Long-term Debt (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) aircraft	Dec. 31, 2018 USD ($)
Debt Instrument [Line Items]
Total long-term debt	$ 3,017,523	$ 3,185,438
Current portion of long-term debt	(367,954)	(354,072)
Less long-term portion of unamortized debt issue cost, net	(20,580)	(21,598)
Long-term debt, net of current maturities and debt issue costs	2,628,989	2,809,768
Current portion of long-term debt	367,954	354,072
Less current portion of unamortized debt issue cost, net	(3,828)	(3,866)
Current portion of long-term debt, net of debt issue costs	364,126	350,206
Debt issued to purchase of aircraft	$ 14,500
Effective interest rate (as a percent)	4.20%
Aggregate amounts of principal maturities of long-term debt
2020	$ 367,954
2021	352,005
2022	365,907
2023	374,906
2024	327,630
Thereafter	1,229,121
Repayment of debt in cash	30,100	43,500
Total repayment of debt	30,100	43,500
Letters of credit	8,800	9,700
Current borrowing capacity	66,200	65,300
Letters of credit and surety bonds outstanding with various banks and surety institutions	$ 61,700	78,700
E175
Debt Instrument [Line Items]
Number of aircraft delivered | aircraft	10
Purchase of aircraft financed through long-term debt	$ 200,000
Debt instrument, term	12 years
Purchased, previously-leased aircraft
Debt Instrument [Line Items]
Interest rate (as a percent)	2.00%
Debt instrument, term	18 months
Number of aircraft purchased | aircraft	2
Minimum | E175
Debt Instrument [Line Items]
Interest rate (as a percent)	3.40%
Maximum | E175
Debt Instrument [Line Items]
Interest rate (as a percent)	4.10%
Notes payable to banks, due in semi-annual installments, variable interest based on LIBOR, or with an interest rate of 4.00% through 2019, secured by aircraft
Debt Instrument [Line Items]
Interest rate (as a percent)	4.00%
Total long-term debt		6,429
Notes payable to a financing company, due in semi-annual installments, with a fixed interest rate of 3.25% through 2021, secured by aircraft
Debt Instrument [Line Items]
Interest rate (as a percent)	3.25%
Total long-term debt	$ 18,412	36,324
Notes payable to banks, due in semi-annual installments plus interest at 6.10% to 6.51% through 2021, secured by aircraft
Debt Instrument [Line Items]
Total long-term debt	$ 22,557	41,592
Notes payable to banks, due in semi-annual installments plus interest at 6.10% to 6.51% through 2021, secured by aircraft | Minimum
Debt Instrument [Line Items]
Interest rate (as a percent)	6.10%
Notes payable to banks, due in semi-annual installments plus interest at 6.10% to 6.51% through 2021, secured by aircraft | Maximum
Debt Instrument [Line Items]
Interest rate (as a percent)	6.51%
Notes payable to banks, due in monthly installments, plus interest at 1.95% to 6.86% through 2029, secured by aircraft
Debt Instrument [Line Items]
Total long-term debt	$ 438,878	476,369
Notes payable to banks, due in monthly installments, plus interest at 1.95% to 6.86% through 2029, secured by aircraft | Minimum
Debt Instrument [Line Items]
Interest rate (as a percent)	1.95%
Notes payable to banks, due in monthly installments, plus interest at 1.95% to 6.86% through 2029, secured by aircraft | Maximum
Debt Instrument [Line Items]
Interest rate (as a percent)	6.86%
Notes payable to banks, due in quarterly installments, plus interest at 3.39% to 5.08% through 2031, secured by aircraft
Debt Instrument [Line Items]
Total long-term debt	$ 2,537,676	2,621,416
Notes payable to banks, due in quarterly installments, plus interest at 3.39% to 5.08% through 2031, secured by aircraft | Minimum
Debt Instrument [Line Items]
Interest rate (as a percent)	3.39%
Notes payable to banks, due in quarterly installments, plus interest at 3.39% to 5.08% through 2031, secured by aircraft | Maximum
Debt Instrument [Line Items]
Interest rate (as a percent)	5.08%
Notes payable to banks due in monthly installments, interest based on LIBOR interest at 3.30% through 2019, secured by spare engines
Debt Instrument [Line Items]
Interest rate (as a percent)	3.30%
Total long-term debt		3,308
Line of credit
Aggregate amounts of principal maturities of long-term debt
Maximum borrowing capacity	$ 75,000	75,000
Amount outstanding	$ 0	$ 0
LIBOR | Line of credit
Debt Instrument [Line Items]
Basis spread on variable rate	2.50%