Intangible Assets

3 Months Ended

Mar. 31, 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 range from 10 to 20 years.

The following table represents our change in carrying value of intangible assets (in millions):

	Gross Carrying Amount		Accumulated Amortization		Net Carrying Amount
Three Months Ended March 31, 2021
Customer relationships, beginning of period	$	1,794.2	$	(668.8)	$	1,125.4
Amortization expense	—		(30.9)		(30.9)
Customer relationships, end of period	$	1,794.2	$	(699.7)	$	1,094.5

The weighted average amortization period for intangible assets is 15.0 years. Amortization expense was $30.9 million for each of the three months ended March 31, 2021 and 2020, respectively.

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

2021 (remaining)	$	92.5
2022	123.4
2023	123.4
2024	123.4
2025	106.1
Thereafter	525.7
Total	$	1,094.5