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Summary of Significant Accounting Policies - Cash and Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Thousands		12 Months Ended
	Dec. 19, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 23, 2020
Public Utilities, General Disclosures [Line Items]
Long-term borrowings held as restricted cash		$ 4,000	$ 5,000	$ 0
Long-term debt		1,352,320	1,292,586
Maine Water Company
Public Utilities, General Disclosures [Line Items]
Long-term borrowings held as restricted cash	$ 5,000
Long-term debt		$ 39,751	$ 38,032
Maine Water Company | State Revolving Fund Loans Series T | Public utility, bonds
Public Utilities, General Disclosures [Line Items]
Long-term debt					$ 4,000

X

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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