Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Depreciable lives used for the principal depreciable asset classifications

The depreciable lives used for the principal depreciable asset classifications are:

	Estimated Useful Life
Aircraft, engines and flight simulators	25 years
Spare rotables and flight assemblies	7 to 15 years
Other equipment and vehicles	5 to 7 years
Internal use software	3 to 10 years
Capital leases	Lease term
Leasehold improvements	Lesser of lease term or estimated useful life of the improvement
Buildings	Lesser of lease term or 30 years

The following table illustrates the components of depreciation and amortization expense:

	Year Ended December 31,
	2017		2016		2015
	(in thousands)
Depreciation	$	86,297	$	57,325	$	30,797
Amortization of heavy maintenance	53,855		43,811		43,111
Total depreciation and amortization	$	140,152	$	101,136	$	73,908

Total cash proceeds received from the sale of mileage credit

The following table illustrates total cash proceeds received from the sale of mileage credits and the portion of such proceeds recognized in non-ticket revenue immediately as marketing component:

	Consideration received from credit card mile programs		Portion of proceeds recognized immediately as marketing component
Year Ended	(in thousands)
December 31, 2017	$	49,453	$	37,960
December 31, 2016	48,882		36,640
December 31, 2015	58,005		35,938

Primary components of non-ticket revenue and the revenue recognition method utilized for each service or product

The following table summarizes the primary components of non-ticket revenue and the revenue recognition method utilized for each service or product:

		Year Ended December 31,
Non-ticket revenue	Recognition method	2017		2016		2015
		(in thousands)
Baggage	Time of departure	$	488,434	$	434,269	$	381,386
Passenger usage fee	Time of departure	411,742		358,920		298,092
Advance seat selection	Time of departure	131,821		110,966		97,786
Other		249,635		217,180		194,861
Non-ticket revenue		$	1,281,632	$	1,121,335	$	972,125

Aircraft maintenance expense

The table below summarizes the extent to which the Company’s maintenance costs are rate capped due to flight hour maintenance contracts:

	Year Ended December 31,
	2017		2016		2015
	(in thousands)
Flight hour-based maintenance expense	$	54,802	$	48,471	$	41,818
Non-flight hour-based maintenance expense	55,637		50,116		38,630
Total maintenance, materials and repairs	$	110,439	$	98,587	$	80,448