Long-term Debt (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Long-term debt, gross	$ 196.1	$ 196.7
Long-term debt	195.5	196.1
Unamortized debt issuance costs	0.6	0.6
Current portion of long-term debt	0.0	0.1
Long-term debt, excluding current portion	195.5	196.0
Notes payable, Due 2019 [Member]
Debt Instrument [Line Items]
Long-term debt, gross	0.0	$ 0.1
Revolving credit facility, Due 2020 [Member]
Debt Instrument [Line Items]
Debt instrument, stated interest rate		1.00%
Long-term debt, gross	$ 0.0	$ 28.6
Revolving credit facility, Due 2024 [Member]
Debt Instrument [Line Items]
Debt instrument, stated interest rate	0.875%
Long-term debt, gross	$ 28.1
Series A Notes, Due 2022 [Member]
Debt Instrument [Line Items]
Debt instrument, stated interest rate	3.67%	3.67%
Long-term debt, gross	$ 42.0	$ 42.0
Series B Notes, Due 2024 [Member]
Debt Instrument [Line Items]
Debt instrument, stated interest rate	3.82%	3.82%
Long-term debt, gross	$ 53.0	$ 53.0
Series C Notes, Due 2027 [Member]
Debt Instrument [Line Items]
Debt instrument, stated interest rate	4.02%	4.02%
Long-term debt, gross	$ 73.0	$ 73.0

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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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.

+ References

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

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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 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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