Intangible Assets

12 Months Ended

Dec. 31, 2023

Goodwill and Intangible Assets Disclosure [Abstract]

Intangible Assets

(4) 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 weighted average amortization period for intangible assets is 14.9 years.

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

	Gross Carrying Amount		Accumulated Amortization		Net Carrying Amount
Year Ended December 31, 2023
Customer relationships, beginning of period	$	1,844.8	$	(923.6)	$	921.2
Amortization expense	—		(127.6)		(127.6)
Customer relationships, end of period	$	1,844.8	$	(1,051.2)	$	793.6
Year Ended December 31, 2022
Customer relationships, beginning of period	$	1,844.8	$	(795.1)	$	1,049.7
Amortization expense	—		(128.5)		(128.5)
Customer relationships, end of period	$	1,844.8	$	(923.6)	$	921.2
Year Ended December 31, 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	—		(126.3)		(126.3)
Customer relationships, end of period	$	1,844.8	$	(795.1)	$	1,049.7

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

2024	$	127.6
2025	110.2
2026	106.3
2027	106.3
2028	106.3
Thereafter	236.9
Total	$	793.6