
(in millions)	Useful Lives	June 30, 2020		December 31, 2019

Land		$	236	$	—
Buildings and equipment	Up to 40 years	3,554		2,587
Wireless communications systems	Up to 20 years	43,876		34,353
Leasehold improvements	Up to 12 years	1,666		1,345
Capitalized software	Up to 10 years	15,020		12,705
Leased wireless devices	Up to 19 months	7,605		1,139
Construction in progress		3,933		2,973
Accumulated depreciation and amortization		(37,089)		(33,118)

Property and equipment, net		$	38,801	$	21,984

We capitalize interest associated with the acquisition or construction of certain property and equipment and spectrum intangible assets. We recognized capitalized interest of $119 million and $125 million for the three months ended June 30, 2020 and 2019, respectively, and $231 million and $243 million for the six months ended June 30, 2020 and 2019, respectively.

Total depreciation expense relating to property and equipment and financing lease right-of-use assets was $3.7 billion and $1.6 billion for the three months ended June 30, 2020 and 2019, respectively, and $5.4 billion and $3.1 billion for the six months ended June 30, 2020 and 2019, respectively. These amounts include depreciation expense related to leased wireless devices of $947 million and $125 million for the three months ended June 30, 2020 and 2019 respectively, and $1.1 billion and $309 million for the six months ended June 30, 2020 and 2019, 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)	Six Months Ended June 30, 2020		Twelve Months Ended December 31, 2019

Asset retirement obligations, beginning of year	$	659	$	609
Fair value of liabilities acquired through Merger	1,062		—
Liabilities incurred	6		35
Liabilities settled	—		(2)
Accretion expense	22		32
Changes in estimated cash flows	—		(15)

Asset retirement obligations, end of period	$	1,749	$	659

Classified on the balance sheet as:


Other long-term liabilities	$	1,749	$	659

The corresponding assets, net of accumulated depreciation, related to asset retirement obligations were $1.1 billion and $159 million as of June 30, 2020 and December 31, 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 three and six months ended June 30, 2020. The expense is included within Impairment expense in our Condensed Consolidated Statements of Comprehensive Income. There were no impairments recognized for the three and six months ended June 30, 2019.