Long-Term Debt (Summary of Long-Term Debt) (Details) - USD ($)	Dec. 31, 2015	Dec. 31, 2014	Aug. 14, 2013
Debt Instrument [Line Items]
6% senior notes due 2023 (net of unamortized discount of $6,692 at 2015 and $7,557 at 2014)	$ 493,308,000	$ 492,443,000	$ 500,000,000
Less unamortized debt issuance costs	(3,526,000)	(4,193,000)
Total notes payable, net	489,782,000	488,250,000
Capitalized lease obligations, vehicles, due through 2018	600,000	0
Less current maturities	(222,000)	0
Long-term debt	490,160,000	488,250,000
Senior Notes [Member]
Debt Instrument [Line Items]
Interest rate (percent)			6.00%
Unamortized Discount	$ 6,692,000	$ 7,557,000
Senior Notes [Member] \| 6% Senior Notes Due 2023 [Member]
Debt Instrument [Line Items]
Interest rate (percent)	6.00%	6.00%

Amount equal to the present value (the principal) at the beginning of the lease term of minimum lease payments during the lease term (excluding that portion of the payments representing executory costs such as insurance, maintenance, and taxes to be paid by the lessor, together with any profit thereon) net of payments or other amounts applied to the principal through the balance sheet date.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6455398&loc=d3e45280-112737

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
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-Number 210
-Section 03
-Paragraph 16
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Contractual interest rate for funds borrowed, under the debt agreement.

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The amount of debt discount that was originally recognized at the issuance of the instrument that has yet to be amortized.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Amount of long-term debt and capital lease obligation due after one year or beyond the normal operating cycle, if longer.

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

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Amount of long-term debt and capital leases due within one year or the normal operating cycle, if longer.

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Carrying value as of the balance sheet date of notes payable (with maturities initially due after one year or beyond the operating cycle if longer), excluding current portion.

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Including the current and noncurrent portions, carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders.

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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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