

	Three Months Ended June 30,				Six Months Ended June 30,
(in millions)	2018		2017		2018		2017
Ticket	$	9,308	$	8,606	$	16,961	$	15,711
Loyalty travel awards	680		622		1,298		1,204
Travel-related services	558		540		1,052		1,031
Total passenger revenue	$	10,546	$	9,768	$	19,311	$	17,946

Passenger tickets. We record sales of passenger tickets to be flown by us or that we sell on behalf of other airlines in air traffic liability. Passenger revenue is recognized when we provide transportation or when ticket breakage occurs. For tickets that we sell on behalf of other airlines, we reduce the air traffic liability when consideration is remitted to those airlines. We periodically evaluate the estimated air traffic liability and record any adjustments in our income statement. These adjustments relate primarily to refunds, exchanges, ticket breakage, transactions with other airlines and other items for which final settlement occurs in periods subsequent to the sale of the related tickets at amounts other than the original sales price.

We recognized approximately $3 billion in passenger revenue during each of the six months ended June 30, 2018 and 2017 that was recorded in our air traffic liability balances at December 31, 2017 and 2016, respectively. We expect the remaining balance of the December 31, 2017 liability to be recognized during 2018.

Regional carriers. Our regional carriers include both our contract carrier agreements with third-party regional carriers ("contract carriers") and Endeavor Air, Inc., our wholly owned subsidiary. Our contract carrier agreements are primarily structured as capacity purchase agreements where we purchase all or a portion of the contract carrier's capacity and are responsible for selling the seat inventory we purchase. We record revenue related to our capacity purchase agreements in passenger revenue and the related expenses in regional carriers expense, excluding fuel.

Loyalty travel awards revenue is related to the redemption of mileage credits for travel. We recognize loyalty travel awards revenue in passenger revenue as mileage credits are redeemed and travel is provided. See below for discussion of our frequent flyer program accounting policies.

Travel-related services are primarily composed of services performed in conjunction with a passenger’s flight, including administrative fees (such as ticket change fees), baggage fees and on-board sales. We recognize revenue for these services when the related transportation service is provided. Prior to the adoption of the new standard, the majority of these fees were classified in other revenue.

Our frequent flyer program (the "SkyMiles program") generates customer loyalty by rewarding customers with incentives to travel on Delta. This program allows customers to earn mileage credits by flying on Delta, Delta Connection and other airlines that participate in the SkyMiles program. When traveling, customers earn redeemable mileage credits based on the passenger's loyalty program status and travel fare paid. Customers can also earn mileage credits through participating companies such as credit card companies, hotels and car rental agencies. To facilitate transactions with participating companies, we sell mileage credits to non-airline businesses, customers and other airlines. Mileage credits are redeemable by customers in future periods for air travel on Delta and other participating airlines, membership in our Sky Club and other program awards.

To reflect the mileage credits earned, the SkyMiles program includes two types of transactions that are considered revenue arrangements with multiple performance obligations: (1) mileage credit earned with travel and (2) mileage credit sold to participating companies.

Passenger ticket sales earning mileage credits. Passenger ticket sales earning mileage credits under our SkyMiles program provide customers with (1) mileage credits earned and (2) air transportation. We value each performance obligation on a standalone basis. To value the mileage credits earned, we consider the quantitative value a passenger receives by redeeming miles for a ticket rather than paying cash, which is referred to as equivalent ticket value ("ETV").

We defer revenue for the mileage credits when earned and recognize loyalty travel awards in passenger revenue as the miles are redeemed and services are provided. We record the air transportation portion of the passenger ticket sales in air traffic liability and recognize passenger revenue when we provide transportation or if the ticket goes unused.

Sale of mileage credits. Customers may earn mileage credits based on their spending with participating companies such as credit card companies, hotels and car rental agencies with which we have marketing agreements to sell mileage credits. Our contracts to sell mileage credits under these marketing agreements have multiple performance obligations. During the six months ended June 30, 2018 and 2017, total cash sales from marketing agreements were $1.7 billion and $1.5 billion, respectively, which are allocated to travel and other performance obligations, as discussed below.

Our most significant contract to sell mileage credits relates to our co-brand credit card relationship with American Express. Our agreements with American Express provide for joint marketing, grant certain benefits to Delta-American Express co-branded credit card holders ("Cardholders") and American Express Membership Rewards program participants, and allow American Express to market using our customer database. Cardholders earn mileage credits for making purchases using co-branded cards, may check their first bag for free, are granted discounted access to Delta Sky Club lounges and receive other benefits while traveling on Delta. Additionally, participants in the American Express Membership Rewards program may exchange their points for mileage credits under the SkyMiles program. We sell mileage credits at agreed-upon rates to American Express which are then provided to their customers under the co-brand credit card program and the Membership Rewards program.

