Debt (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Long-term debt	$ 197.0	$ 228.6
Unamortized debt issuance expense	0.7	0.9
Current portion of long-term debt	0.0	2.4
Long-term debt, excluding current portion	$ 197.0	226.2
Senior A Notes Due 2022 [Member]
Debt Instrument [Line Items]
Debt instrument, stated interest rate	3.67%
Senior B Notes Due 2024 [Member]
Debt Instrument [Line Items]
Debt instrument, stated interest rate	3.82%
Senior C Notes Due 2027 [Member]
Debt Instrument [Line Items]
Debt instrument, stated interest rate	4.02%
Term Loan Due 2018 [Member]
Debt Instrument [Line Items]
Long-term debt		34.9
Debt instrument, stated interest rate	0.00%
Note Payable Due 2019 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 0.1	0.2
Revolving credit facility, due 2020 [Member]
Debt Instrument [Line Items]
Line of credit, noncurrent	$ 29.6	26.4
Debt instrument, stated interest rate	1.00%
Senior Notes [Member] \| Senior A Notes Due 2022 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 42.0	42.0
Debt instrument, stated interest rate	3.67%
Senior Notes [Member] \| Senior B Notes Due 2024 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 53.0	53.0
Debt instrument, stated interest rate	3.82%
Senior Notes [Member] \| Senior C Notes Due 2027 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 73.0	$ 73.0
Debt instrument, stated interest rate	4.02%

Amount of short-term debt and current maturity of long-term debt and capital lease obligations due within one year or the normal operating cycle, if longer.

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- Definition

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

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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- Definition

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.

+ References

Reference 1: 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(a)(16))
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-Name Accounting Standards Codification
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-Name Accounting Standards Codification
-Topic 210
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-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

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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- Definition

The carrying value as of the balance sheet date of the noncurrent portion 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.

+ References

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

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 10
-Section 45
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=99376301&loc=d3e1314-112600

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- Definition

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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