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Intangible Assets
 12 Months Ended
Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]  
Intangible Assets INTANGIBLE ASSETS

Indefinite-Lived Intangible Assets
 
Carrying Value at December 31,
(in millions)
2017
2016
International routes and slots
$
2,583

$
2,563

Delta tradename
850

850

SkyTeam-related assets
661

661

Domestic slots
622

622

Total
$
4,716

$
4,696



International Routes and Slots. Our international routes and slots primarily relate to Pacific route authorities and slots at capacity-constrained airports in Asia, and slots at London-Heathrow airport.

Domestic Slots. Our domestic slots relate to our slots at New York-LaGuardia and Washington-Reagan National airports.

Definite-Lived Intangible Assets
 
December 31, 2017
 
December 31, 2016
(in millions)
Gross
Carrying
Value
 
Accumulated
Amortization
 
Gross
Carrying
Value
 
Accumulated
Amortization
Marketing agreements
$
730

$
(677
)
 
$
730

$
(667
)
Contracts
193

(115
)
 
193

(108
)
Other
53

(53
)
 
53

(53
)
Total
$
976

$
(845
)
 
$
976

$
(828
)


Amortization expense was $17 million, $17 million and $18 million for the years ended December 31, 2017, 2016 and 2015, respectively. We estimate that we will incur approximately $16 million of amortization expense annually through 2022.