	September 30, 2020			December 31, 2019
Computer and communications equipment	$	97,999		$	95,115
Software, including software development costs		249,514			235,230
Leasehold improvements and other fixed assets		126,556			116,231
		474,069			446,576
Less: accumulated depreciation and amortization		(255,415	)		(241,735	)
Fixed assets, net	$	218,654		$	204,841

Depreciation expense was $6.3 million and $4.8 million for the three months ended September 30, 2020 and 2019, respectively. Depreciation expense was $18.3 million and $15.2 million for the nine months ended September 30, 2020 and 2019, respectively. Depreciation is included as part of “Occupancy and equipment” in the Company’s unaudited condensed consolidated statements of operations.

The Company has $5.6 million of asset retirement obligations related to certain of its leasehold improvements as of September 30, 2020. The associated asset retirement cost is capitalized as part of the carrying amount of the long-lived asset. The liability is discounted and accretion expense is recognized using the credit adjusted risk-free interest rate in effect when the liability was initially recognized.

For the three months ended September 30, 2020 and 2019, software development costs totaling $12.8 million and $14.5 million, respectively, were capitalized. Amortization of software development costs totaled $7.7 million and $7.2 million for the three months ended September 30, 2020 and 2019, respectively. For the nine months ended September 30, 2020 and 2019, software development costs totaling $41.1 million and $30.0 million, respectively, were capitalized. Amortization of software development costs totaled $23.0 million and $20.4 million for the nine months ended September 30, 2020 and 2019, respectively. Amortization of software development costs is included as part of “Occupancy and equipment” in the Company’s unaudited condensed consolidated statements of operations.

Impairment charges of $0.1 million and $0.4 million were recorded for the three months ended September 30, 2020 and 2019, respectively, related to the evaluation of capitalized software projects for future benefit and for fixed assets no longer in service. Impairment charges of $5.1 million and $1.3 million were recorded for the nine months ended September 30, 2020 and 2019, respectively, related to the evaluation of capitalized software projects for future benefit and for fixed assets no longer in service. Impairment charges related to

capitalized software and fixed assets are reflected in “Occupancy and equipment” in the Company’s unaudited condensed consolidated statements of operations.