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Intangible Assets
 9 Months Ended
Sep. 30, 2019
Goodwill and Intangible Assets Disclosure [Abstract]  
Intangible Assets Intangible Assets

The following table provides the gross carrying value and accumulated amortization for each major class of intangible asset at September 30, 2019 and December 31, 2018 (in millions, except weighted average useful life):
 
 
 
 
September 30, 2019
 
December 31, 2018

 
Weighted Average Useful Life
 
Gross Carrying Value
 
Accumulated Amortization
 
Net Book Value
 
Gross Carrying Value
 
Accumulated Amortization
 
Net Book Value
Customer relationships
 
17 years
 
$
160.9

 
$
(13.2
)
 
$
147.7

 
$
160.9

 
$
(6.1
)
 
$
154.8

Tradename
 
1 year
 
—

 
—

 
—

 
1.1

 
(1.1
)
 
—

Technology
 
6 years
 
26.8

 
(6.3
)
 
20.5

 
26.8

 
(2.9
)
 
23.9

Favorable leasehold interests
 
1-8 years
 
—

 
—

 
—

 
2.3

 
(0.5
)
 
1.8

Total intangible assets
 
 
 
$
187.7

 
$
(19.5
)
 
$
168.2

 
$
191.1

 
$
(10.6
)
 
$
180.5



Intangible asset amortization expense was $3.5 million and $10.5 million for the three and nine months ended September 30, 2019, and $4.5 million and $6.7 million for the three and nine months ended September 30, 2018.

Estimated annual amortization expense related to intangible assets with definite lives as of September 30, 2019 is as follows (in millions):
Remainder of 2019
 
$
3.5

2020
 
13.9

2021
 
13.9

2022
 
13.9

2023
 
13.9

Thereafter
 
109.1

Total
 
$
168.2