Total Debt - Schedule of Total Debt Outstanding (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Outstanding Principal	$ 896.0	$ 1,142.0
Issuance Costs and Fees	11.7	12.8
Long-Term Debt, Net	884.3	1,129.2
Current maturities of long-term debt	4.0	4.0
Total principal amount of debt, net of current maturities	892.0	1,138.0
Total debt, net of current maturities	880.3	1,125.2
2017 Credit Agreement \| Line of Credit
Debt Instrument [Line Items]
Outstanding Principal	396.0	642.0
Issuance Costs and Fees	4.7	5.1
Long-Term Debt, Net	391.3	636.9
2028 Senior Notes \| Senior Notes
Debt Instrument [Line Items]
Outstanding Principal	500.0	500.0
Issuance Costs and Fees	7.0	7.7
Long-Term Debt, Net	493.0	492.3
Term Loan B \| 2017 Credit Agreement \| Line of Credit
Debt Instrument [Line Items]
Outstanding Principal	396.0	400.0
Issuance Costs and Fees	4.7	5.1
Long-Term Debt, Net	391.3	394.9
Revolver \| 2017 Credit Agreement \| Line of Credit
Debt Instrument [Line Items]
Outstanding Principal	0.0	239.0
Issuance Costs and Fees	0.0	0.0
Long-Term Debt, Net	0.0	239.0
Swing line of credit \| 2017 Credit Agreement \| Line of Credit
Debt Instrument [Line Items]
Outstanding Principal	0.0	3.0
Issuance Costs and Fees	0.0	0.0
Long-Term Debt, Net	$ 0.0	$ 3.0

Long-term Debt, Excluding Current Maturities, Gross

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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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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.

+ References

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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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