Debt (Schedule Of Additional Information Regarding Series A Bonds) (Details) - USD ($)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Carrying Value	$ 485,605,998
Term Loan Facility [Member]
Debt Instrument [Line Items]
Issue Amount	610,600,000
Carrying Value	$ 485,606,000
2011 Series A Bond Of 4.20% Maturing 2031 [Member] \| Term Loan Facility [Member]
Debt Instrument [Line Items]
Maturing	Mar. 15, 2031
Average Life (Years)	6 years 8 months 12 days
Interest Rate	4.20%	4.20%
Issue Amount	$ 90,000,000
Carrying Value	$ 63,000,000
2011 Series A Bond Of 4.75% Maturing 2041 [Member] \| Term Loan Facility [Member]
Debt Instrument [Line Items]
Maturing	Mar. 15, 2041
Average Life (Years)	11 years 8 months 12 days
Interest Rate	4.75%	4.75%
Issue Amount	$ 185,000,000
Carrying Value	$ 148,000,000
2012 Series A Bond Of 4.01% Maturing 2032 [Member] \| Term Loan Facility [Member]
Debt Instrument [Line Items]
Maturing	Mar. 15, 2032
Average Life (Years)	7 years 2 months 12 days
Interest Rate	4.01%	4.01%
Issue Amount	$ 75,000,000
Carrying Value	$ 56,250,000
2012 Series A Bond Of 4.41% Maturing 2042 [Member] \| Term Loan Facility [Member]
Debt Instrument [Line Items]
Maturing	Mar. 15, 2042
Average Life (Years)	15 years
Interest Rate	4.41%	4.41%
Issue Amount	$ 125,000,000
Carrying Value	$ 88,000,000
2012 Series A Bond Of 4.78% Maturing 2042 [Member] \| Term Loan Facility [Member]
Debt Instrument [Line Items]
Maturing	Mar. 15, 2042
Average Life (Years)	14 years 8 months 12 days
Interest Rate	4.78%	4.78%
Issue Amount	$ 50,000,000
Carrying Value	50,000,000
2017 Series A Bonds [Member]
Debt Instrument [Line Items]
Carrying Value	$ 40,000,000
2017 Series A Bonds [Member] \| Term Loan Facility [Member]
Debt Instrument [Line Items]
Maturing	Mar. 15, 2037
Average Life (Years)	10 years 2 months 12 days
Interest Rate	3.43%
Issue Amount	$ 40,000,000
Carrying Value	$ 40,000,000
2016 CoBank Note [Member]
Debt Instrument [Line Items]
Maturing	Mar. 15, 2031
Average Life (Years)	5 years 8 months 12 days
Interest Rate	2.58%	2.58%
Issue Amount	$ 45,600,000
Carrying Value	$ 40,356,000

Face (par) amount of debt instrument at time of issuance.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ 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 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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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

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ 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 210.5-02.22(a)(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: 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(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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