Long-Term Debt - Schedule of Long-Term Debt (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Gross amount	$ 758,000	$ 785,000
Less unamortized debt discount	353	360
Less unamortized debt issuance costs	1,683	1,779
Total	755,964	782,861
Credit facility
Debt Instrument [Line Items]
Gross amount	228,000	255,000
Notes Payable \| 4.625% Notes, due September 1, 2021
Debt Instrument [Line Items]
Gross amount	$ 400,000	$ 400,000
Stated interest rate	4.625%	4.625%
Medium-term Notes \| 7.32% Medium-term Notes, Series A, due July 28, 2022
Debt Instrument [Line Items]
Gross amount	$ 20,000	$ 20,000
Stated interest rate	7.32%	7.32%
Medium-term Notes \| 7.35% Medium-term Notes, Series A, due July 28, 2027
Debt Instrument [Line Items]
Gross amount	$ 10,000	$ 10,000
Stated interest rate	7.35%	7.35%
Medium-term Notes \| 7.125% Medium-term Notes, Series B, due February 15, 2028
Debt Instrument [Line Items]
Gross amount	$ 100,000	$ 100,000
Stated interest rate	7.125%	7.125%

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
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28541-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

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Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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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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