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Investment Securities (Debt securities issued by corporation) (Details) - USD ($) $ in Millions | Mar. 31, 2016 | Dec. 31, 2015 | Mar. 31, 2015 | Dec. 31, 2014
--- | --- | --- | --- | ---
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | $ 3,360.2 | $ 4,050.9 | $ 3,355.3 | $ 3,343.6
Debt securities issued by corporations |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 910.4 | 1,000.0 |  |
AAA | Debt securities issued by corporations |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 9.4 | 0.0 |  |
AA | Debt securities issued by corporations |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 107.4 | 95.2 |  |
A | Debt securities issued by corporations |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 364.2 | 397.7 |  |
BBB | Debt securities issued by corporations |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 429.4 | 507.1 |  |
Other | Debt securities issued by corporations |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | $ 0.0 | $ 0.0 |  |

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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 -Glossary Trading Securities -URI http://asc.fasb.org/extlink&oid=6526789 Reference 2: 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=27723805&loc=d3e24584-111560 Reference 3: 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=51813488&loc=d3e22054-111558 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Available-for-Sale Securities -URI http://asc.fasb.org/extlink&oid=6505594 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Subparagraph a, f, g -Article 7 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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