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Long-Term Debt (Summary of Long-Term Debt) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Sep. 13, 2019	Apr. 25, 2017
Debt Instrument [Line Items]
Capitalized lease obligations, vehicles, due through 2024	$ 2.1	$ 2.4
Unamortized debt issuance costs	(4.4)	(5.5)
Total long-term debt	1,002.4	1,038.1
Less current maturities	51.2	38.8
Total long-term debt, net of current	951.2	999.3
Senior Notes | 5.625% senior notes due 2027 (net of unamortized discount of $2.4 at 2020 and $2.7 at 2019)
Debt Instrument [Line Items]
Long-term debt	297.6	297.3
Interest rate (percent)				5.625%
Unamortized discount	2.4	2.7
Senior Notes | 4.75% senior notes due 2029 (net of unamortized discount of $5.4 at 2020 and $6.1 at 2019)
Debt Instrument [Line Items]
Long-term debt	494.6	493.9
Interest rate (percent)			4.75%
Unamortized discount	5.4	6.1
Secured Debt | Term loan due 2023 (effective interest rate of 2.67% at 2020 and 4.31% at 2019)
Debt Instrument [Line Items]
Long-term debt	$ 212.5	$ 250.0
Effective interest rate	2.67%	4.31%

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Amount of short-term and long-term debt and lease obligation.

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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.

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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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Present value of lessee's discounted obligation for lease payments from finance lease.

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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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