6. PROPERTY, PLANT AND EQUIPMENT

A summary of the historical cost of WES’s property, plant and equipment is as follows:



thousands	Estimated Useful Life	September 30, 2017		December 31, 2016
Land	n/a	$	4,271	$	4,012
Gathering systems and processing complexes	3 to 47 years	6,972,302		6,462,053
Pipelines and equipment	15 to 45 years	139,344		139,646
Assets under construction	n/a	434,432		226,626
Other	3 to 40 years	31,829		29,605
Total property, plant and equipment		7,582,178		6,861,942
Accumulated depreciation		2,074,464		1,812,010
Net property, plant and equipment		$	5,507,714	$	5,049,932

The cost of property classified as “Assets under construction” is excluded from capitalized costs being depreciated. These amounts represent property that is not yet suitable to be placed into productive service as of the respective balance sheet date.

Impairments. During the nine months ended September 30, 2017, WES recognized impairments of $170.1 million, including an impairment of $158.8 million at the Granger complex, which was impaired to its estimated fair value of $48.5 million using the income approach and Level 3 fair value inputs, due to a reduced throughput fee as a result of a producer’s bankruptcy. Also during the period, WES recognized additional impairments of $11.3 million, primarily related to (i) a $3.7 million impairment at the Granger straddle plant, which was impaired to its estimated salvage value of $0.6 million using the income approach and Level 3 fair value inputs, (ii) a $3.1 million impairment of the Fort Union equity investment (see Note 7), (iii) a $2.0 million impairment of an idle facility in northeast Wyoming, which was impaired to its estimated salvage value of $0.4 million using the market approach and Level 3 fair value inputs, and (iv) the cancellation of a pipeline project in West Texas.

During 2016, WES recognized impairments of $15.5 million, including an impairment of $6.1 million at the Newcastle system, which was impaired to its estimated fair value of $3.1 million using the income approach and Level 3 fair value inputs, due to a reduction in estimated future cash flows caused by the low commodity price environment. Also during 2016, WES recognized impairments of $9.4 million, primarily related to the cancellation of projects at the DJ Basin complex and Springfield and DBJV systems, and the abandonment of compressors at the MIGC system.