Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2016

Depreciable lives used for the principal depreciable asset classifications

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



	Estimated Useful Life
Aircraft	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 lease	Lease term
Leasehold improvements	Lesser of lease term or estimated useful life of the improvement

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



	Year Ended December 31,
	2016		2015		2014
	(in thousands)
Depreciation	$	57,325	$	30,797	$	11,169
Amortization of heavy maintenance	43,811		43,111		35,802
Total depreciation and amortization	$	101,136	$	73,908	$	46,971

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 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, 2016	$	48,882	$	36,640
December 31, 2015	58,005		35,938
December 31, 2014	33,819		28,140

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	2016		2015		2014
		(in thousands)
Baggage	Time of departure	$	434,269	$	381,386	$	318,103
Passenger usage fee	Time of departure	358,920		298,092		221,992
Advance seat selection	Time of departure	110,966		97,786		76,270
Service charges for changes and cancellations	When itinerary is changed	42,823		43,756		38,388
Other		174,357		151,105		131,855
Non-ticket revenue		$	1,121,335	$	972,125	$	786,608

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,
	2016		2015		2014
	(in thousands)
Flight hour-based maintenance expense	$	48,471	$	41,818	$	35,675
Non-flight hour-based maintenance expense	50,116		38,630		38,281
Total maintenance, materials and repairs	$	98,587	$	80,448	$	73,956