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Intangible Assets
6 Months Ended
Jun. 30, 2021
Goodwill and Intangible Assets Disclosure [Abstract]  
Goodwill and Intangible Assets
(3) Intangible Assets

Intangible assets associated with customer relationships are amortized on a straight-line basis over the expected period of benefits of the customer relationships, which ranged from 10 to 20 years at the time the intangible assets were originally recorded.

The following table represents our change in carrying value of intangible assets (in millions):
Gross Carrying AmountAccumulated AmortizationNet Carrying Amount
Six Months Ended June 30, 2021
Customer relationships, beginning of period$1,794.2 $(668.8)$1,125.4 
Customer relationships obtained from acquisition of business57.3 — 57.3 
Amortization expense— (62.5)(62.5)
Customer relationships, end of period$1,851.5 $(731.3)$1,120.2 

The weighted average amortization period for intangible assets is 14.9 years. Amortization expense was $31.6 million and $30.9 million for the three months ended June 30, 2021 and 2020, respectively, and $62.5 million and $61.8 million for the six months ended June 30, 2021 and 2020, respectively.

The following table summarizes our estimated aggregate amortization expense for the next five years and thereafter (in millions):

2021 (remaining)$64.1 
2022128.2 
2023128.2 
2024128.2 
2025110.8 
Thereafter560.7 
Total$1,120.2