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Intangible Assets
 3 Months Ended
Mar. 31, 2020
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 March 31, 2020 and December 31, 2019 (in millions, except weighted average useful life):
March 31, 2020December 31, 2019
Weighted Average Useful LifeGross Carrying ValueAccumulated AmortizationNet Book ValueGross Carrying ValueAccumulated AmortizationNet Book Value
Customer relationships17 years$160.9  $(18.0) $142.9  $160.9  $(15.6) $145.3  
Technology6 years26.8  (8.4) 18.4  26.8  (7.4) 19.4  
Total intangible assets$187.7  $(26.4) $161.3  $187.7  $(23.0) $164.7  

Intangible asset amortization expense was $3.5 million and $3.5 million for the three months ended March 31, 2020 and 2019, respectively.

Estimated annual amortization expense related to intangible assets with definite lives as of March 31, 2020 is as follows (in millions):

Remainder of 2020$10.4  
202113.9  
202213.9  
202313.9  
202411.0  
20259.5  
Thereafter88.7  
Total$161.3