	June 30, 2021			December 31, 2020			Estimated Useful Lives (In Years)
Gathering systems	$	9,205.4		$	9,216.1		5 to 20
Processing and fractionation facilities		6,275.4			6,276.8		5 to 25
Terminaling and storage facilities		1,313.0			1,555.1		5 to 25
Transportation assets		2,616.0			2,567.7		10 to 50
Other property, plant and equipment		331.0			32.4		3 to 50
Land		160.8			160.8		—
Construction in progress		358.6			324.3		—
Finance lease right-of-use assets		52.9			51.8		—
Property, plant and equipment		20,313.1			20,185.0
Accumulated depreciation, amortization and impairment		(8,316.5	)		(8,011.4	)
Property, plant and equipment, net	$	11,996.6		$	12,173.6

Intangible assets	$	2,643.5		$	2,643.5		10 to 20
Accumulated amortization and impairment		(1,326.6	)		(1,261.1	)
Intangible assets, net	$	1,316.9		$	1,382.4

During the three and six months ended June 30, 2021, depreciation expense was $179.1 million and $362.5 million, respectively. During the three and six months ended June 30, 2020, depreciation expense was $169.3 million and $370.0 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, including changes to our estimates that could have an impact on our assessment of asset recoverability.

During the first quarter 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 of 2020 from certain major oil producing nations increasing production also significantly contributed to the sharp drop in commodity prices. 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. As a result, we determined that indicators of impairment existed for certain asset groups reported primarily within our Gathering and Processing segment, and recorded non-cash pre-tax impairments of $2,442.8 million (inclusive of impairments of intangible assets) primarily associated with the partial impairment of certain gas processing facilities and gathering systems associated with our Central operations and full impairment of our Coastal operations. Our first quarter 2020 impairment assessment forecasted continuing decline in natural gas production across the Mid-Continent and Gulf of Mexico regions. The carrying value adjustments are included in Impairment of long-lived assets in our Consolidated Statements of Operations.

We determined fair value through the use of discounted estimated cash flows to measure the impairment loss for each asset group for which undiscounted future net cash flows were not sufficient to recover the net book value.

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 terminal value and 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 determination of terminal values. 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 terminal values reflect management’s best estimate of inputs a market participant would utilize.

While commodity prices remain volatile and uncertainties associated with the impacts of COVID-19 continue, production from wells that were previously shut-in during the first half of 2020 across our operating areas has largely resumed. There were no indicators of impairment identified during the remainder of 2020 or first half of 2021.

We may identify additional triggering events in the future, which will require additional evaluations of the recoverability of the carrying value of our long-lived assets and may result in future impairments.

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 of $208.6 million 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 $130.9 million, $122.7 million, $117.5 million, $113.7 million and $110.6 million for each of the years 2021 through 2025, respectively.