Debt (Detail) - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Debt Instrument, Repayment Terms, Percentage Of Principal	0.25%
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate	3.30%
Line of Credit, Current	$ 4.4
Finance Lease, Liability	21.6	$ 22.0
Long-term Debt, Gross	835.9	910.5
Unamortized Debt Issuance Expense	(6.4)	(8.1)
Potential additional lines of credit	$ 100.0
Lien leverage ratio	3.59
Line of Credit Facility, Remaining Borrowing Capacity	$ 220.6
Long-term Debt and Lease Obligation, Including Current Maturities	829.5	902.4
Long-term Debt and Lease Obligation, Current	(1.6)	(17.9)
Long-term Debt and Lease Obligation	834.2	892.6
Long-term Debt, Excluding Current Maturities	$ 827.9	884.5
Fixed Charge Coverage Ratio	1.10
Minimum
Debt Instrument [Line Items]
Debt Instrument, Repayment Terms, Asset Sales, Percentage Of Principal	0.00%
Maximum
Debt Instrument [Line Items]
Debt Instrument, Repayment Terms, Asset Sales, Percentage Of Principal	50.00%
Lien leverage ratio, maximum	4.25
LIBOR
Debt Instrument [Line Items]
Debt, weighted average interest rate	3.00%
LIBOR \| Minimum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	3.00%
LIBOR \| Maximum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	3.25%
Base Rate \| Minimum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	2.00%
Base Rate \| Maximum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	2.25%
Net of Letters of Credit [Member]
Debt Instrument [Line Items]
Line of Credit, Current	$ 0.0	13.5
Senior Note Due 2023 [Member]
Debt Instrument [Line Items]
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate	4.50%
Long-term Debt, Gross	$ 275.0	275.0
Long-term Debt	273.8	273.5
Unamortized Debt Issuance Expense	$ (1.2)	(1.5)
Term Loan [Member]
Debt Instrument [Line Items]
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate	4.30%
Long-term Debt, Gross	$ 239.3	300.0
Long-term Debt	235.4	294.9
Unamortized Debt Issuance Expense	$ (3.8)	(5.1)
Senior Note Due 2025 [Member]
Debt Instrument [Line Items]
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate	5.40%
Long-term Debt, Gross	$ 300.0	300.0
Long-term Debt	298.7	298.5
Unamortized Debt Issuance Expense	(1.3)	$ (1.5)
Line of Credit [Member]
Debt Instrument [Line Items]
Outstanding stand by letters of credit	$ 4.4
Pro Forma [Member]
Debt Instrument [Line Items]
Lien leverage ratio, maximum	2.00
Lien leverage ratio	0.97

Debt Instrument, Repayment Terms, Asset Sales, Percentage Of Principal

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Debt Instrument, Repayment Terms, Percentage Of Principal

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- Definition

Fixed Charge Coverage Ratio

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Lien leverage ratio

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Lien leverage ratio, maximum

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Potential additional lines of credit

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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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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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Weighted average interest rate of debt outstanding.

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- Definition

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

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- Definition

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

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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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- Definition

The carrying value as of the balance sheet date of the current portion of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the 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 of long-term debt and lease obligation, classified as noncurrent.

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Amount of long-term debt and lease obligation, classified as current.

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Amount of long-term debt and lease obligation, including portion classified as current.

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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 interest rate applicable to the portion of the carrying amount of long-term borrowings outstanding as of the balance sheet date, including current maturities, which accrues interest at a rate subject to change from time to time.

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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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