Financial Instruments and Risk Management - Narrative (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2015 aircraft derivative_instrument	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Derivative [Line Items]
Interest rate derivative losses reclassified into earnings	$ 58	$ 53
Loss reclassified from AOCI into earnings, tax	21	31
Accumulated other comprehensive loss	1,429			$ 1,464	$ 1,345
Airbus A321 [Member]
Derivative [Line Items]
Number of aircraft protected by interest rate derivatives schedule for delivery | aircraft			3
Interest Rate Swap [Member]
Derivative [Line Items]
Number of interest rate derivatives settled | derivative_instrument			6
Accumulated other comprehensive loss	1,300			$ 1,400
Interest Rate Swap [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Interest rate derivative losses reclassified into earnings	58	53
Loss reclassified from AOCI into earnings, tax	$ 21	$ 31