Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2019

Accounting Policies [Abstract]

Depreciable lives used for the principal depreciable asset classifications and components of depreciation and amortization

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

	Year Ended December 31,
	2019		2018		2017
	(in thousands)
Depreciation	$	155,326	$	129,412	$	83,154
Amortization of heavy maintenance	63,364		41,286		53,855
Amortization of capitalized software	6,574		6,029		3,143
Total depreciation and amortization	$	225,264	$	176,727	$	140,152

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
Finance leases	Lease term or estimated useful life of the asset
Leasehold improvements	Lesser of lease term or estimated useful life of the improvement
Buildings	Lesser of lease term or 30 years

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	2019		2018		2017
		(in thousands)
Baggage	Time of departure	$	734,243	$	620,154	$	488,434
Passenger usage fee	Time of departure	669,177		531,459		411,742
Advance seat selection	Time of departure	228,876		180,012		131,821
Other		238,454		224,283		177,495
Non-fare revenue		$	1,870,750	$	1,555,908	$	1,209,492

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, 2019	$	48,136	$	37,151
December 31, 2018	39,194		30,353
December 31, 2017	49,453		37,960

Aircraft maintenance expense

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

	Year Ended December 31,
	2019		2018		2017
	(in thousands)
Flight hour-based maintenance expense	$	78,253	$	68,039	$	54,802
Non-flight hour-based maintenance expense	65,322		61,039		55,637
Total maintenance, materials and repairs	$	143,575	$	129,078	$	110,439