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Debt (Narrative) (Details) - USD ($) $ in Millions | 1 Months Ended
--- | ---
Sep. 30, 2015 | Aug. 31, 2015 | Jun. 30, 2015
Debt [Line Items] |  |  |
Commercial paper classified as long-term debt |  | $ 111.0 |
Senior Debentures [Member] | 3.3% senior debenture, due 2015 [Member] |  |  |
Debt [Line Items] |  |  |
Payments to retire long-term debt |  |  | $ 150.0
Interest rate |  |  | 3.30%
Senior Debentures [Member] | 3.4% senior debenture, due 2025 [Member] |  |  |
Debt [Line Items] |  |  |
Interest rate |  | 3.40% |
Proceeds from issuance of long-term debt |  | $ 250.0 |
Pollution Control Revenue Bonds [Member] | 5.375% pollution control revenue bonds, due 2015 [Member] |  |  |
Debt [Line Items] |  |  |
Payments to retire long-term debt |  | $ 14.6 |
Interest rate |  | 5.375% |
Pollution Control Revenue Bonds [Member] | 5% pollution control revenue bonds, due 2015 [Member] |  |  |
Debt [Line Items] |  |  |
Payments to retire long-term debt | $ 16.0 |  |
Interest rate | 5.00% |  |

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- 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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5 + Details
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