Debt instrument basis spread on reference rate.

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Debt instrument, maturity extension fee percentage.

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Debt instrument maturity extension period.

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Debt instrument number of option available to extend maturity period.

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- Definition

Line of credit facility additional borrowing capacity.

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Line of credit facility, amount available for issuance of letters of credit.

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Line of credit facility maximum accordion feature.

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Line of credit facility, sublimit for swingline loans.

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Number of participating banks as co-documentation agents.

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Number of participating banks as co-syndication agents.

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Number of participating banks as joint lead arrangers and joint bookrunners.

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One time non pro rata pay down amount

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One time pro rata pay down amount..

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Unamortized debt issuance costs.

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Amount of amortization expense attributable to debt issuance costs.

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

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

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

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Date the debt instrument was issued, in CCYY-MM-DD format.

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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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Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

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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.

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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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Weighted average interest rate of debt outstanding.

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Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

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The fee, expressed as a percentage of the line of credit facility, for the line of credit facility regardless of whether the facility has been used.

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Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.

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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 cash outflow for cost from early extinguishment and prepayment of debt. Includes, but is not limited to, third-party cost, premium paid, and other fee paid to lender directly for debt extinguishment or debt prepayment. Excludes accrued interest.

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The cash outflow paid to third parties in connection with debt origination, which will be amortized over the remaining maturity period of the associated long-term debt.

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The cash inflow during the period from additional borrowings in aggregate debt. Includes proceeds from short-term and long-term debt.

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The cash inflow from the issuance of long-term debt that is not secured by collateral. Excludes proceeds from tax exempt unsecured debt.

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The cash outflow during the period from the repayment of aggregate short-term and long-term debt. Excludes payment of capital lease obligations.

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The cash outflow to repay long-term debt that is not secured by collateral. Excludes repayments of tax exempt unsecured debt.

