Debt instrument basis spread on reference rate.

No definition available.

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- Definition

Debt instrument, maturity extension fee percentage.

+ References

No definition available.

+ Details

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- Definition

Debt instrument maturity extension period.

+ References

No definition available.

+ Details

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- Definition

Debt instrument number of option available to extend maturity period.

+ References

No definition available.

+ Details

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- Definition

Line of credit facility maximum accordion feature.

+ References

No definition available.

+ Details

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Balance Type:	credit
Period Type:	instant

- Definition

Number of participating banks as co-documentation agents.

+ References

No definition available.

+ Details

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- Definition

Number of participating banks as co-syndication agents.

+ References

No definition available.

+ Details

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- Definition

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

+ References

No definition available.

+ Details

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Period Type:	instant

- Definition

Amount of amortization expense attributable to debt issuance costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28555-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
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-Section 45
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-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(8))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
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-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(8))
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

+ Details

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

- Definition

Contractual interest rate for funds borrowed, under the debt 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(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Balance Type:	na
Period Type:	instant

- Definition

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

+ 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(a))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Balance Type:	na
Period Type:	duration

- 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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Period Type:	duration

- 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/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Balance Type:	na
Period Type:	duration

- 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

No definition available.

+ Details

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Period Type:	duration

- Definition

Weighted average interest rate of debt outstanding.

+ References

No definition available.

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

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.

+ References

No definition available.

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- 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

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 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3255-108585

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Balance Type:	debit
Period Type:	duration

- Definition

The cash outflow for the settlement of obligation drawn from a contractual arrangement with the lender, including letter of credit, standby letter of credit and revolving credit arrangements, under which borrowings can be made up to a specific amount at any point in time with maturities due beyond one year or the operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3291-108585

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Period Type:	duration

- Definition

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

+ References

+ Details

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Period Type:	duration

- 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

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 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Unsecured Credit Agreements - Additional Information (Detail)			3 Months Ended		6 Months Ended
	Jun. 23, 2017 USD ($) Bank Option	Apr. 18, 2017 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2016
Debt Instrument [Line Items]
Repayment of line of credit					$ 187,000,000	$ 36,000,000
Weighted average interest rate on all outstanding borrowings			2.86%		2.86%		2.14%
Amortization of debt issuance costs charged to interest expense					$ 787,000	728,000
Interest Expense
Debt Instrument [Line Items]
Amortization of debt issuance costs charged to interest expense			$ 433,000	$ 444,000	857,000	$ 876,000
Other Income (Expenses) - Cost of Debt Extinguishment
Debt Instrument [Line Items]
Amortization of debt issuance costs charged to interest expense					2,901,000
Unamortized debt issuance costs			654,000		654,000
Other Income (Expenses) - Cost of Debt Extinguishment \| Senior Notes
Debt Instrument [Line Items]
Amortization of debt issuance costs charged to interest expense			$ 8,344,000		$ 8,344,000
Minimum
Debt Instrument [Line Items]
Debt instrument, term					59 months
Maximum
Debt Instrument [Line Items]
Debt instrument, term					239 months
Revolver \| Baa3 Rating
Debt Instrument [Line Items]
Basis spread on variable rate	1.20%
5.5 Year Term Loan
Debt Instrument [Line Items]
Commitment fee percentage	0.25%
5.5 Year Term Loan \| Baa3 Rating
Debt Instrument [Line Items]
Basis spread on variable rate	1.35%
7 Year Term Loan
Debt Instrument [Line Items]
Commitment fee percentage	0.25%
7 Year Term Loan \| Baa3 Rating
Debt Instrument [Line Items]
Basis spread on variable rate	1.90%
Unsecured Revolving Credit Facility \| M&T and Regions Bank
Debt Instrument [Line Items]
Repayment of line of credit	$ 5,000,000
Unsecured Term Note \| Regions Bank
Debt Instrument [Line Items]
Repayment of unsecured long-term debt	185,000,000
Unsecured Term Note \| Sun Trust Bank
Debt Instrument [Line Items]
Repayment of unsecured long-term debt	50,000,000
Unsecured Term Note \| M&T and Regions Bank
Debt Instrument [Line Items]
Repayment of unsecured long-term debt	100,000,000
2017 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 accordion feature	$ 1,000,000,000
2017 Unsecured Revolving Credit and Term Loan Agreement \| Revolver
Debt Instrument [Line Items]
Line of credit, maximum borrowing capacity	$ 400,000,000
Maturity date	Jan. 31, 2022
Number of option available to extend maturity period \| Option	1
Maturity extension period	5 months
Maturity extension fee percentage	0.0625%
Commitment fee percentage	0.25%
2017 Unsecured Revolving Credit and Term Loan Agreement \| Revolver \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	0.825%
2017 Unsecured Revolving Credit and Term Loan Agreement \| Revolver \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	1.55%
2017 Unsecured Revolving Credit and Term Loan Agreement \| 5.5 Year Term Loan
Debt Instrument [Line Items]
Debt instrument, face amount	$ 250,000,000
Debt instrument, term	5 years 6 months
Maturity date	Jan. 31, 2023
2017 Unsecured Revolving Credit and Term Loan Agreement \| 5.5 Year Term Loan \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	0.90%
2017 Unsecured Revolving Credit and Term Loan Agreement \| 5.5 Year Term Loan \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	1.75%
2017 Unsecured Revolving Credit and Term Loan Agreement \| 7 Year Term Loan
Debt Instrument [Line Items]
Debt instrument, face amount	$ 150,000,000
Debt instrument, term	7 years
Maturity date	Jun. 30, 2024
2017 Unsecured Revolving Credit and Term Loan Agreement \| 7 Year Term Loan \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	1.50%
2017 Unsecured Revolving Credit and Term Loan Agreement \| 7 Year Term Loan \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	2.45%
2015 Unsecured Term Loan Agreement
Debt Instrument [Line Items]
Maturity date	Feb. 28, 2019
Number of option available to extend maturity period \| Option	2
Maturity extension period	1 year
Maturity extension fee percentage	0.01%
2015 Unsecured Term Loan Agreement \| Baa3 Rating
Debt Instrument [Line Items]
Basis spread on variable rate	1.40%
2015 Unsecured Term Loan Agreement \| 1 Month LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	0.90%
2015 Unsecured Term Loan Agreement \| 1 Month LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	1.75%
2017 Senior Notes
Debt Instrument [Line Items]
Debt instrument, term					10 years
Maturity date					Apr. 18, 2027
Proceeds from issuance of debt		$ 150,000,000
Debt instrument, interest rate		4.84%
Issuance date		Apr. 18, 2017
2017 Senior Notes \| Other Income (Expenses) - Cost of Debt Extinguishment
Debt Instrument [Line Items]
Amortization of debt issuance costs charged to interest expense					$ 5,443,000
2017 Senior Notes \| 10-Year U.S. Treasury Yield
Debt Instrument [Line Items]
Debt instrument basis spread on reference rate		2.40%