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	March 31, 2017		December 31, 2016
Plants, pipelines and facilities (1)	3-45 (5)	$	35,382.6	$	35,124.6
Underground and other storage facilities (2)	5-40 (6)		3,352.8		3,326.9
Transportation equipment (3)	3-10		169.2		165.8
Marine vessels (4)	15-30		801.2		800.7
Land			265.1		264.6
Construction in progress			3,565.8		3,320.7
Total			43,536.7		43,003.3
Less accumulated depreciation			9,980.6		9,710.8
Property, plant and equipment, net		$	33,556.1	$	33,292.5

(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.

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

	For the Three Months Ended March 31,
	2017		2016
Depreciation expense (1)	$	317.5	$	295.9
Capitalized interest (2)		39.6		42.5

(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

We record asset retirement obligations ("AROs") in connection with legal requirements to perform specified retirement activities under contractual arrangements and/or governmental regulations. Our contractual AROs primarily result from right-of-way agreements associated with our pipeline operations and real estate leases associated with our plant sites. In addition, we record AROs in connection with governmental regulations associated with the abandonment or retirement of above-ground brine storage pits and certain marine vessels. We also record AROs in connection with regulatory requirements associated with the renovation or demolition of certain assets containing hazardous substances such as asbestos. We typically fund our AROs using cash flow from operations.

Property, plant and equipment at March 31, 2017 and December 31, 2016 includes $44.2 million and $44.9 million, respectively, of asset retirement costs capitalized as an increase in the associated long-lived asset.

The following table presents information regarding our AROs since January 1, 2017:

ARO liability balance, January 1, 2017	$	85.4
Liabilities settled		(1.0	)
Accretion expense		1.3
ARO liability balance, March 31, 2017	$	85.7

Azure Acquisition

In March 2017, we announced the execution of a definitive agreement to acquire the midstream business and assets of Azure Midstream Partners, L.P. and its operating subsidiaries ("Azure"). The purchase agreement was the result of Azure's bankruptcy auction proceedings, which we won with a bid price of $189 million. The sale was approved by the U.S. Bankruptcy Court for the Southern District of Texas on March 15, 2017.

Azure's assets, which are located in East Texas and North Louisiana, include over 960 miles of natural gas gathering pipelines, three natural gas processing facilities with an aggregate processing capacity of approximately 210 million cubic feet per day, and two NGL pipelines with throughput capacities of 10 thousand barrels per day each. The Azure assets serve production from the Haynesville Shale and Bossier, Cotton Valley and Travis Peak formations.

The transaction closed in April 2017 after receiving final regulatory approvals and the satisfaction of other closing conditions. Under terms of the definitive agreement, we paid a deposit of $16.0 million in March 2017, which was applied to the purchase price of $189 million at closing. The deposit is presented on our Unaudited Condensed Statement of Consolidated Cash Flows under the Investing Activities section as "Cash used for pending business combination." The remainder of the accounting for the Azure acquisition, including development of the purchase price allocation, will take place during the second quarter of 2017 when the necessary information is obtained.