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Investment Securities (Other Long-Term Investments) (Details) - USD ($) $ in Millions | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | ---
Investment securities |  |
Cost | $ 5,308.8 |
Other Investments | 323.3 | $ 315.8
Other long-term investments |  |
Investment securities |  |
Other Investments | 2.0 | 3.2
Limited liability companies and private equity securities [Member] |  |
Investment securities |  |
Other Investments | 72.0 | 82.1
Surplus Note |  |
Investment securities |  |
Other Investments | 71.9 | 51.5
Convertible Preferred Stock |  |
Investment securities |  |
Other Investments | 30.6 | 32.7
Affordable Housing Development Fund [Member] |  |
Investment securities |  |
Other Investments | 12.3 | 14.7
Partnership investments [Member] |  |
Investment securities |  |
Other Investments | 3.5 | 3.8
Total hedge and private equity funds included in other long-term investments |  |
Investment securities |  |
Other Investments | $ 131.0 | $ 127.8

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- Definition Amount of investments classified as other. + 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 210.5-02.12) -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 210.7-03.1(f,g)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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- Definition This item represents the original cost of the entity's investments including investments in equity securities and, as to fixed maturities, original cost reduced by repayments and adjusted for amortization of premiums or accrual of discounts. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -Section S99 -Paragraph 7 -Subparagraph (SX 210.12-15) -URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611379-123010 + Details
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