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Intangible Assets, net
 12 Months Ended
Dec. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]  
Intangible Assets, net
9. Intangible Assets, net

Intangible assets include customer relationships and contracts associated with various business and asset acquisitions. These acquired intangible assets were recorded at fair value determined as of the acquisition date and are being amortized over the period we expect to benefit from the assets. Intangible assets, net consisted of the following (in thousands):

 
December 31, 2018
 
December 31, 2017
 
Gross
 Carrying
 Amount
 
Accumulated
Amortization
 
Gross
 Carrying
 Amount
 
Accumulated
Amortization
Customer related (10-25 year life)
$
107,008

 
$
(69,678
)
 
$
107,008

 
$
(64,887
)
Contract based (5-7 year life)
64,556

 
(49,516
)
 
68,395

 
(41,644
)
Intangible assets
$
171,564

 
$
(119,194
)
 
$
175,403

 
$
(106,531
)


Amortization expense of intangible assets totaled $16.5 million, $17.8 million and $17.9 million during the years ended December 31, 2018, 2017 and 2016, respectively.

Estimated future intangible amortization expense is as follows (in thousands):

2019
$
13,047

2020
9,562

2021
4,687

2022
3,496

2023
3,251

Thereafter
18,327

Total
$
52,370