	March 31, 2021			December 31, 2020
Computer and communications equipment	$	93,813		$	92,565
Software, including software development costs		269,608			259,439
Leasehold improvements and other fixed assets		118,796			120,951
		482,217			472,955
Less: accumulated depreciation and amortization		(270,984	)		(258,173	)
Fixed assets, net	$	211,233		$	214,782

Depreciation expense was $6.3 million and $6.1 million for the three months ended March 31, 2021 and 2020, respectively. Depreciation is included as part of “Occupancy and equipment” in the Company’s unaudited condensed consolidated statements of operations.

The Company has $5.9 million of asset retirement obligations related to certain of its leasehold improvements as of March 31, 2021 and December 31, 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 March 31, 2021 and 2020, software development costs totaling $11.2 million and $12.9 million, respectively, were capitalized. Amortization of software development costs totaled $8.3 million and $7.6 million for the three months ended March 31, 2021 and 2020, 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 $2.0 million and $4.8 million were recorded for the three months ended March 31, 2021 and 2020, 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.