Financial Instruments and Risk Management - Narrative (Details)	3 Months Ended	12 Months Ended
	Sep. 30, 2015 derivative_instrument	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) aircraft derivative_instrument
Derivative [Line Items]
Unrealized gain (loss) on interest rate derivative instruments		$ 0	$ 0	$ (910,000)
Interest rate derivative losses reclassified into earnings, net of tax		(37,000)	224,000	82,000
Loss reclassified from AOCI into income, tax		372,000	130,000	$ 50,000
Accumulated other comprehensive loss		1,464,000	1,345,000
Airbus A321
Derivative [Line Items]
Number of aircraft protected by interest rate derivatives delivered | aircraft				3
Interest rate swap
Derivative [Line Items]
Number of interest rate derivatives settled | derivative_instrument	6			6
Accumulated other comprehensive loss		1,400,000	1,300,000
Interest rate swap | Cash flow hedge
Derivative [Line Items]
Unrealized gain (loss) on interest rate derivative instruments		0	0
Loss reclassified from AOCI into earnings		335,000	354,000
Interest rate derivative losses reclassified into earnings, net of tax		$ (37,000)	$ 224,000