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Long-Term Debt (Details Textual) (USD $) In Millions, unless otherwise specified				3 Months Ended				3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Mar. 31, 2013 5.375% Notes [Member]	Oct. 03, 2012 5.375% Notes [Member]	Mar. 31, 2013 Letter of Credit [Member]	Dec. 31, 2012 Letter of Credit [Member]	Mar. 31, 2013 Revolving Credit Facility [Member]	Nov. 21, 2011 Revolving Credit Facility [Member]	Mar. 31, 2013 Maximum [Member] Revolving Credit Facility [Member]	Mar. 31, 2013 Minimum [Member] Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity									$ 350
Debt instrument maturity date				Oct. 15, 2020				Nov. 21, 2016
Federal fund rate Spread										1.25%	0.25%
Spread, Euro-rate										2.25%	1.25%
Outstanding Amount of Debt					250.0
Letter of credit outstanding						5.2	5.2
Percentage of aggregate principal amount of outstanding	5.375%	5.375%		5.375%	5.375%
Long-Term Debt (Textual) [Abstract]
Margin over federal fund rate	0.50%
Margin over Euro-rate	1.00%
Unamortized deferred debt issuance costs		$ 4.8	$ 4.7

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- Definition

Debt instrument basis spread on variable rate under condition two.

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- Definition

Margin over Euro-rate.

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No definition available.

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- Definition

Margin over federal fund rate.

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No definition available.

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- Definition

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

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No definition available.

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- Definition

The stated principal amount of the debt instrument at time of issuance, which may vary from the carrying amount because of unamortized premium or discount.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399



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-Name Accounting Standards Codification

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-Name Accounting Principles Board Opinion (APB)

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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- Definition

Interest rate stated in the contractual debt agreement.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5



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- Definition

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

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Amount borrowed under the credit facility as of the balance sheet date.

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-Number 210

-Section 02

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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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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 129

-Paragraph 2, 4

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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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