Commitments and Contingencies - Commitments and Obligations (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Operating lease payments for aircraft and facility obligations
Oct - Dec 2018	$ 25,327
2019	83,268
2020	98,790
2021	90,378
2022	72,438
Thereafter	125,570
Total	495,771
Firm aircraft and spare engine commitments
Oct - Dec 2018	211,485
2019	27,500
2020	27,500
2021	86,440
Total	352,925
Interest commitments
Oct - Dec 2018	60,498
2019	111,075
2020	96,051
2021	82,139
2022	69,227
Thereafter	181,423
Total	600,413
Principal maturities on long-term debt
October - 2018	90,653
2019	360,599
2020	342,080
2021	336,897
2022	342,439
Thereafter	1,662,100
Total	3,134,768	$ 2,712,350
Total commitments and obligations
Oct - Dec 2018	387,963
2019	582,442
2020	564,421
2021	595,854
2022	484,104
Thereafter	1,969,093
Total	$ 4,583,877
Variable interest rate (as percent)	1.00%