Identifiable intangible assets consist of acquired customer contracts and relationships, trademarks and non-compete agreements. Amortization expense related to intangible assets was $13.4 and $13.9 for the three months ended December 31, 2021 and 2020, respectively. These assets are amortized over their estimated useful lives of which the reasonableness is continually evaluated by the Company. The weighted average amortization period of intangible assets acquired during the three months ended December 31, 2021 is seven years.

Intangible assets as of December 31, 2021 and September 30, 2021 consisted of the following:


		December 31, 2021					September 30, 2021
	Estimated Useful Life	Gross Carrying Amount		Accumulated Amortization			Gross Carrying Amount		Accumulated Amortization
Customer relationships	6-21 yrs.	$	699.7	$	(513.0	)	$	694.9	$	(499.8	)
Trademarks	12 yrs.		3.8		(2.4	)		3.8		(2.3	)
Non-compete agreements	5 yrs.		2.7		(1.8	)		2.7		(1.7	)
Total intangible assets		$	706.2	$	(517.2	)	$	701.4	$	(503.8	)

The following is a summary of the goodwill activity for the periods ended September 30, 2021 and December 31, 2021:


	Maintenance Services		Development Services			Total
Balance, September 30, 2020	$	1,680.4	$	178.9		$	1,859.3
Acquisitions		77.6		13.9			91.5
Balance, September 30, 2021	$	1,758.0	$	192.8		$	1,950.8
Acquisitions		1.4		(1.7	)		(0.3	)
Balance, December 31, 2021	$	1,759.4	$	191.1		$	1,950.5

During the three months ended December 31, 2020, the Company acquired, through a series of separate transactions, 100% of the operations of three unrelated companies, one of which was allocated between Maintenance Services and Development Services. The Company paid approximately $62.2 in aggregate consideration for the acquisitions, net of cash acquired. The Company accounted for the business combinations under the acquisition method and, accordingly, recorded the assets acquired and liabilities assumed at their estimated fair market values based on management’s preliminary estimates, with the excess allocated to goodwill. The fair values were primarily estimated using Level 3 assumptions within the fair value hierarchy, including estimated future cash flows, discount rates and other factors. The amount allocated to goodwill is reflective of the benefits the Company expects to realize from anticipated synergies and the acquired assembled workforce. The Company expects a portion of the goodwill resulting from these acquisitions will be deductible for tax purposes.