	June 30, 2020			December 31, 2019			Estimated Useful Lives (In Years)
Gathering systems	$	9,117.8		$	8,976.8		5 to 20
Processing and fractionation facilities		5,672.8			5,143.0		5 to 25
Terminaling and storage facilities		1,498.7			1,495.5		5 to 25
Transportation assets		2,316.3			2,292.4		10 to 50
Other property, plant and equipment		196.4			184.1		3 to 25
Land		159.2			159.7		—
Construction in progress		1,304.0			1,576.5		—
Finance lease right-of-use assets		51.0			48.8
Property, plant and equipment		20,316.2			19,876.8
Accumulated depreciation, amortization and impairment		(7,898.8	)		(5,328.3	)
Property, plant and equipment, net	$	12,417.4		$	14,548.5

Intangible assets	$	2,643.5		$	2,643.5		10 to 20
Accumulated amortization and impairment		(1,190.7	)		(908.5	)
Intangible assets, net	$	1,452.8		$	1,735.0

During the preparation of the Company's first quarter 2019 consolidated financial statements, the Company identified an error related to depreciation expense on certain assets that should have been placed in-service during 2018. The Company does not believe this error is material to its previously issued historical consolidated financial statements for any of the periods impacted and accordingly, has not adjusted the historical financial statements. The Company recorded the cumulative impact of a one-time $12.5 million overstatement of depreciation expense during the first quarter of 2019.

During the three and six months ended June 30, 2020, depreciation expense was $169.3 million and $370.0 million, respectively. During the three and six months ended June 30, 2019, depreciation expense was $194.3 million and $388.7 million, respectively.

We review and evaluate our long-lived assets, including intangible assets, for impairment when events or changes in circumstances indicate that the related carrying amount of such assets may not be recoverable, and changes to our estimates could have an impact on our assessment of asset recoverability.

During the first half of 2020, global commodity prices declined due to factors that significantly impacted both demand and supply. As the COVID-19 pandemic spread, causing travel and other restrictions to be implemented globally, the demand for commodities declined. Additionally, the supply shock late in the first quarter from certain major oil producing nations increasing production also significantly contributed to the sharp drop in commodity prices. While these major oil and gas producing countries subsequently agreed to collectively decrease production, these events, combined with the outbreak of the COVID-19 pandemic, contributed to volatility and depressed commodity prices in the first half of 2020. The drop in commodity prices resulted in prompt reactions from some domestic producers, including significantly reducing capital budgets and resultant drilling activity and shutting-in production. Commodity prices remain weak relative to historical levels and have remained volatile as uncertainty around global commodity supply and demand continues due to the COVID-19 pandemic.

In the first quarter of 2020, we determined that indicators of impairment existed for certain asset groups reported primarily within our Gathering and Processing segment. For each asset group for which undiscounted future net cash flows were not sufficient to recover the net book value, fair value was determined through use of discounted estimated cash flows to measure the impairment loss.

The estimated cash flows used to assess recoverability of our long-lived assets and measure fair value of our asset groups are derived from current business plans, which are developed using near-term price and volume projections reflective of the current environment and management's projections for long-term average prices and volumes. In addition to near and long-term price assumptions, other key assumptions include volume projections, operating costs, timing of incurring such costs and the use of an appropriate discount rate. We believe our estimates and models used to determine fair value are similar to what a market participant would use.

The fair value measurement of our long-lived assets was based, in part, on significant inputs not observable in the market (as discussed above) and thus represents a Level 3 measurement. The significant unobservable inputs used include discount rates and terminal value exit multiples. We utilized a weighted average discount rate of 14.0% when deriving the fair value of the asset groups impaired during the first quarter of 2020. The weighted average discount rate and exit multiples reflect management’s best estimate of inputs a market participant would utilize.

In the first quarter of 2020, we recorded non-cash pre-tax impairments of $2,442.8 million primarily associated with the partial impairment of gas processing facilities and gathering systems associated with our Mid-Continent operations and full impairment of our Coastal operations - all of which are in our Gathering and Processing segment. Our first quarter impairment assessment forecasted further decline in natural gas production across the Mid-Continent and Gulf of Mexico. The carrying value adjustments are included in Impairment of long-lived assets in our Consolidated Statements of Operations. There were no indicators of impairment identified during the second quarter of 2020.

Intangible assets consist of customer contracts and customer relationships acquired in prior business combinations. The fair value of these acquired intangible assets were determined at the date of acquisition based on the present values of estimated future cash flows. Amortization expense attributable to these assets is recorded over the periods in which we benefit from services provided to customers.

As a result of the triggering events and analysis described above, in the first quarter of 2020, we recognized a non-cash pre-tax impairment loss associated with certain intangible customer relationships for which undiscounted future net cash flows were not sufficient to recover the net book value.

The estimated annual amortization expense for intangible assets is approximately $144.0 million, $130.9 million, $122.7 million, $117.5 million and $113.7 million for each of the years 2020 through 2024, respectively.