Long-term Debt (Details) $ in Millions	3 Months Ended
Mar. 31, 2019 USD ($) Rate	Dec. 31, 2018 USD ($)
Debt Instrument [Line Items]
Unamortized deferred loan costs \| $	$ 22.3	$ 23.6
Long-term Debt \| $	1,107.7	$ 1,100.4
Contractual Quarterly Payments \| $	$ 2.1
Leverage Ratio	2.65
Maximum [Member]
Debt Instrument [Line Items]
Leverage Ratio in Latest Year of Debt Term	3.75%
Leverage Ratio	4.75%
Notes Payable to Banks [Member] \| Minimum [Member] \| $200 Million Revolving Credit Facility, Due March 2023 [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.25%
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.20%
Notes Payable to Banks [Member] \| Minimum [Member] \| Eurodollar [Member] \| $200 Million Revolving Credit Facility, Due March 2023 [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.25%
Notes Payable to Banks [Member] \| Maximum [Member] \| $200 Million Revolving Credit Facility, Due March 2023 [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.25%
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.35%
Notes Payable to Banks [Member] \| Maximum [Member] \| $825 Million Term B Loan Facility, due June 2022 [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Interest Rate During Period	4.50%
Notes Payable to Banks [Member] \| Maximum [Member] \| Eurodollar [Member] \| $200 Million Revolving Credit Facility, Due March 2023 [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	2.25%
Notes Payable to Banks [Member] \| Maximum [Member] \| Eurodollar [Member] \| $825 Million Term B Loan Facility, due June 2022 [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	2.00%
$200 Million Revolving Credit Facility, Due March 2023 [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity \| $	$ 190.1

Contractual Quarterly Payments

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- Definition

The leverage ratio as defined in the credit agreement.

+ References

No definition available.

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- Definition

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

+ References

No definition available.

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- Definition

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

+ References

No definition available.

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+ References

No definition available.

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- Definition

The effective interest rate during the reporting period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
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- Definition

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

+ References

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- Definition

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

+ References

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
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- Definition

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.

+ References

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