We account for marketing agreements, including American Express, consistent with the accounting method that allocates the consideration received to the individual products and services delivered. We allocate the value based on the relative selling prices of those products and services, which generally consist of award travel, baggage fee waivers, lounge access and the use of our brand. We determined our best estimate of the selling prices by considering discounted cash flow analysis using multiple inputs and assumptions, including: (1) the expected number of miles awarded and number of miles redeemed, (2) ETV for the award travel obligation, (3) published rates on our website for baggage fees, discounted access to Delta Sky Club lounges and other benefits while traveling on Delta and (4) brand value.

We defer the amount for award travel obligation as part of frequent flyer deferred revenue and recognize loyalty travel awards in passenger revenue as the mileage credits are used for travel. Revenue allocated to services performed in conjunction with a passenger’s flight, such as baggage fee waivers, is recognized as travel-related services in passenger revenue when the related service is performed. Revenue allocated to access Delta Sky Club lounges is recognized as miscellaneous in other revenue as access is provided. Revenue allocated to the remaining performance obligations, primarily brand value, is recorded as loyalty program in other revenue over time as miles are delivered.

Mileage breakage. For mileage credits that we estimate are not likely to be redeemed ("breakage"), we recognize the associated value proportionally during the period in which the remaining mileage credits are expected to be redeemed. Management uses statistical models to estimate breakage based on historical redemption patterns. A change in assumptions as to the period over which mileage credits are expected to be redeemed, the actual redemption activity for mileage credits or the estimated fair value of mileage credits expected to be redeemed could have a material impact on our revenue in the year in which the change occurs and in future years.

Current activity of the frequent flyer program. Mileage credits are combined in one homogeneous pool and are not separately identifiable. As such, the revenue is comprised of miles that were part of the frequent flyer deferred revenue balance at the beginning of the period as well as miles that were issued during the period.

The table below presents the activity of the current and noncurrent frequent flyer liability, and includes miles earned through travel and miles sold, which are primarily through marketing agreements.



(in millions)	2018			2017
Balance at January 1	$	6,321		$	5,922
Mileage credits earned	1,550			1,459
Travel mileage credits redeemed	(1,298		)	(1,204		)
Non-travel mileage credits redeemed	(82		)	(81		)
Balance at June 30	$	6,491		$	6,096

Passenger revenue is recognized in a specific geographic region based on the origin and destination of each flight segment. Our passenger revenue by geographic region (as defined by the U.S. Department of Transportation) is summarized in the following table:



	Three Months Ended June 30,				Six Months Ended June 30,
(in millions)	2018		2017		2018		2017
Domestic	$	7,413	$	6,888	$	13,714	$	12,790
Atlantic	1,782		1,566		2,841		2,495
Latin America	709		714		1,536		1,489
Pacific	642		600		1,220		1,172
Total passenger revenue	$	10,546	$	9,768	$	19,311	$	17,946



	Three Months Ended June 30,				Six Months Ended June 30,
(in millions)	2018		2017		2018		2017
Ancillary businesses and refinery	$	522	$	320	$	1,042	$	632
Loyalty program	358		316		705		621
Miscellaneous	126		156		260		299
Total other revenue	$	1,006	$	792	$	2,007	$	1,552

Ancillary businesses and refinery. Ancillary businesses and refinery includes aircraft maintenance and staffing services we provide to third parties, our vacation wholesale operations, our private jet operations and refinery production sales to third parties. Third-party refinery production sales are at or near cost; accordingly, the margin on these sales is de minimis. See Note 10, "Segments," for more information on revenue recognition within our refinery segment.

Loyalty program. Loyalty program revenues relate to brand usage and other performance obligations embedded in mileage credits sold, including redemption of mileage credits for non-travel awards. These revenues are included within the total cash sales from marketing agreements, discussed above.

Accounts receivable primarily consist of amounts due from credit card companies from the sale of passenger tickets, ancillary businesses and refinery sales, and other companies for the purchase of mileage credits under the SkyMiles program. We provide an allowance for uncollectible accounts equal to the estimated losses expected to be incurred based on historical chargebacks, write-offs, bankruptcies and other specific analyses. Bad debt expense was not material in any period presented.