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Investment Securities (Other long-term investments) (Details) - USD ($) $ in Millions | 9 Months Ended |  |
--- | --- | --- | ---
Sep. 30, 2016 | Sep. 30, 2015 | Dec. 31, 2015 | Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Payments to Acquire Other Investments | $ 37.3 | $ 30.5 |  |
Other Investments | 351.4 |  | $ 315.8 |
Hedge and private equity funds included in other long-term investments |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Other Investments | 143.0 |  | 127.8 |
Limited liability companies and private equity securities [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Other Investments | 84.6 |  | 82.1 |
Surplus Note |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Other Investments | 71.5 |  | 51.5 |
Private equity funds |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Other Investments | 33.1 |  | 32.7 |
Affordable Housing Development Fund [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Other Investments | 12.8 | $ 15.2 | 14.7 | $ 16.8
Partnership investments [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Other Investments | 3.2 |  | 3.8 |
Other |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Other Investments | 3.2 |  | 3.2 |
Hedge Funds Equity R E I T [Member] | Hedge funds |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Other long-term investments | 20.4 |  | 20.6 |
Unfunded Commitments | $ 0.0 |  | $ 0.0 |

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- Definition The amount of unfunded commitments for which the entity is obligated, to those certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 6A -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=36462937&loc=SL6742756-110258 + Details
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- Definition Other investments not otherwise specified in the taxonomy. + 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 The cash outflow associated with other investments held by the entity for investment purposes not otherwise defined in the taxonomy. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Investing Activities -URI http://asc.fasb.org/extlink&oid=6516133 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3213-108585 + Details
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