
(in millions)	Useful Lives	December 31, 2020		December 31, 2019

Land		$	236	$	—
Buildings and equipment	Up to 30 years	4,006		2,587
Wireless communications systems	Up to 20 years	49,453		34,353
Leasehold improvements	Up to 12 years	1,879		1,345
Capitalized software	Up to 10 years	16,412		12,705
Leased wireless devices	Up to 19 months	6,989		1,139
Construction in progress		4,595		2,973
Accumulated depreciation and amortization		(42,395)		(33,118)

Property and equipment, net		$	41,175	$	21,984

Total depreciation expense relating to property and equipment and financing lease right-of-use assets was $13.1 billion, $6.5 billion and $6.4 billion for the years ended December 31, 2020, 2019 and 2018, respectively. These amounts include depreciation expense related to leased wireless devices of $3.1 billion, $543 million and $940 million for the years ended December 31, 2020, 2019 and 2018, respectively.

We capitalize interest associated with the acquisition or construction of certain property and equipment and spectrum intangible assets. We recognized capitalized interest of $440 million, $473 million and $362 million for the years ended December 31, 2020, 2019 and 2018, respectively.

Asset retirement obligations are primarily for certain legal obligations to remediate leased property on which our network infrastructure and administrative assets are located.


(in millions)	Year Ended December 31, 2020		Year Ended December 31, 2019

Asset retirement obligations, beginning of year	$	659	$	609
Fair value of liabilities acquired through Merger	1,110		—
Liabilities incurred	16		35
Liabilities settled	(40)		(2)
Accretion expense	55		32
Changes in estimated cash flows	17		(15)

Asset retirement obligations, end of period	$	1,817	$	659

Classified on the balance sheet as:

Other current liabilities	$	14	$	—
Other long-term liabilities	1,803		659

The corresponding assets, net of accumulated depreciation, related to asset retirement obligations were $912 million and $159 million as of December 31, 2020 and 2019, respectively.

In connection with the continuing integration of the businesses following the Merger, we evaluated the long-term billing system architecture strategy for our postpaid customers. In order to facilitate customer migration from the Sprint legacy billing platform, our postpaid billing system replacement plan and associated development will no longer serve our future needs. As a result, we recorded a non-cash impairment of $200 million related to capitalized software development costs for the year ended December 31, 2020, all of which relates to the impairment recognized during the three months ended June 30, 2020. The expense is included within Impairment expense in our Consolidated Statements of Comprehensive Income. There were no impairments recognized for the years ended December 31, 2019 and 2018.