Total Debt - Schedule of Total Debt Outstanding (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Outstanding Principal	$ 1,492.0	$ 896.0
Issuance Costs and Fees	18.1	11.7
Long-Term Debt, Net	1,473.9	884.3
Current maturities of long-term debt	4.0	4.0
Total principal amount of debt, net of current maturities	1,488.0	892.0
Total debt, net of current maturities	1,469.9	880.3
2027 Senior Notes \| Senior Notes
Debt Instrument [Line Items]
Outstanding Principal	600.0
Issuance Costs and Fees	8.0
Long-Term Debt, Net	592.0
2028 Senior Notes \| Senior Notes
Debt Instrument [Line Items]
Outstanding Principal	500.0	500.0
Issuance Costs and Fees	6.1	7.0
Long-Term Debt, Net	493.9	493.0
Term Loan B due 2024 \| 2017 Credit Agreement \| Line of Credit
Debt Instrument [Line Items]
Outstanding Principal	392.0	396.0
Issuance Costs and Fees	4.0	4.7
Long-Term Debt, Net	$ 388.0	$ 391.3

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.

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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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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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