Long-term Debt (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) aircraft	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Debt Instrument [Line Items]
Total long-term debt	$ 2,712,350	$ 2,570,904
Current portion of long-term debt	(313,243)	(308,945)
Less long-term portion of unamortized debt issue cost, net	(21,761)	(21,908)
Long-term debt, net of current maturities and debt issue costs	2,377,346	2,240,051
Current portion of long-term debt	313,243	308,945
Less current portion of unamortized debt issue cost, net	(3,565)	(3,485)
Current portion of long-term debt, net of debt issue costs	$ 309,678	$ 305,460
Effective interest rate (as a percent)	3.90%	3.80%
Aggregate amounts of principal maturities of long-term debt
2018	$ 313,243
2019	315,753
2020	284,998
2021	272,470
2022	280,360
Thereafter	1,245,526
Repayment of debt in cash		$ 16,500	$ 110,800
Total repayment of debt		18,400	145,400
Pre-tax gain		1,279	$ 33,660
Letters of credit	14,800	6,500
Current borrowing capacity	60,200	68,500
Letters of credit and surety bonds outstanding with various banks and surety institutions	$ 87,400	87,700
E 175
Debt Instrument [Line Items]
Number of new aircraft acquired | aircraft	21
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 interest rates ranging from 2.91% to 3.43% through 2020, secured by aircraft
Debt Instrument [Line Items]
Total long-term debt	$ 34,905	63,408
Notes payable to banks, due in semi-annual installments, variable interest based on LIBOR, or with interest rates ranging from 2.91% to 3.43% through 2020, secured by aircraft | Minimum
Debt Instrument [Line Items]
Interest rate (as a percent)	2.91%
Notes payable to banks, due in semi-annual installments, variable interest based on LIBOR, or with interest rates ranging from 2.91% to 3.43% through 2020, secured by aircraft | Maximum
Debt Instrument [Line Items]
Interest rate (as a percent)	3.43%
Notes payable to a financing company, due in semi-annual installments, variable interest based on LIBOR, or with interest rates ranging from 2.85% to 3.25% through 2021, secured by aircraft
Debt Instrument [Line Items]
Total long-term debt	$ 97,612	146,266
Notes payable to a financing company, due in semi-annual installments, variable interest based on LIBOR, or with interest rates ranging from 2.85% to 3.25% through 2021, secured by aircraft | Minimum
Debt Instrument [Line Items]
Interest rate (as a percent)	2.85%
Notes payable to a financing company, due in semi-annual installments, variable interest based on LIBOR, or with interest rates ranging from 2.85% to 3.25% through 2021, secured by aircraft | Maximum
Debt Instrument [Line Items]
Interest rate (as a percent)	3.25%
Notes payable to banks, due in semi-annual installments plus interest at 6.06% to 6.51% through 2021, secured by aircraft
Debt Instrument [Line Items]
Total long-term debt	$ 63,090	86,052
Notes payable to banks, due in semi-annual installments plus interest at 6.06% to 6.51% through 2021, secured by aircraft | Minimum
Debt Instrument [Line Items]
Interest rate (as a percent)	6.06%
Notes payable to banks, due in semi-annual installments plus interest at 6.06% to 6.51% through 2021, secured by aircraft | Maximum
Debt Instrument [Line Items]
Interest rate (as a percent)	6.51%
Notes payable to a financing company, due in semi-annual installments plus interest at 5.78% to 6.23% through 2017, secured by aircraft
Debt Instrument [Line Items]
Total long-term debt		8,854
Notes payable to a financing company, due in semi-annual installments plus interest at 5.78% to 6.23% through 2017, secured by aircraft | Minimum
Debt Instrument [Line Items]
Interest rate (as a percent)	5.78%
Notes payable to a financing company, due in semi-annual installments plus interest at 5.78% to 6.23% through 2017, secured by aircraft | Maximum
Debt Instrument [Line Items]
Interest rate (as a percent)	6.23%
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	$ 372,157	426,389
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 4.16% to 6.05% through 2029, secured by aircraft
Debt Instrument [Line Items]
Total long-term debt	$ 49,001	8,998
Notes payable to banks, due in monthly installments, plus interest at 4.16% to 6.05% through 2029, secured by aircraft | Minimum
Debt Instrument [Line Items]
Interest rate (as a percent)	4.16%
Notes payable to banks, due in monthly installments, plus interest at 4.16% 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 monthly installments, plus interest at 3.44% through 2019, secured by aircraft
Debt Instrument [Line Items]
Interest rate (as a percent)	3.44%
Total long-term debt		3,280
Notes payable to banks, due in quarterly installments plus interest at 3.39% to 4.73% through 2029, secured by aircraft
Debt Instrument [Line Items]
Total long-term debt	$ 2,085,822	1,806,981
Notes payable to banks, due in quarterly installments plus interest at 3.39% to 4.73% through 2029, 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 4.73% through 2029, secured by aircraft | Maximum
Debt Instrument [Line Items]
Interest rate (as a percent)	4.73%
Notes payable to banks, due in monthly installments, plus interest based on LIBOR at 3.54% through 2017, secured by aircraft
Debt Instrument [Line Items]
Interest rate (as a percent)	3.54%
Total long-term debt		4,666
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	$ 9,763	16,010
Line of credit
Debt Instrument [Line Items]
Effective interest rate (as a percent)	4.10%
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%