Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2018

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,
	2018		2017		2016
	(in thousands)
Depreciation	$	129,412	$	83,154	$	54,171
Amortization of heavy maintenance	41,286		53,855		43,811
Amortization of capitalized software	6,029		3,143		3,154
Total depreciation and amortization	$	176,727	$	140,152	$	101,136

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

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

		Year Ended December 31,
Non-fare revenue	Recognition method	2018		2017		2016
		(in thousands)
Baggage	Time of departure	$	620,154	$	488,434	$	434,269
Passenger usage fee	Time of departure	531,459		411,742		358,920
Advance seat selection	Time of departure	180,012		131,821		110,966
Other		224,283		177,495		155,539
Non-fare revenue		$	1,555,908	$	1,209,492	$	1,059,694

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, 2018	$	39,194	$	30,353
December 31, 2017	49,453		37,960
December 31, 2016	48,882		36,640

Aircraft maintenance expense

The table below summarizes the components of the Company’s maintenance cost:

	Year Ended December 31,
	2018		2017		2016
	(in thousands)
Flight hour-based maintenance expense	$	68,039	$	54,802	$	48,471
Non-flight hour-based maintenance expense	61,039		55,637		50,116
Total maintenance, materials and repairs	$	129,078	$	110,439	$	98,587