Long-term Debt (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) aircraft	Dec. 31, 2015 USD ($)
Long-term Debt
Total long-term debt	$ 2,570,904	$ 1,948,803
Less current maturities	(308,945)	(272,027)
Long-term debt, net of current maturities	$ 2,261,959	$ 1,676,776
Effective interest rate (as a percent)	3.80%	3.70%
Aggregate amounts of principal maturities of long-term debt
2017	$ 308,945
2018	286,164
2019	286,266
2020	251,265
2021	236,896
Thereafter	1,201,368
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 and surety bonds outstanding with various banks and surety institutions	$ 87,700	88,900
E 175
Long-term Debt
Number of new aircraft acquired | aircraft	41
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 2.64% to 2.82% through 2017 to 2020, secured by aircraft
Long-term Debt
Total long-term debt	$ 63,408	108,348
Notes payable to banks, due in semi-annual installments, variable interest based on LIBOR, or with interest rates ranging from 2.64% to 2.82% through 2017 to 2020, secured by aircraft | Minimum
Long-term Debt
Interest rate (as a percent)	2.64%
Notes payable to banks, due in semi-annual installments, variable interest based on LIBOR, or with interest rates ranging from 2.64% to 2.82% through 2017 to 2020, secured by aircraft | Maximum
Long-term Debt
Interest rate (as a percent)	2.82%
Notes payable to a financing company, due in semi-annual installments, variable interest based on LIBOR, or with interest rates ranging from 2.27% to 3.25% through 2017 to 2021, secured by aircraft
Long-term Debt
Total long-term debt	$ 146,266	217,341
Notes payable to a financing company, due in semi-annual installments, variable interest based on LIBOR, or with interest rates ranging from 2.27% to 3.25% through 2017 to 2021, secured by aircraft | Minimum
Long-term Debt
Interest rate (as a percent)	2.27%
Notes payable to a financing company, due in semi-annual installments, variable interest based on LIBOR, or with interest rates ranging from 2.27% to 3.25% through 2017 to 2021, secured by aircraft | Maximum
Long-term Debt
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
Long-term Debt
Total long-term debt	$ 86,052	108,069
Notes payable to banks, due in semi-annual installments plus interest at 6.06% to 6.51% through 2021, secured by aircraft | Minimum
Long-term Debt
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
Long-term Debt
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
Long-term Debt
Total long-term debt	$ 8,854	17,208
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
Long-term Debt
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
Long-term Debt
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
Long-term Debt
Total long-term debt	$ 426,389	479,170
Notes payable to banks, due in monthly installments plus interest of 2.68% to 6.86% through 2025, secured by aircraft | Minimum
Long-term Debt
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
Long-term Debt
Interest rate (as a percent)	6.86%
Notes payable to banks, due in monthly installments, plus interest at 6.05% through 2020, secured by aircraft
Long-term Debt
Interest rate (as a percent)	6.05%
Total long-term debt	$ 8,998	11,304
Notes payable to banks, due in monthly installments, plus interest at 3.44% through 2019, secured by aircraft
Long-term Debt
Interest rate (as a percent)	3.44%
Total long-term debt	$ 3,280	4,615
Notes payable to banks, due in quarterly installments plus interest at 3.39% to 4.68% through 2028, secured by aircraft
Long-term Debt
Total long-term debt	$ 1,806,981	966,156
Notes payable to banks, due in quarterly installments plus interest at 3.39% to 4.68% through 2028, secured by aircraft | Minimum
Long-term Debt
Interest rate (as a percent)	3.39%
Notes payable to banks, due in quarterly installments plus interest at 3.39% to 4.68% through 2028, secured by aircraft | Maximum
Long-term Debt
Interest rate (as a percent)	4.68%
Notes payable to banks, due in monthly installments, plus interest based on LIBOR at 3.54% through 2017, secured by aircraft
Long-term Debt
Interest rate (as a percent)	3.54%
Total long-term debt	$ 4,666	14,538
Notes payable to banks due in monthly installments, interest based on LIBOR interest at 3.30% through 2019, secured by spare engines
Long-term Debt
Interest rate (as a percent)	3.30%
Total long-term debt	$ 16,010	22,054
Line of credit
Long-term Debt
Effective interest rate (as a percent)	3.30%
Aggregate amounts of principal maturities of long-term debt
Maximum borrowing capacity	$ 75,000	$ 25,000
Amount outstanding	0
Letters of credit	6,500
Current borrowing capacity	$ 68,500
LIBOR | Line of credit
Long-term Debt
Basis spread on variable rate	2.50%