Debt (Details) - USD ($)	3 Months Ended
Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Debt Disclosure [Abstract]
Short-term Debt	$ 0	$ 53,300,000	$ 164,900,000
Short-term Debt, Weighted Average Interest Rate			5.083%
Debt Outstanding
Long-term Debt	719,710,000	719,323,000	$ 594,725,000
Unamortized Debt Issuance Expense	(6,990,000)	(7,377,000)	(6,975,000)
Long-term Debt, Gross	$ 726,700,000	726,700,000	601,700,000
Debt Instrument, Maturity Date Range, Start	Aug. 01, 2017
Debt Instrument, Maturity Date Range, End	Dec. 05, 2046
Liabilities, Fair Value Disclosure [Abstract]
Debt Instrument, Fair Value Disclosure	$ 785,980,000	$ 793,339,000	$ 686,159,000
Minimum [Member]
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	1.545%
Maximum [Member]
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	9.05%

No definition available.

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- Definition

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

+ 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))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
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-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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-Publisher FASB
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-Section S99
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-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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- Definition

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13433-108611

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

+ References

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- Definition

Latest date the outstanding debt instruments are required to be repaid, in CCYY-MM-DD format.

+ References

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Earliest date the outstanding debt instruments are required to be repaid, in CCYY-MM-DD format.

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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.

+ 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

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-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Weighted average interest rate of long-term debt outstanding.

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Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.

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

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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

+ References

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