Long-Term Debt - Additional Information (Detail) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Document Period End Date	Sep. 30, 2019
Long-term debt including current maturities	$ 452,100	$ 327,200
Debt Instrument, Unused Borrowing Capacity, Amount	250,000
Uncollateralized Senior Note Due December Two Thousand Thirty Four [Member]
Debt Instrument [Line Items]
Long-term debt including current maturities	$ 70,000
Debt Instrument, Interest Rate, Stated Percentage	2.98%
Term Note Due January Two Thousand Twenty [Member]
Debt Instrument [Line Items]
Long-term debt including current maturities	$ 30,000	$ 30,000
Debt Instrument, Interest Rate, Stated Percentage	2.80%	3.23%
Term Note Due February Two Thousand Twenty [Member]
Debt Instrument [Line Items]
Long-term debt including current maturities	$ 30,000
Debt Instrument, Interest Rate, Stated Percentage	2.84%
3.48% note, due May 31, 2038 [Member]
Debt Instrument [Line Items]
Long-term debt including current maturities	$ 50,000	$ 50,000
3.25% note, due April 30, 2032 [Member]
Debt Instrument [Line Items]
Long-term debt including current maturities	$ 70,000	$ 70,000
MetLife [Member] \| Notes Payable, Other Payables [Member] \| Shelf Notes [Member]
Debt Instrument [Line Items]
Maturity date term	20 years
New York Life [Member] \| Notes Payable, Other Payables [Member] \| Shelf Notes [Member]
Debt Instrument [Line Items]
Maturity date term	20 years
Prudential [Member] \| Notes Payable, Other Payables [Member] \| Shelf Notes [Member]
Debt Instrument [Line Items]
Senior notes	$ 150,000
Maturity date term	20 years
Debt Instrument, Interest Rate, Stated Percentage	3.25%
Prudential [Member] \| Shelf Notes [Member]
Debt Instrument [Line Items]
Senior notes	$ 150,000
New York Life [Member]
Debt Instrument [Line Items]
Senior notes	100,000
Debt Instrument, Unused Borrowing Capacity, Amount	50,000
Electric Limited Proceedings [Member]
Debt Instrument [Line Items]
Combined Dollar Amount of Term Notes to Finance Hurricane Michael Costs	$ 60,000

Combined Dollar Amount of Term Notes to Finance Hurricane Michael Costs

No definition available.

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- Definition

The end date of the period reflected on the cover page if a periodic report. For all other reports and registration statements containing historical data, it is the date up through which that historical data is presented. If there is no historical data in the report, use the filing date. The format of the date is CCYY-MM-DD.

+ References

No definition available.

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- Definition

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

+ References

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- Definition

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

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- Definition

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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- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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- Definition

Amount of unused borrowing capacity under the long-term financing arrangement that is available to the entity as of the balance sheet date.

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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.

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