The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

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Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Goodwill [Line Items]
Goodwill	$ 706,960,000		$ 706,960,000
Amortization expense	1,500,000	$ 1,300,000
Impairment of intangible assets (excluding goodwill)	$ 0	$ 0
Minimum
Goodwill [Line Items]
Useful life of finite-lived intangible assets	2 years
Maximum
Goodwill [Line Items]
Useful life of finite-lived intangible assets	20 years
ETG Utility Operations
Goodwill [Line Items]
Goodwill	$ 700,200,000		700,200,000
Corporate and Other
Goodwill [Line Items]
Goodwill	$ 6,800,000		$ 6,800,000