Note 4. Property, Plant and Equipment

The historical costs of our property, plant and equipment and related accumulated depreciation balances were as follows at the dates indicated:

	Estimated Useful Life in Years	June 30, 2018		December 31, 2017
Plants, pipelines and facilities (1)	3-45 (5)	$	41,446.7	$	37,132.2
Underground and other storage facilities (2)	5-40 (6)		3,508.6		3,460.9
Transportation equipment (3)	3-10		185.6		177.1
Marine vessels (4)	15-30		807.5		803.8
Land			360.3		273.1
Construction in progress			2,345.2		4,698.1
Total			48,653.9		46,545.2
Less accumulated depreciation			11,599.4		10,924.8
Property, plant and equipment, net		$	37,054.5	$	35,620.4

(1) Plants, pipelines and facilities include processing plants; NGL, natural gas, crude oil and petrochemical and refined products pipelines; terminal loading and unloading facilities; buildings; office furniture and equipment; laboratory and shop equipment and related assets. (2) Underground and other storage facilities include underground product storage caverns; above ground storage tanks; water wells and related assets. (3) Transportation equipment includes tractor-trailer tank trucks and other vehicles and similar assets used in our operations. (4) Marine vessels include tow boats, barges and related equipment used in our marine transportation business. (5) In general, the estimated useful lives of major assets within this category are: processing plants, 20-35 years; pipelines and related equipment, 5-45 years; terminal facilities, 10-35 years; buildings, 20-40 years; office furniture and equipment, 3-20 years; and laboratory and shop equipment, 5-35 years. (6) In general, the estimated useful lives of assets within this category are: underground storage facilities, 5-35 years; storage tanks, 10-40 years; and water wells, 5-35 years.

In March 2018, we acquired the remaining 50% member interest of our Delaware Processing joint venture, which resulted in the consolidation of approximately $200 million of property, plant and equipment. See Note 11 for information regarding this recent acquisition.

In April 2018, we acquired 65-acres of waterfront property on the Houston Ship Channel for approximately $85.2 million, all of which was recorded as land. The purchase price consisted of $55.2 million in cash with the balance funded through 1,223,242 newly-issued Enterprise common units. The land is located immediately to the east of our Enterprise Hydrocarbons Terminal (“EHT”) and is expected to facilitate future expansion projects at EHT.

The following table summarizes our depreciation expense and capitalized interest amounts for the periods indicated:

	For the Three Months Ended June 30,				For the Six Months Ended June 30,
	2018		2017		2018		2017
Depreciation expense (1)	$	361.0	$	321.1	$	692.8	$	638.6
Capitalized interest (2)		27.1		44.5		85.3		84.1

(1) Depreciation expense is a component of “Costs and expenses” as presented on our Unaudited Condensed Statements of Consolidated Operations. (2) We capitalize interest costs incurred on funds used to construct property, plant and equipment while the asset is in its construction phase. The capitalized interest is recorded as part of the asset to which it relates and is amortized over the asset’s estimated useful life as a component of depreciation expense. When capitalized interest is recorded, it reduces interest expense from what it would be otherwise.

Asset Retirement Obligations

Property, plant and equipment at June 30, 2018 and December 31, 2017 includes $50.1 million and $39.9 million, respectively, of asset retirement costs capitalized as an increase in the associated long-lived asset. The following table presents information regarding our asset retirement obligations, or AROs, since January 1, 2018:

ARO liability balance, January 1, 2018	$	86.7
Liabilities incurred		0.5
Liabilities settled		(1.5	)
Revisions in estimated cash flows		11.7
Accretion expense		2.9
ARO liability balance, June 30, 2018	$	100.3