Debt - Long-term Debt (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Long-term debt, gross	$ 254.6	$ 255.7
Less: unamortized debt issuance costs	0.5	0.5
Total debt	254.1	255.2
Less: current portion of long-term debt	44.2	2.3
Long-term debt, net	$ 209.9	252.9
Term Loan, due December 31, 2024 (1.01%)
Debt Instrument [Line Items]
Debt instrument, stated interest rate	1.01%
Long-term debt, gross	$ 86.6	87.7
Long-term debt, net	$ 84.4
Series A notes, due July 5, 2022 (3.67%)
Debt Instrument [Line Items]
Debt instrument, stated interest rate	3.67%
Long-term debt, gross	$ 42.0	42.0
Series B notes, due July 5, 2024 (3.82%)
Debt Instrument [Line Items]
Debt instrument, stated interest rate	3.82%
Long-term debt, gross	$ 53.0	53.0
Series C notes, due July 5, 2027 (4.02%)
Debt Instrument [Line Items]
Debt instrument, stated interest rate	4.02%
Long-term debt, gross	$ 73.0	$ 73.0

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.

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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 current. 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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