Long-term Debt (Details) $ in Millions	3 Months Ended
Mar. 31, 2020 USD ($) Rate	Dec. 31, 2019 USD ($)
Debt Instrument [Line Items]
Senior Notes \| $	$ 550.0	$ 550.0
Long-term Debt, Gross \| $	1,073.8	1,040.8
Unamortized Debt Issuance Expense \| $	8.3	8.5
Long-term Debt \| $	$ 1,065.5	1,032.3
Leverage Ratio	1.14
Letters of Credit Outstanding, Amount \| $	$ 3.9
Maximum [Member]
Debt Instrument [Line Items]
Leverage Ratio in Latest Year of Debt Term	3.75%
Leverage Ratio	4.25%
$250 Million Revolving Credit Facility, Due November 2024 [Member]
Debt Instrument [Line Items]
Secured Debt \| $	$ 33.0	0.0
Debt Instrument, Interest Rate, Increase (Decrease)	0.25%
Line of Credit Facility, Maximum Borrowing Capacity \| $	$ 250.0
$822 Million Term B Loan Facility, due April 2025 [Member] [Domain]
Debt Instrument [Line Items]
Secured Debt \| $	490.8	$ 490.8
Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount \| $	$ 550.0
Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.625%
Notes Payable to Banks [Member] \| $250 Million Revolving Credit Facility, Due November 2024 [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.25%
Notes Payable to Banks [Member] \| $250 Million Revolving Credit Facility, Due November 2024 [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.25%
Notes Payable to Banks [Member] \| $822 Million Term B Loan Facility, due April 2025 [Member] [Domain] \| Minimum [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.20%
Notes Payable to Banks [Member] \| $822 Million Term B Loan Facility, due April 2025 [Member] [Domain] \| Maximum [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.75%
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.35%
Notes Payable to Banks [Member] \| Long-term Debt [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Interest Rate During Period	2.78%
Notes Payable to Banks [Member] \| Eurodollar [Member] \| $250 Million Revolving Credit Facility, Due November 2024 [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.25%
Notes Payable to Banks [Member] \| Eurodollar [Member] \| $250 Million Revolving Credit Facility, Due November 2024 [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	2.25%
Notes Payable to Banks [Member] \| Eurodollar [Member] \| $822 Million Term B Loan Facility, due April 2025 [Member] [Domain] \| Maximum [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.75%
$250 Million Revolving Credit Facility, Due November 2024 [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity \| $	$ 213.1

The leverage ratio as defined in the credit agreement.

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Leverage ratio limit in latest year of debt facility, as described in debt agreement.

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Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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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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Face (par) amount of debt instrument at time of issuance.

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Incremental percentage increase (decrease) in the stated rate on a debt instrument.

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

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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.

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The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

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The effective interest rate during the reporting period.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

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The fee, expressed as a percentage of the line of credit facility, for available but unused credit capacity under the credit facility.

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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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Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

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Including the current and noncurrent portions, carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders.

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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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