Long-Term Debt - Outstanding Debt (Detail) - USD ($)	6 Months Ended
Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Principal	$ 800,000,000		$ 765,000,000
Debt Issuance Costs	(5,327,000)		(6,089,000)
Carrying Amount	$ 794,673,000		758,911,000
Amended Credit Facility
Debt Instrument [Line Items]
Maturity Date	Sep. 14, 2023
Principal	$ 175,000,000		140,000,000
Debt Issuance Costs	0		0
Carrying Amount	175,000,000		140,000,000
Line of credit facility, maximum borrowing capacity	$ 300,000,000.0	$ 500,000,000.0
7.0% Senior Notes
Debt Instrument [Line Items]
Maturity Date	Oct. 15, 2022
Principal	$ 350,000,000		350,000,000
Debt Issuance Costs	(1,954,000)		(2,372,000)
Carrying Amount	$ 348,046,000		347,628,000
Debt, stated interest rate	7.00%
8.75% Senior Notes
Debt Instrument [Line Items]
Maturity Date	Jun. 15, 2025
Principal	$ 275,000,000		275,000,000
Debt Issuance Costs	(3,373,000)		(3,717,000)
Carrying Amount	$ 271,627,000		$ 271,283,000
Debt, stated interest rate	8.75%

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.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
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-Section 50
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- Definition

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

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-Publisher FASB
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- Definition

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

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-Publisher FASB
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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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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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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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