Commitments and Contingencies - Future Minimum Lease Payments Under Capital Leases and Noncancelable Operating Leases (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
Operating Leased Assets [Line Items]
Lease term	3 years
Total Operating and Capital Lease Obligations
remainder of 2018	$ 114,461
2019	211,490
2020	190,523
2021	177,150
2022	163,154
2023 and thereafter	519,669
Total minimum lease payments	1,376,447
Fixed Maintenance Reserve Payments, Aircraft and Related Flight Equipment [Member]
Committed Expenditures
Committed expenditures, remainder of 2018	3,700
Committed expenditures, 2019	5,800
Committed expenditures, 2020	5,600
Committed expenditures, 2021	5,700
Committed expenditures, 2022	4,900
Committed expenditures, 2023 and beyond	12,900
Non-aircraft Related Commitments [Member]
Committed Expenditures
Committed expenditures, remainder of 2018	5,300
Committed expenditures, 2019	12,500
Committed expenditures, 2020	12,500
Committed expenditures, 2021	9,800
Committed expenditures, 2022	9,900
Committed expenditures, 2023 and beyond	65,800
Capital Lease Obligations [Member]
Capital Leases
remainder of 2018	329
2019	625
2020	249
2021	28
2022	0
2023 and thereafter	0
Total minimum lease payments	1,231
Less amount representing interest	65
Present value of minimum lease payments	1,166
Less current portion	599
Long-term portion	567
Aircraft and Spare Engine Leases [Member]
Operating Leases
remainder of 2018	86,675
2019	167,360
2020	161,876
2021	160,185
2022	146,904
2023 and thereafter	419,925
Total minimum lease payments	1,142,925
Property Facility Leases [Member]
Operating Leases
remainder of 2018	27,457
2019	43,505
2020	28,398
2021	16,937
2022	16,250
2023 and thereafter	99,744
Total minimum lease payments	$ 232,291