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Fair Value of Financial Instruments (Details) - USD ($) $ in Millions | Sep. 30, 2016 | Aug. 18, 2014 | Dec. 21, 2012 | Oct. 17, 2012
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Debt Instrument [Line Items] |  |  |  |
Fair value of notes | $ 984.0 |  |  |
October Notes |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Unamortized discount | 2.1 |  |  |
Net unamortized debt issuance costs | 4.4 |  |  |
December Notes |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Unamortized premium | 0.3 |  |  |
Net unamortized debt issuance costs | 1.0 |  |  |
August Notes |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Unamortized premium | 12.6 |  |  |
Net unamortized debt issuance costs | 4.3 |  |  |
April Notes |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Net unamortized debt issuance costs | 6.2 |  |  |
Senior Notes |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Carrying value of debt | $ 944.9 |  |  |
Unamortized discount |  |  |  | $ 3.7
Unamortized premium |  | $ 18.0 | $ 0.5 |

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- Definition Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=49121117&loc=d3e13433-108611 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=49121117&loc=d3e13476-108611 + Details
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- Definition The amount of debt discount that was originally recognized at the issuance of the instrument that has yet to be amortized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28541-108399 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=34725769&loc=d3e28878-108400 + Details
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- Definition The amount of debt premium that was originally recognized at the issuance of the instrument that has yet to be amortized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28541-108399 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=34725769&loc=d3e28878-108400 + 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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