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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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Unsecured Credit Agreements - Additional Information (Detail)										6 Months Ended		12 Months Ended
	Aug. 28, 2019 USD ($)	Aug. 02, 2019 USD ($)	Jul. 01, 2019	Feb. 27, 2019 USD ($)	Sep. 27, 2018 USD ($)	Sep. 13, 2018 USD ($)	Jul. 02, 2018 USD ($)	Jun. 23, 2017 USD ($) Bank Option	Apr. 18, 2017 USD ($)	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Feb. 28, 2019 USD ($)	Jan. 31, 2019 Option	Nov. 20, 2017 USD ($)
Debt Instrument [Line Items]
Additional borrowing capacity																	$ 80,000,000
Revolving credit facility, available borrowing capacity																	880,000,000
Debt instrument, term												119 months
Weighted average interest rate on all outstanding borrowings										3.68%		3.68%	4.23%
Cost of debt extinguishment												$ (1,176,000)	$ (101,000)	$ (5,151,000)
Deferred debt issuance costs, charged to interest expense												2,542,000	$ 1,776,000	1,893,000
Debt Issuance Costs
Debt Instrument [Line Items]
Cost of debt extinguishment												6,000
Other Nonoperating Income (Expense)
Debt Instrument [Line Items]
Cost of debt extinguishment												$ 328,000
Unamortized debt issuance costs														$ 654,000
Revolving Credit Facility
Debt Instrument [Line Items]
Debt instrument, face amount																	425,000,000
2023 Unsecured Term Loan
Debt Instrument [Line Items]
Debt instrument, face amount																	265,000,000
Commitment fee percentage								0.25%
2024 Unsecured Term Loan
Debt Instrument [Line Items]
Debt instrument, face amount																	$ 190,000,000
Commitment fee percentage								0.25%
Senior Guaranteed Notes Series B and Senior Guaranteed Notes Series C \| Unsecured Debt
Debt Instrument [Line Items]
Debt instrument, face amount							$ 325,000,000
Senior Guaranteed Notes Series B \| Unsecured Debt
Debt Instrument [Line Items]
Maturity date							Jul. 02, 2028					Jul. 31, 2028
Debt instrument, face amount							$ 225,000,000
Proceeds from issuance of debt						$ 125,000,000	$ 100,000,000
Debt instrument, interest rate							5.09%			5.09%		5.09%
Senior Guaranteed Notes Series C \| Unsecured Debt
Debt Instrument [Line Items]
Maturity date							Jul. 02, 2030					Jul. 31, 2030
Debt instrument, face amount							$ 100,000,000
Proceeds from issuance of debt						$ 50,000,000	$ 50,000,000
Debt instrument, interest rate							5.19%			5.19%		5.19%
Baa3 Rating \| Revolving Credit Facility
Debt Instrument [Line Items]
Basis spread on variable rate								1.20%
Commitment fee percentage								0.25%
Baa3 Rating \| 2023 Unsecured Term Loan
Debt Instrument [Line Items]
Basis spread on variable rate								1.35%
Baa3 Rating \| 2024 Unsecured Term Loan
Debt Instrument [Line Items]
Basis spread on variable rate			1.25%
2019 Unsecured Term Loan Agreement
Debt Instrument [Line Items]
One time non pro rata paydown amount								$ 50,000,000
One Time Pro Rata Pay Down Amount					$ 25,000,000
Repayment of borrowing				$ 300,000,000
2019 Unsecured Term Loan Agreement \| Unsecured Debt
Debt Instrument [Line Items]
Maturity date												Feb. 29, 2020
Repayments on unsecured term loan							$ 25,000,000
2019 Unsecured Term Loan Agreement \| 1 Month LIBOR
Debt Instrument [Line Items]
Number of option available to extend maturity period \| Option																2
2019 Unsecured Term Loan Agreement \| 1 Month LIBOR \| Unsecured Debt
Debt Instrument [Line Items]
Basis spread on variable rate												1.40%
Two Thousand Nineteen Unsecured Term Loan Agreement
Debt Instrument [Line Items]
Maturity date								Feb. 28, 2019
Two Thousand Nineteen Unsecured Term Loan Agreement \| Baa3 Rating
Debt Instrument [Line Items]
Basis spread on variable rate								1.40%
Two Thousand Nineteen Unsecured Term Loan Agreement \| 1 Month LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate								0.90%
Two Thousand Nineteen Unsecured Term Loan Agreement \| 1 Month LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate								1.75%
Unsecured Revolving Credit and Term Loan Agreement
Debt Instrument [Line Items]
Debt instrument, face amount								$ 800,000,000
Number of participating banks as joint lead arrangers and joint bookrunners \| Bank								4
Number of participating banks as co-syndication agents \| Bank								4
Number of participating banks as co-documentation agents \| Bank								4
Line of credit, maximum borrowing capacity															$ 1,055,000,000
Line of credit, maximum accordion feature								$ 1,000,000,000
Revolving credit facility, available borrowing capacity				$ 425,000,000											$ 600,000,000
Unsecured Revolving Credit and Term Loan Agreement \| Revolving Credit Facility
Debt Instrument [Line Items]
Maturity date								Jan. 21, 2022
Line of credit, maximum borrowing capacity								$ 400,000,000
Sublimit for swingline loans								35,000,000
Amount available for issuance of letters of credit								$ 20,000,000
Number of option available to extend maturity period \| Option								1
Maturity extension period								5 months
Maturity extension fee percentage								0.0625%
Unsecured Revolving Credit and Term Loan Agreement \| Revolving Credit Facility \| Unsecured Debt
Debt Instrument [Line Items]
Maturity date												Jan. 31, 2022
Unsecured Revolving Credit and Term Loan Agreement \| 5.5 Year Term Loan
Debt Instrument [Line Items]
Debt instrument, face amount								$ 250,000,000
Unsecured Revolving Credit and Term Loan Agreement \| 7 Year Term Loan
Debt Instrument [Line Items]
