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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 210
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Contractual interest rate for funds borrowed, under the debt agreement.

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

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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.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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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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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.

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Debt (Details) - USD ($)	9 Months Ended
Debt (Details) - USD ($)	Sep. 30, 2017	Sep. 13, 2017	Aug. 01, 2017	Dec. 31, 2016	Sep. 30, 2016
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 786,700,000	$ 100,000,000	$ 40,000,000	$ 726,700,000	$ 601,700,000
Short-term debt	0			53,300,000	194,900,000
Long-term Debt	779,424,000			719,323,000	595,213,000
Unamortized debt issuance costs	$ (7,276,000)			$ (7,377,000)	$ (6,487,000)
Maturity start date range	Mar. 16, 2018
Maturity end date range	Sep. 13, 2047
Stated interest rate			7.00%
Weighted average interest rate	4.78%
Minimum
Debt Instrument [Line Items]
Stated interest rate	1.545%
Maximum
Debt Instrument [Line Items]
Stated interest rate	9.05%
$25M [Domain]
Debt Instrument [Line Items]
Stated interest rate		2.822%
Debt Instrument, Maturity Date	Sep. 13, 2027
$75M [Domain]
Debt Instrument [Line Items]
Stated interest rate		3.685%
Debt Instrument, Maturity Date	Sep. 13, 2047
$75M [Domain]
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 75,000,000
$25M [Domain]
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 25,000,000