Intangible Assets

9 Months Ended

Sep. 30, 2021

Goodwill and Intangible Assets Disclosure [Abstract]

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 Amount		Accumulated Amortization		Net Carrying Amount
Nine Months Ended September 30, 2021
Customer relationships, beginning of period	$	1,794.2	$	(668.8)	$	1,125.4
Customer relationships obtained from acquisition of business	50.6		—		50.6
Amortization expense	—		(94.4)		(94.4)
Customer relationships, end of period	$	1,844.8	$	(763.2)	$	1,081.6

The weighted average amortization period for intangible assets is 14.9 years. Amortization expense was $31.9 million and $30.9 million for the three months ended September 30, 2021 and 2020, respectively, and $94.4 million and $92.7 million for the nine months ended September 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)	$	31.9
2022	127.6
2023	127.6
2024	127.6
2025	110.3
Thereafter	556.6
Total	$	1,081.6