Debt (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Maturities of Long-term Debt [Abstract]
Long-Term Debt, Current Maturities, Gross	$ 97,000	$ 40,000
Long-term Debt, Maturities, Repayments of Principal in Year Two	30,000
Long-term Debt, Maturities, Repayments of Principal in Year Three	75,000
Long-term Debt, Maturities, Repayments of Principal in Year Four	60,000
Long-term Debt, Maturities, Repayments of Principal in Year Five	0
Long-term Debt, Maturities, Repayments of Principal after Year Five	524,700
Debt Instruments [Abstract]
Long-term Debt, Gross	786,700	726,700
Long-Term Debt, Current Maturities, Gross	97,000	40,000
Long-term Debt, Excluding Current Maturities, Gross	689,700	686,700
Debt Outstanding
Proceeds from Issuance of Debt	100,000
Short-term Debt, Other Disclosures [Abstract]
Short-term Debt	54,200	53,300
Bank Loans	$ 0	$ 0
Short-term Debt, Weighted Average Interest Rate, at Point in Time	1.90%	0.80%
Commercial Paper, Maximum Maturity	11 days
Commercial Paper, Average Maturity	6 days
Liabilities, Fair Value Disclosure [Abstract]
Long-term Debt, Gross	$ 786,700	$ 726,700
Unamortized Debt Issuance Expense	(6,813)	(7,377)
Long-term Debt	779,887	719,323
Debt Instrument, Fair Value Disclosure	853,339	793,339
Note 7000 Series B Due 2017 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 0	$ 40,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	0.00%	7.00%
Repayments of Long-term Debt [Abstract]
Extinguishment of Debt, Amount	$ 40,000
Note 1545 Series Due 2018 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 75,000	$ 75,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	1.545%	1.55%
Note 6600 Series B Due 2018 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 22,000	$ 22,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	6.60%	6.60%
Note 8310 Series B Due 2019 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 10,000	$ 10,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	8.31%	8.31%
Note 7630 Series B Due 2019 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 20,000	$ 20,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	7.63%	7.63%
Note 5370 Series B Due 2020 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 75,000	$ 75,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	5.37%	5.37%
Note 9050 Series A Due 2021 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 10,000	$ 10,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	9.05%	9.05%
Note 3176 Series B Due 2021 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 50,000	$ 50,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	3.176%	3.18%
Note 3542 Series B Due 2023 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 50,000	$ 50,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	3.542%	3.542%
Note 5620 Series B Due 2023 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 40,000	$ 40,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	5.62%	5.62%
Note 7720 Series B Due 2025 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 20,000	$ 20,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	7.72%	7.72%
Note 6520 Series B Due 2025 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 10,000	$ 10,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	6.52%	6.52%
Note 7050 Series B Due 2026 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 20,000	$ 20,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	7.05%	7.05%
Note 3211 Series B Due 2026 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 35,000	$ 35,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	3.211%	3.21%
Note 7000 Series Due 2027 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 20,000	$ 20,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	7.00%	7.00%
Note 2822 Series Due 2027 [Member] [Domain]
Debt Instruments [Abstract]
Secured Debt	$ 25,000	$ 0
Debt Outstanding
Proceeds from Issuance of Debt	$ 25,000
Debt Instrument, Interest Rate, Stated Percentage	2.822%	0.00%
Note 6650 Series Due 2027 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 19,700	$ 19,700
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	6.65%	6.65%
Note 6650 Series Due 2028 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 10,000	$ 10,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	6.65%	6.65%
Note 7740 Series Due 2030 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 20,000	$ 20,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	7.74%	7.74%
Note 7850 Series Due 2030 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 10,000	$ 10,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	7.85%	7.85%
Note 5820 Series Due 2032 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 30,000	$ 30,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	5.82%	5.82%
Note 5660 Series Due 2032 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 40,000	$ 40,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	5.66%	5.66%
Note 5250 Series Due 2035 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 10,000	$ 10,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	5.25%	5.25%
Note 4000 Series Due 2042 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 50,000	$ 50,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	4.00%	4.00%
Note 4136 Series Due 2046 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 40,000	$ 40,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	4.136%	4.14%
Note 3685 Series Due 2047 [Member] [Domain]
Debt Instruments [Abstract]
Secured Debt	$ 75,000	$ 0
Debt Outstanding
Proceeds from Issuance of Debt	$ 75,000
Debt Instrument, Interest Rate, Stated Percentage	3.685%	0.00%
Line of Credit [Member]
Short-term Debt, Other Disclosures [Abstract]
Line of Credit Facility, Maximum Borrowing Capacity	$ 300,000
Line of Credit Facility Contingent Increase To Maximum Borrowing Capacity	450,000
Line of Credit Facility, Amount Outstanding	$ 0	$ 0
Debt Instrument Covenant Consolidated Indebtedness To Capitalization Ratio	70.00%
Letter of Credit [Member]
Short-term Debt, Other Disclosures [Abstract]
Line of Credit Facility, Maximum Borrowing Capacity	$ 100,000
Letters of Credit Outstanding, Amount	$ 0	$ 0

Commercial Paper, Average Maturity (duration)

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Commercial Paper, Maximum Maturity (duration)

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Represents the maximum ratio of consolidated indebtedness to total capitalizationwhich is necessary to be maintained under the terms of the line of credit facility.

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Increased maximum borrowing capacity under the credit facility which the reporting entity may request.

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Long-Term Debt, Current Maturities, Gross

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Long-term Debt, Excluding Current Maturities, Gross

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Amounts due on short-term bank loans secured by customers' securities on the balance sheet date.

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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.

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

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

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Gross amount of debt extinguished.

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The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

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The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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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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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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The cash inflow during the period from additional borrowings in aggregate debt. Includes proceeds from short-term and long-term debt.

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Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

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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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Weighted average interest rate of short-term debt outstanding calculated at point in time.

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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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