Long-term Debt (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) aircraft	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Total long-term debt	$ 3,185,438	$ 2,712,350
Current portion of long-term debt	(354,072)	(313,243)
Less long-term portion of unamortized debt issue cost, net	(21,598)	(21,761)
Long-term debt, net of current maturities and debt issue costs	2,809,768	2,377,346
Current portion of long-term debt	354,072	313,243
Long-term debt, net of current maturities	354,072	313,243
Less current portion of unamortized debt issue cost, net	(3,866)	(3,565)
Current portion of long-term debt, net of debt issue costs	$ 350,206	$ 309,678
Effective interest rate (as a percent)	4.20%	3.90%
Aggregate amounts of principal maturities of long-term debt
2019	$ 354,072
2020	351,738
2021	347,835
2022	353,935
2023	359,677
Thereafter	1,418,181
Repayment of debt in cash	43,500	$ 16,500
Total repayment of debt	43,500	18,400
Pre-tax gain			$ 1,279
Letters of credit	9,700	14,800
Current borrowing capacity	65,300	60,200
Letters of credit and surety bonds outstanding with various banks and surety institutions	$ 78,700	87,400
E 175
Debt Instrument [Line Items]
Number of new aircraft acquired | aircraft	39
Purchase price portion financed through debt issuance (as a percent)	85.00%
Purchase price portion paid with cash (as a percent)	15.00%
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	34,905
Notes payable to a financing company, due in semi-annual installments, variable interest based on LIBOR, or with an 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	$ 36,324	97,612
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	$ 41,592	63,090
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 of 2.68% to 6.86% through 2025, secured by aircraft
Debt Instrument [Line Items]
Total long-term debt	$ 371,300	372,157
Notes payable to banks, due in monthly installments plus interest of 2.68% to 6.86% through 2025, secured by aircraft | Minimum
Debt Instrument [Line Items]
Interest rate (as a percent)	2.68%
Notes payable to banks, due in monthly installments plus interest of 2.68% to 6.86% through 2025, secured by aircraft | Maximum
Debt Instrument [Line Items]
Interest rate (as a percent)	6.86%
Notes payable to banks, due in monthly installments, plus interest at 4.07% to 6.05% through 2029, secured by aircraft
Debt Instrument [Line Items]
Total long-term debt	$ 105,069	49,001
Notes payable to banks, due in monthly installments, plus interest at 4.07% to 6.05% through 2029, secured by aircraft | Minimum
Debt Instrument [Line Items]
Interest rate (as a percent)	4.07%
Notes payable to banks, due in monthly installments, plus interest at 4.07% to 6.05% through 2029, secured by aircraft | Maximum
Debt Instrument [Line Items]
Interest rate (as a percent)	6.05%
Notes payable to banks, due in quarterly installments, plus interest at 3.39% to 5.08% through 2030, secured by aircraft
Debt Instrument [Line Items]
Total long-term debt	$ 2,621,416	2,085,822
Notes payable to banks, due in quarterly installments, plus interest at 3.39% to 5.08% through 2030, 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 2030, 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	9,763
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%