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Debt - Impact of Accounting Changes (Details) - USD ($) $ in Millions | Dec. 31, 2016 | Dec. 31, 2015
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New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |
Total long-term debt | $ 1,178.7 | $ 1,211.8
As previously reported |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |
Total long-term debt |  | 1,223.0
Accounting Standards Update 2015-03 | Adjustment |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |
Total long-term debt |  | (11.2)
5% Convertible Senior Notes due 2017 |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |
Total long-term debt |  | 331.5
5% Convertible Senior Notes due 2017 | As previously reported |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |
Total long-term debt |  | 333.5
5% Convertible Senior Notes due 2017 | Accounting Standards Update 2015-03 | Adjustment |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |
Total long-term debt |  | (2.0)
2% Convertible Senior Notes due 2020 |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |
Total long-term debt |  | 490.8
2% Convertible Senior Notes due 2020 | As previously reported |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |
Total long-term debt |  | 500.0
2% Convertible Senior Notes due 2020 | Accounting Standards Update 2015-03 | Adjustment |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |
Total long-term debt |  | (9.2)
Convertible Junior Subordinated Debentures Due 2063 |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |
Total long-term debt |  | 389.5
Convertible Junior Subordinated Debentures Due 2063 | As previously reported |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |
Total long-term debt |  | 389.5
Convertible Junior Subordinated Debentures Due 2063 | Accounting Standards Update 2015-03 | Adjustment |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |
Total long-term debt |  | $ 0.0

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- Definition Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 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) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 20, 22 -Article 5 + Details
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