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Long-Term Debt (Details Textual) (USD $) In Millions, unless otherwise specified |  |  |  | 3 Months Ended |  |  |  | 3 Months Ended |  |
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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. + References No definition available. + Details
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- Details
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- Definition Margin over Euro-rate. + References No definition available. + Details
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- Definition Margin over federal fund rate. + References No definition available. + Details
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- Definition The 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 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 Reference 2: 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=6584090&loc=d3e28878-108400 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Principles Board Opinion (APB) -Number 21 -Paragraph 16, 20 -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. + Details
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- Definition Interest rate stated in the contractual debt agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5 Reference 2: 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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- 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 SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5 Reference 2: 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 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 129 -Paragraph 2, 4 -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. + Details
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- Definition Amount borrowed under the credit facility as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5 + 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 Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 129 -Paragraph 2, 4 -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. Reference 2: 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 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5 + 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 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 + Details
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