Fair Value of Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Fair Value of Financial Assets and Liabilities
Risk premium (as a percent)	1.70%
Advances for construction	$ 184,479	$ 182,502
Level 1
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities	0	0
Advances for construction	0	0
Total	0	0
Level 2
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities	584,764	607,492
Advances for construction	77,621	75,083
Total	662,385	682,575
Level 3
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities	0	0
Advances for construction	0	0
Total	0	0
Cost
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities	521,594	531,713
Advances for construction	184,479	182,502
Total	706,073	714,215
Total
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities	584,764	607,492
Advances for construction	77,621	75,083
Total	$ 662,385	$ 682,575

Represents the risk premium rate as an input to measure fair value.

No definition available.

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- Definition

For utilities only, represents the carrying amount of the liability as of the balance sheet date for payments received by a utility from its customers in advance of performing its obligations under terms of its construction agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.24)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 910
-SubTopic 405
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=68049106&loc=d3e50628-109371

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- Definition

Represents the aggregate of total long-term debt, including current maturities and short-term debt.

+ References

No definition available.

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- Definition

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.

+ References

No definition available.

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- Definition

Fair value of financial and nonfinancial obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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