Debt instrument, face amount								$ 150,000,000
Unsecured Revolving Credit and Term Loan Agreement \| 2023 Unsecured Term Loan
Debt Instrument [Line Items]
Maturity date								Jan. 31, 2023
Unsecured Revolving Credit and Term Loan Agreement \| 2023 Unsecured Term Loan \| Unsecured Debt
Debt Instrument [Line Items]
Maturity date												Jan. 31, 2023
Unsecured Revolving Credit and Term Loan Agreement \| 2024 Unsecured Term Loan
Debt Instrument [Line Items]
Basis spread on variable rate													1.90%
Unsecured Revolving Credit and Term Loan Agreement \| 2024 Unsecured Term Loan \| Unsecured Debt
Debt Instrument [Line Items]
Maturity date												Jun. 30, 2024
Unsecured Revolving Credit and Term Loan Agreement \| Minimum \| Revolving Credit Facility
Debt Instrument [Line Items]
Commitment fee percentage								0.13%
Unsecured Revolving Credit and Term Loan Agreement \| Minimum \| 2024 Unsecured Term Loan
Debt Instrument [Line Items]
Basis spread on variable rate										0.85%	1.50%		1.90%
Unsecured Revolving Credit and Term Loan Agreement \| Maximum \| Revolving Credit Facility
Debt Instrument [Line Items]
Commitment fee percentage								0.30%
Unsecured Revolving Credit and Term Loan Agreement \| Maximum \| 2024 Unsecured Term Loan
Debt Instrument [Line Items]
Basis spread on variable rate										1.65%	2.45%
Unsecured Revolving Credit and Term Loan Agreement \| 1 Month LIBOR \| Revolving Credit Facility \| Unsecured Debt
Debt Instrument [Line Items]
Basis spread on variable rate												1.20%
Unsecured Revolving Credit and Term Loan Agreement \| 1 Month LIBOR \| 2023 Unsecured Term Loan \| Unsecured Debt
Debt Instrument [Line Items]
Basis spread on variable rate												1.35%
Unsecured Revolving Credit and Term Loan Agreement \| 1 Month LIBOR \| 2024 Unsecured Term Loan \| Unsecured Debt
Debt Instrument [Line Items]
Basis spread on variable rate												1.25%
Unsecured Revolving Credit and Term Loan Agreement \| LIBOR \| Minimum \| Revolving Credit Facility
Debt Instrument [Line Items]
Basis spread on variable rate								0.825%
Unsecured Revolving Credit and Term Loan Agreement \| LIBOR \| Minimum \| 2023 Unsecured Term Loan
Debt Instrument [Line Items]
Basis spread on variable rate								0.90%
Unsecured Revolving Credit and Term Loan Agreement \| LIBOR \| Minimum \| 2024 Unsecured Term Loan
Debt Instrument [Line Items]
Basis spread on variable rate			0.85%										1.50%
Unsecured Revolving Credit and Term Loan Agreement \| LIBOR \| Maximum \| Revolving Credit Facility
Debt Instrument [Line Items]
Basis spread on variable rate								1.55%
Unsecured Revolving Credit and Term Loan Agreement \| LIBOR \| Maximum \| 2023 Unsecured Term Loan
Debt Instrument [Line Items]
Basis spread on variable rate								1.75%
Unsecured Revolving Credit and Term Loan Agreement \| LIBOR \| Maximum \| 2024 Unsecured Term Loan
Debt Instrument [Line Items]
Basis spread on variable rate			1.65%										2.45%
2026 Unsecured Term Loan
Debt Instrument [Line Items]
Maturity date				Feb. 27, 2026
Debt instrument, face amount				$ 450,000,000
Line of credit, maximum accordion feature				$ 550,000,000
Commitment fee percentage				0.25%
Debt instrument, term				7 years
Remaining borrowing capacity				$ 150,000,000
2026 Unsecured Term Loan \| Unsecured Debt
Debt Instrument [Line Items]
Maturity date												Feb. 28, 2026
2026 Unsecured Term Loan \| 1 Month LIBOR \| Unsecured Debt
Debt Instrument [Line Items]
Basis spread on variable rate												1.85%
2026 Unsecured Term Loan \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate				1.85%
2026 Unsecured Term Loan \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate				1.45%
2026 Unsecured Term Loan \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate				2.40%
2020 Unsecured Term Loan
Debt Instrument [Line Items]
Maturity date		Aug. 02, 2020
Debt instrument, face amount		$ 300,000,000
Commitment fee percentage		0.25%
Proceeds from issuance of debt	$ 300,000,000
2020 Unsecured Term Loan \| Unsecured Debt
Debt Instrument [Line Items]
Maturity date												Aug. 31, 2020
Basis spread on variable rate												1.25%
2020 Unsecured Term Loan \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate		1.25%
2020 Unsecured Term Loan \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate		0.85%
2020 Unsecured Term Loan \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate		1.65%
2017 Senior Notes
Debt Instrument [Line Items]
Maturity date														Apr. 18, 2017
Debt instrument, term														10 years
Proceeds from issuance of debt									$ 150,000,000
Debt instrument, interest rate									4.84%
Issuance date														Apr. 18, 2027
2017 Senior Notes \| 10-Year U.S. Treasury Yield
Debt Instrument [Line Items]
Debt instrument basis spread on reference rate									2.40%
Two Thousand Twenty Six Unsecured Term Loan And Amended Unsecured Revolving Credit And Term Loan Agreement And Two Thousand Twenty Unsecured Term Loan
Debt Instrument [Line Items]
Debt issuance costs incurred												$ 6,549,000
Senior Guaranteed Notes Series B and Senior Guaranteed Notes Series C
Debt Instrument [Line Items]
Debt issuance costs incurred													$ 2,209,000
Senior Guaranteed Notes Series A
Debt Instrument [Line Items]
Debt issuance costs incurred														$ 8,711,000
New Debt Agreements [Member]
Debt Instrument [Line Items]
Unamortized debt issuance costs												$ 6,543,000
Deferred debt issuance costs, charged to interest expense														5,810,000
Restated Term Loan Agreement
Debt Instrument [Line Items]
Cost of debt extinguishment														$ 2,901,000