SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Interchange Power (Details) - Accounts Payable - Interchange Power - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
INTERCHANGE POWER:
Interchange liability	$ 1.6	$ 2.3
Purchased Power Expense
INTERCHANGE POWER:
Net interchange activity, credit (expense)	$ 0.4	$ (0.6)	$ 0.4

Carrying value as of the balance sheet date of obligations incurred and payable for energy purchases, typically by a utility or an entity engaged in selling energy contracts.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(a)(7))
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Disclosure of the difference in values (gain (loss)) between the nonmonetary assets or liabilities exchanged with another entity inclusive of boot (small monetary consideration).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 845
-SubTopic 10
-Section 50
-Paragraph 1
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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