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Intangible Assets
3 Months Ended
Mar. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]  
Intangible Assets
Intangible Assets
Intangible assets that are being amortized on a straight-line basis over their estimated useful lives are detailed below:
 
 

 
As of March 31, 2018
 
As of December 31, 2017
(In millions)
 
Useful
Lives
 
Gross
Carrying
Amount
 
Accumulated
Amortization
 
Net
Amount
 
Gross
Carrying
Amount
 
Accumulated
Amortization
 
Net
Amount
Customer relationships
 
22 Years
 
$
132

 
$
66

 
$
66

 
$
132

 
$
64

 
$
68

Patents
 
10-15 Years

22

 
5

 
17

 
22

 
5

 
17

Other
 
4-20 Years
 
15

 
8

 
7

 
15

 
8

 
7

Total amortizable intangible assets
 

 
$
169

 
$
79

 
$
90

 
$
169

 
$
77

 
$
92


Identifiable intangible assets with finite lives are reviewed for impairment whenever events or circumstances indicate that the carrying values may not be recoverable.
Amortization expense was $2 million and $3 million in the three months ended March 31, 2018 and March 31, 2017, respectively. The estimated future amortization expense of identifiable intangible assets during the next five years is $6 million for the remaining portion of 2018, $9 million in each year from 2019 to 2021, and $8 million in 2022.
In addition, the carrying amount of acquired water rights with indefinite lives as of March 31, 2018 and December 31, 2017 totaled $75 million. The acquired water rights are tested for impairment annually in the third quarter, or whenever events or circumstances indicate the carrying value may not be recoverable. U. S. Steel performed a quantitative impairment evaluation of its acquired water rights during the third quarter of 2017. Based on the results of the evaluation, the water rights were not impaired.