Long-Term Debt - Additional Information (Detail) - USD ($)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Unamortized debt issue costs	$ 3,716,000	$ 4,472,000
Senior Notes \| 6% senior notes due 2022
Debt Instrument [Line Items]
Debt maturity period	2022
Interest rate (as a percent)	6.00%
Debt redemption period start date	Nov. 15, 2022
Senior Notes \| 6% senior notes due 2022 \| Minimum
Debt Instrument [Line Items]
Number of days prior notice required for redemption of notes	30 days
Senior Notes \| 6% senior notes due 2022 \| Maximum
Debt Instrument [Line Items]
Number of days prior notice required for redemption of notes	60 days
Line of Credit \| Credit facility
Debt Instrument [Line Items]
Senior secured revolving line of credit	$ 550,000,000
Line of credit, maturity date	Jun. 26, 2020
Senior secured revolving line of credit	$ 100,000,000
Leverage ratio	350.00%
Borrowings outstanding	$ 100,000,000
Borrowing limit used	1,000,000
Unamortized debt issue costs	$ 3,100,000	$ 4,300,000
Line of Credit \| Credit facility \| Minimum
Debt Instrument [Line Items]
Commitment fee (as a percent)	0.25%
Fronting fees (as a percent)	1.375%
Line of Credit \| Credit facility \| Minimum \| London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread (as a percent)	1.375%
Line of Credit \| Credit facility \| Minimum \| Base Rate
Debt Instrument [Line Items]
Basis spread (as a percent)	0.375%
Line of Credit \| Credit facility \| Maximum
Debt Instrument [Line Items]
Commitment fee (as a percent)	0.35%
Fronting fees (as a percent)	2.00%
Line of Credit \| Credit facility \| Maximum \| London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread (as a percent)	2.00%
Line of Credit \| Credit facility \| Maximum \| Base Rate
Debt Instrument [Line Items]
Basis spread (as a percent)	1.00%

Debt instrument maturity year.

No definition available.

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- Definition

Line of credit facility expected increase amount.

+ References

No definition available.

+ Details

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- Definition

Line Of Credit Facility Fronting Fee Percentage

+ References

No definition available.

+ Details

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- Definition

Line Of Credit Facility Maximum Leverage Percentage

+ References

No definition available.

+ Details

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- Definition

Period of prior notice required for redemption of notes.

+ References

No definition available.

+ Details

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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.

+ Details

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- Definition

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

+ References

+ Details

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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

No definition available.

+ Details

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- Definition

Start date of debt instrument redemption period, in CCYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph e
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

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- Definition

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

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions 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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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- Definition

Date the credit facility terminates, in CCYY-MM-DD format.

+ References

+ Details

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- Definition

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.

+ References

+ Details

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Period Type:	instant

- Definition

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

+ References

No definition available.

+ Details

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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

+ Details

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