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Investment Securities (Fair Value Measurement by Level) (Details) $ in Millions | Dec. 31, 2017 USD ($) Investment | Dec. 31, 2016 USD ($) Investment | Dec. 31, 2015 USD ($)
--- | --- | --- | ---
Investment securities |  |  |
Fair value investments | $ 3,208.3 | $ 2,547.3 | $ 1,543.5
Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 880.9 | 746.3 |
Mortgage and asset-backed securities |  |  |
Investment securities |  |  |
Fair value investments | $ 694.7 | 979.8 |
Common Stock |  |  |
Investment securities |  |  |
Number of investments | Investment | 2 |  |
Hedge and private equity funds included in other long-term investments |  |  |
Investment securities |  |  |
Carrying value | $ 125.3 | 82.6 |
Level 1 |  |  |
Investment securities |  |  |
Fair value investments | 890.4 | 279.5 | 789.0
Level 2 |  |  |
Investment securities |  |  |
Fair value investments | 2,105.4 | 2,093.8 | 585.6
Level 2 | Mortgage and asset-backed securities |  |  |
Investment securities |  |  |
Fair value investments | 694.7 | 979.8 |
Level 3 Inputs | Mortgage and asset-backed securities |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Level 3 Inputs | Fixed maturity investments |  |  |
Investment securities |  |  |
Fair value investments | $ 0.0 | $ 0.0 | 0.0
Number of investments | Investment | 3 | 3 |
Level 3 Inputs | Other long-term investments |  |  |
Investment securities |  |  |
Fair value investments | $ 87.2 | $ 91.4 | 103.6
Level 3 Inputs | Hedge and private equity funds included in other long-term investments |  |  |
Investment securities |  |  |
Fair value investments | 125.3 | 82.6 | 65.3
Fair value measured on a recurring basis |  |  |
Investment securities |  |  |
Fair value investments | 3,259.1 | 2,639.7 |
Fair value measured on a recurring basis | U.S. Government and agency obligations |  |  |
Investment securities |  |  |
Fair value investments | 296.5 | 111.0 |
Fair value measured on a recurring basis | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 880.9 | 746.3 |
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer |  |  |
Investment securities |  |  |
Fair value investments | 185.1 | 190.8 |
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications |  |  |
Investment securities |  |  |
Fair value investments | 127.8 | 72.0 |
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials |  |  |
Investment securities |  |  |
Fair value investments | 114.8 | 79.7 |
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities |  |  |
Investment securities |  |  |
Fair value investments | 108.9 | 140.8 |
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials |  |  |
Investment securities |  |  |
Fair value investments | 95.5 | 65.0 |
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care |  |  |
Investment securities |  |  |
Fair value investments | 94.3 | 114.9 |
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology |  |  |
Investment securities |  |  |
Fair value investments | 80.5 | 48.8 |
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy |  |  |
Investment securities |  |  |
Fair value investments | 48.1 | 6.1 |
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial |  |  |
Investment securities |  |  |
Fair value investments | 25.9 | 28.2 |
Fair value measured on a recurring basis | Mortgage and asset-backed securities |  |  |
Investment securities |  |  |
Fair value investments | 694.7 | 979.8 |
Fair value measured on a recurring basis | Municipal obligations |  |  |
Investment securities |  |  |
Fair value investments | 254.9 | 238.5 |
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations |  |  |
Investment securities |  |  |
Fair value investments | 2.7 | 12.1 |
Fair value measured on a recurring basis | Fixed maturity investments |  |  |
Investment securities |  |  |
Fair value investments | 2,129.7 | 2,087.7 |
Fair value measured on a recurring basis | Short-term investments |  |  |
Investment securities |  |  |
Fair value investments | 176.1 | 175.0 | $ 142.0
Fair value measured on a recurring basis | Common equity securities | Consumer |  |  |
Investment securities |  |  |
Fair value investments | 10.7 | 8.6 |
Fair value measured on a recurring basis | Common equity securities | Communications |  |  |
Investment securities |  |  |
Fair value investments | 10.9 | 7.0 |
Fair value measured on a recurring basis | Common equity securities | Financials |  |  |
Investment securities |  |  |
Fair value investments | 16.3 | 7.7 |
Fair value measured on a recurring basis | Common equity securities | Health Care |  |  |
Investment securities |  |  |
Fair value investments | 17.1 | 13.9 |
Fair value measured on a recurring basis | Common equity securities | Technology |  |  |
Investment securities |  |  |
Fair value investments | 15.1 | 7.3 |
Fair value measured on a recurring basis | Common equity securities | Energy |  |  |
Investment securities |  |  |
Fair value investments | 3.8 | 2.5 |
Fair value measured on a recurring basis | Common equity securities | Industrial |  |  |
Investment securities |  |  |
Fair value investments | 11.9 | 1.5 |
Fair value measured on a recurring basis | Common equity securities | Other |  |  |
Investment securities |  |  |
Fair value investments | 210.6 | 79.9 |
Fair value measured on a recurring basis | Common equity securities | Exchange Traded Funds |  |  |
Investment securities |  |  |
Fair value investments | 569.7 | 157.2 |
