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Investment Securities (Cost or Amortized Cost, Gross Unrealized Investment Gains and Losses) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Investment securities |  |  |
Fixed maturity investments, at fair value | $ 4,250.2 | $ 2,630.2 |
Sales and maturities of investment securities | 5,385.3 | 2,162.8 | $ 3,337.5
Common Stock |  |  |
Investment securities |  |  |
Trading Securities, Cost | 440.8 | 822.5 |
Net foreign currency gains | 0.0 | 0.0 |
Fixed maturity investments, at fair value | 474.3 | 1,113.9 |
Trading Securities, Unrealized Holding Gain | 35.9 | 302.8 |
Trading Securities, Unrealized Holding Loss | (2.4) | (11.4) |
Other long-term investments |  |  |
Investment securities |  |  |
Trading Securities, Cost | 314.9 | 304.5 |
Net foreign currency gains | (3.9) | (2.3) |
Fixed maturity investments, at fair value | 323.3 | 315.8 |
Trading Securities, Unrealized Holding Gain | 40.3 | 32.0 |
Trading Securities, Unrealized Holding Loss | $ (28.0) | $ (18.4) |

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- Definition Amount of investments in debt securities classified as trading. + 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 210.9-03.4) -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 320 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26626-111562 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Trading Securities -URI http://asc.fasb.org/extlink&oid=6526789 Reference 4: 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.(a),1) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26610-111562 + Details
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