Debt Debt (Schedule of Long Term Debt Maturities) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Long-term Debt, Maturities, Repayments of Principal in Next Rolling Twelve Months	$ 1,144
Long-term Debt, Maturities, Repayments of Principal in Rolling Year Two	1,188
Long-term Debt, Maturities, Repayments of Principal in Rolling Year Three	1,230
Long-term Debt, Maturities, Repayments of Principal in Rolling Year Four	148,854
Long-term Debt, Maturities, Repayments of Principal in Rolling Year Five	472,328
Long-term Debt, Maturities, Repayments of Principal in Rolling after Year Five	535,245
Long-term Debt, Gross	1,159,989
Debt Instrument, Unamortized Discount	(4,039)
Less: unamortized loan costs	10,453	$ 9,740
Debt, Long-term and Short-term, Combined Amount	1,145,497	1,115,981
Private Placement [Member]
Unsecured Debt	400,000	$ 400,000
January 2023 [Member] \| Private Placement [Member]
Unsecured Debt	$ 125,000
Debt Instrument, Interest Rate, Stated Percentage	3.99%
November 2023 [Member] \| Private Placement [Member]
Unsecured Debt	$ 50,000
Debt Instrument, Interest Rate, Stated Percentage	3.99%
September 2024 [Member] \| Private Placement [Member]
Unsecured Debt	$ 75,000
Debt Instrument, Interest Rate, Stated Percentage	3.93%
November 2025 [Member] \| Private Placement [Member]
Unsecured Debt	$ 50,000
Debt Instrument, Interest Rate, Stated Percentage	4.33%
January 2027 [Member] \| Private Placement [Member]
Unsecured Debt	$ 100,000
Debt Instrument, Interest Rate, Stated Percentage	4.51%

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
-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

Reference 2: 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(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 210
-Section S99
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-Subparagraph (SX 210.9-03(16))
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Contractual interest rate for funds borrowed, under the debt agreement.

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Amount, after accumulated amortization, of debt discount.

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-SubTopic 30
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Represents the aggregate of total long-term debt, including current maturities and short-term debt.

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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 next rolling twelve months following the latest balance sheet. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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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 rolling twelve months following the latest balance sheet. For 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 rolling twelve months following the latest balance sheet. For 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 rolling twelve months following the latest balance sheet. For 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 rolling twelve months following the latest balance sheet. For 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 rolling twelve months following the latest balance sheet. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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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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Including the current and noncurrent portions, carrying value as of the balance sheet date of uncollateralized debt obligations (with maturities initially due after one year or beyond the operating cycle if longer).

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section S99
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-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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

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