Fair value measured on a recurring basis | Common equity securities | Common equity securities |  |  |
Investment securities |  |  |
Fair value investments | 866.1 | 285.6 |
Fair value measured on a recurring basis | Other long-term investments |  |  |
Investment securities |  |  |
Fair value investments | 87.2 | 91.4 |
Fair value measured on a recurring basis | Level 1 |  |  |
Investment securities |  |  |
Fair value investments | 1,041.4 | 441.7 |
Fair value measured on a recurring basis | Level 1 | U.S. Government and agency obligations |  |  |
Investment securities |  |  |
Fair value investments | 296.5 | 101.5 |
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Consumer |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Communications |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Financials |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Utilities |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Materials |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Health Care |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Technology |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Energy |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Industrial |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 1 | Mortgage and asset-backed securities |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 1 | Municipal obligations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 1 | Foreign government, agency and provincial obligations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 1 | Fixed maturity investments |  |  |
Investment securities |  |  |
Fair value investments | 296.5 | 101.5 |
Fair value measured on a recurring basis | Level 1 | Short-term investments |  |  |
Investment securities |  |  |
Fair value investments | 151.0 | 162.3 |
Fair value measured on a recurring basis | Level 1 | Common equity securities | Consumer |  |  |
Investment securities |  |  |
Fair value investments | 10.7 | 8.6 |
Fair value measured on a recurring basis | Level 1 | Common equity securities | Communications |  |  |
Investment securities |  |  |
Fair value investments | 10.9 | 7.0 |
Fair value measured on a recurring basis | Level 1 | Common equity securities | Financials |  |  |
Investment securities |  |  |
Fair value investments | 16.3 | 7.7 |
Fair value measured on a recurring basis | Level 1 | Common equity securities | Health Care |  |  |
Investment securities |  |  |
Fair value investments | 17.1 | 13.9 |
Fair value measured on a recurring basis | Level 1 | Common equity securities | Technology |  |  |
Investment securities |  |  |
Fair value investments | 15.1 | 7.3 |
Fair value measured on a recurring basis | Level 1 | Common equity securities | Energy |  |  |
Investment securities |  |  |
Fair value investments | 3.8 | 2.5 |
Fair value measured on a recurring basis | Level 1 | Common equity securities | Industrial |  |  |
Investment securities |  |  |
Fair value investments | 11.9 | 1.5 |
Fair value measured on a recurring basis | Level 1 | Common equity securities | Other |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 1 | Common equity securities | Exchange Traded Funds |  |  |
Investment securities |  |  |
Fair value investments | 508.1 | 129.4 |
Fair value measured on a recurring basis | Level 1 | Common equity securities | Common equity securities |  |  |
Investment securities |  |  |
Fair value investments | 593.9 | 177.9 |
Fair value measured on a recurring basis | Level 1 | Other long-term investments |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 2 |  |  |
Investment securities |  |  |
Fair value investments | 2,130.5 | 2,106.6 |
Fair value measured on a recurring basis | Level 2 | U.S. Government and agency obligations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 9.5 |
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 880.9 | 746.3 |
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Consumer |  |  |
Investment securities |  |  |
Fair value investments | 185.1 | 190.8 |
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Communications |  |  |
Investment securities |  |  |
Fair value investments | 127.8 | 72.0 |
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Financials |  |  |
Investment securities |  |  |
Fair value investments | 114.8 | 79.7 |
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Utilities |  |  |
Investment securities |  |  |
Fair value investments | 108.9 | 140.8 |
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Materials |  |  |
Investment securities |  |  |
Fair value investments | 95.5 | 65.0 |
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Health Care |  |  |
Investment securities |  |  |
Fair value investments | 94.3 | 114.9 |
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Technology |  |  |
Investment securities |  |  |
Fair value investments | 80.5 | 48.8 |
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Energy |  |  |
Investment securities |  |  |
Fair value investments | 48.1 | 6.1 |
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Industrial |  |  |
Investment securities |  |  |
Fair value investments | 25.9 | 28.2 |
Fair value measured on a recurring basis | Level 2 | Mortgage and asset-backed securities |  |  |
Investment securities |  |  |
Fair value investments | 694.7 | 979.8 |
Fair value measured on a recurring basis | Level 2 | Municipal obligations |  |  |
Investment securities |  |  |
Fair value investments | 254.9 | 238.5 |
