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Debt Summary of Obligations (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Debt Instrument [Line Items] |  |
Long-term debt, gross | $ 836.9 | $ 1,189.5
Deferred issuance costs | (6.5) | (10.8)
Total long-term debt | 830.4 | 1,178.7
Interest payments made | 57.8 | 49.5
Federal Home Loan Bank Advances (FHLB) |  |
Debt Instrument [Line Items] |  |
Long-term debt, gross | 155.0 | 155.0
Interest payments made | 3.0 | 2.4
5% Convertible Senior Notes due 2017 |  |
Debt Instrument [Line Items] |  |
Long-term debt, gross | $ 0.0 | 145.0
Stated interest rate (in hundredths) | 5.00% |
Interest payments made | $ 3.6 | 10.6
2% Convertible Senior Notes due 2020 |  |
Debt Instrument [Line Items] |  |
Long-term debt, gross | $ 0.0 | 207.6
Stated interest rate (in hundredths) | 2.00% |
Interest payments made | $ 2.1 | 9.1
Senior Notes | 5.75% Senior Notes due 2023 |  |
Debt Instrument [Line Items] |  |
Long-term debt, gross | $ 425.0 | 425.0
Stated interest rate (in hundredths) | 5.75% |
Interest payments made | $ 25.1 | 0.0
Convertible Junior Subordinated Debentures, at 9% per annum, Due 2063 |  |
Debt Instrument [Line Items] |  |
Long-term debt, gross | $ 256.9 | 256.9
Stated interest rate (in hundredths) | 9.00% |
Interest payments made | $ 23.1 | 27.4
Line of Credit |  |
Debt Instrument [Line Items] |  |
Interest payments made | $ 0.9 | $ 0.0

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- Definition Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt. + 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)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: 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 3: 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 + Details
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