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Investment Securities (Cost and Amortized Cost Maturity Schedule) (Details) - USD ($) $ in Millions | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
--- | --- | --- | ---
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |
Fixed maturity investments, at fair value | $ 4,250.2 | $ 2,630.2 |
Trading Securities by Maturity Cost or Amortized Cost |  |  |
Cost or amortized cost | 4,266.0 |  |
Trading Securities by Maturity, Fair Value |  |  |
Trading Securities | 4,256.8 |  |
Fixed Maturities and Convertible Fixed Maturities Excluding A B S M B S and Preferred Stock |  |  |
Trading Securities by Maturity Cost or Amortized Cost |  |  |
Due in one year or less | 212.8 |  |
Due after one year through five years | 1,272.6 |  |
Due after five years through ten years | 462.8 |  |
Due after ten years | 167.8 |  |
Trading Securities by Maturity, Fair Value |  |  |
Due in one year or less | 213.3 |  |
Due after one year through five years | 1,270.2 |  |
Due after five years through ten years | 458.4 |  |
Due after ten years | 168.0 |  |
Mortgage and asset-backed securities |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |
Fixed maturity investments, at fair value | 2,132.9 | 1,167.0 |
Trading Securities by Maturity Cost or Amortized Cost |  |  |
Trading Securities, Maturities without Single Maturity Date, Amortized Cost | 2,141.7 |  |
Trading Securities by Maturity, Fair Value |  |  |
Trading Securities, Maturities without Single Maturity Date, Carrying Value | 2,132.9 |  |
Preferred stocks |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |
Fixed maturity investments, at fair value | 14.0 | 82.7 |
Trading Securities by Maturity Cost or Amortized Cost |  |  |
Trading Securities, Maturities without Single Maturity Date, Amortized Cost | 8.3 |  |
Trading Securities by Maturity, Fair Value |  |  |
Trading Securities, Maturities without Single Maturity Date, Carrying Value | 14.0 |  |
Fixed maturity investments | Star & Shield Insurance Exchange [Member] |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |
Fixed maturity investments, at fair value | $ 6.6 | $ 9.5 | $ 10.1

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- Definition Amount of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading. + References Reference 1: 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 2: 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 3: 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 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Trading Securities -URI http://asc.fasb.org/extlink&oid=6526789 + Details
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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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