
Accounts receivable, net	61	61
Prepaid expenses and other	84	7
Total	67,526	69,488
Deferred tax assets, net	28,141	26,059
Investment in consolidated subsidiaries	840,109	596,570
Total Assets	935,776	692,117
Current liabilities:
Accounts payable	476	755
Other accrued liabilities	2,974	530
Current maturities of long-term debt, less discount, and capital lease obligations		288
Total	3,450	1,573
Intercompany payable	266,478	242,248
Other liabilities and deferred credits:
Other liabilities and deferred credits	164	2,234
Total	164	2,234
Shareholders’ equity	665,684	446,062
Total Liabilities and Shareholders’ Equity	935,776	692,117
Subsidiary Issuer / Guarantor
Current assets:
Cash and cash equivalents	330,326	203,406	271,376	179,676
Restricted cash	5,000	5,000
Short-term investments	287,995	278,545
Accounts receivable, net	84,980	81,248
Spare parts and supplies, net	18,238	19,164
Prepaid expenses and other	57,688	74,948
Total	784,227	662,311
Property and equipment at cost	2,006,345	1,927,126
Less accumulated depreciation and amortization	(486,588)	(427,315)
Property and equipment, net	1,519,757	1,499,811
Long-term prepayments and other	67,468	70,373
Goodwill and other intangible assets, net	123,655	125,323
Intercompany receivable	277,511	242,248
Total Assets	2,772,618	2,600,066
Current liabilities:
Accounts payable	106,520	100,007
Air traffic liability	512,271	427,302
Other accrued liabilities	147,598	159,583
Current maturities of long-term debt, less discount, and capital lease obligations	58,349	74,153
Total	824,738	761,045
Long-term debt and capital lease obligations	506,328	677,915
Other liabilities and deferred credits:
Accumulated pension and other postretirement benefit obligations	363,732	372,700
Other liabilities and deferred credits	99,680	86,861
Deferred tax liabilities, net	193,818	162,684
Total	657,230	622,245
Shareholders’ equity	784,322	538,861
Total Liabilities and Shareholders’ Equity	2,772,618	2,600,066
Non-Guarantor Subsidiaries
Current assets:
Cash and cash equivalents	8,006	8,676	$ 7,849	$ 4,879
Accounts receivable, net	528	625
Prepaid expenses and other	146	95
Total	8,680	9,396
Property and equipment at cost	68,803	58,126
Less accumulated depreciation and amortization	(7,533)	(5,195)
Property and equipment, net	61,270	52,931
Long-term prepayments and other		500
Goodwill and other intangible assets, net	1,734
Total Assets	71,684	62,827
Current liabilities:
Accounts payable	641	759
Air traffic liability	3,117	3,464
Other accrued liabilities	276	145
Total	4,034	4,368
Intercompany payable	11,033
Other liabilities and deferred credits:
Other liabilities and deferred credits	830	750
Total	830	750
Shareholders’ equity	55,787	57,709
Total Liabilities and Shareholders’ Equity	$ 71,684	$ 62,827