	December 31, 2020			December 31, 2019			Estimated Useful Lives (In Years)
Gathering systems	$	9,216.1		$	8,976.8		5 to 20
Processing and fractionation facilities		6,276.8			5,143.0		5 to 25
Terminaling and storage facilities		1,555.1			1,495.5		5 to 25
Transportation assets		2,567.7			2,292.4		10 to 50
Other property, plant and equipment		32.4			184.1		3 to 50
Land		160.8			159.7		—
Construction in progress		324.3			1,576.5		—
Finance lease right-of-use assets		51.8			48.8
Property, plant and equipment		20,185.0			19,876.8
Accumulated depreciation, amortization and impairment		(8,011.4	)		(5,328.3	)
Property, plant and equipment, net	$	12,173.6		$	14,548.5

Intangible assets	$	2,643.5		$	2,643.5		10 to 20
Accumulated amortization and impairment		(1,261.1	)		(908.5	)
Intangible assets, net	$	1,382.4		$	1,735.0

During the preparation of the Company's 2020 consolidated financial statements, the Company identified certain gathering pipelines that should not have had value ascribed to them as part of a prior acquisition as these assets were inactive. 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 wrote these assets down in 2020 and recognized a non-cash loss of $32.4 million in Other operating (income) expense in our Consolidated Statements of Operations.

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.

For each of the years ended December 31, 2020, 2019, and 2018 depreciation expense was $721.1 million, $800.0 million and $633.3 million.

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 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 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 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.

While commodity prices remain low relative to historical levels 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.

In the fourth quarter of 2019, we recorded a non-cash pre-tax impairment charge of $225.3 million for the partial impairment of certain gas processing facilities and gathering systems associated with our Central and Coastal operations in our Gathering and Processing segment. The impairment was a result of our assessment that forecasted undiscounted future net cash flows from operations, while positive, will not be sufficient to recover the existing total net book value of the underlying assets. Underlying our assessment was the expected continuing decline in natural gas production across the Barnett Shale in North Texas and Gulf of Mexico due to a sustained low commodity price environment.

For both the 2020 and 2019 impairment assessments discussed above, 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% and 8.5% when deriving the fair value of the asset groups impaired during the first quarter of 2020 and the fourth quarter of 2019, respectively. The weighted average discount rate and terminal values reflect management’s best estimate of inputs a market participant would utilize.

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 associated with certain intangible customer relationships for which undiscounted future net cash flows were not sufficient to recover the net book value.

For each of the years ended December 31, 2020, 2019, and 2018 amortization expense for our intangible assets was $144.0 million, $171.6 million and $182.6 million. 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. As of December 31, 2020, the weighted average amortization period for our intangible assets was approximately 13 years.

	December 31, 2020			December 31, 2019
Balance at beginning of period	$	1,735.0		$	1,983.2
Held for sale assets		—			(76.6	)
Impairment		(208.6	)		—
Amortization		(144.0	)		(171.6	)
Balance at end of period	$	1,382.4		$	1,735.0