Fair value measured on a recurring basis | Level 2 | Foreign government, agency and provincial obligations |  |  |
Investment securities |  |  |
Fair value investments | 2.7 | 12.1 |
Fair value measured on a recurring basis | Level 2 | Fixed maturity investments |  |  |
Investment securities |  |  |
Fair value investments | 1,833.2 | 1,986.2 |
Fair value measured on a recurring basis | Level 2 | Short-term investments |  |  |
Investment securities |  |  |
Fair value investments | 25.1 | 12.7 |
Fair value measured on a recurring basis | Level 2 | Common equity securities | Consumer |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 2 | Common equity securities | Communications |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 2 | Common equity securities | Financials |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 2 | Common equity securities | Health Care |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 2 | Common equity securities | Technology |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 2 | Common equity securities | Energy |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 2 | Common equity securities | Industrial |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 2 | Common equity securities | Other |  |  |
Investment securities |  |  |
Fair value investments | 210.6 | 79.9 |
Fair value measured on a recurring basis | Level 2 | Common equity securities | Exchange Traded Funds |  |  |
Investment securities |  |  |
Fair value investments | 61.6 | 27.8 |
Fair value measured on a recurring basis | Level 2 | Common equity securities | Common equity securities |  |  |
Investment securities |  |  |
Fair value investments | 272.2 | 107.7 |
Fair value measured on a recurring basis | Level 2 | Other long-term investments |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 Inputs |  |  |
Investment securities |  |  |
Fair value investments | 87.2 | 91.4 |
Fair value measured on a recurring basis | Level 3 Inputs | Foreign Exchange Forward [Member] |  |  |
Investment securities |  |  |
Fair value investments | (3.7) | (1.2) |
Fair value measured on a recurring basis | Level 3 Inputs | U.S. Government and agency obligations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations | Consumer |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations | Communications |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations | Financials |  |  |
Investment securities |  |  |
Fair value investments |  | 0.0 |
Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations | Utilities |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations | Materials |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations | Health Care |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations | Technology |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations | Energy |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations | Industrial |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 Inputs | Mortgage and asset-backed securities |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 Inputs | Municipal obligations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 Inputs | Foreign government, agency and provincial obligations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 Inputs | Fixed maturity investments |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 Inputs | Short-term investments |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities | Consumer |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities | Communications |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities | Financials |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities | Health Care |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities | Technology |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities | Energy |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities | Industrial |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities | Other |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities | Exchange Traded Funds |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities | Common equity securities |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 Inputs | Other long-term investments |  |  |
Investment securities |  |  |
Fair value investments | 87.2 | 91.4 |
Forward Contracts | Fair value measured on a recurring basis | Level 3 Inputs | Other long-term investments |  |  |
Investment securities |  |  |
Carrying value | $ (3.7) | (1.2) |
Star & Shield Insurance Exchange | Fixed maturity investments |  |  |
Investment securities |  |  |
Fair value investments |  | 6.6 |
Star & Shield Insurance Exchange | Short-term investments |  |  |
Investment securities |  |  |
Fair value investments |  | $ 0.1 |

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- Definition Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method. + 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 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(5)) -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 35 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82895926&loc=d3e24584-111560 Reference 4: 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(6)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 5: 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(2)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558 Reference 7: 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 + Details
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