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Investment Securities (Fair Value of Hedge Funds and Private Equity Funds) (Details) - USD ($) $ in Millions | Sep. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Private equity funds |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Other long-term investments | $ 81.1 |
Hedge and private equity funds included in other long-term investments |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Other long-term investments | 154.9 | $ 82.6
Unfunded Commitments | 105.2 | 123.6
Hedge funds | Hedge Fund, Long/ Short Equity Banks and Financial |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Other long-term investments | 54.7 | 21.5
Unfunded Commitments | 0.0 | 0.0
Hedge funds | Hedge Funds Equity R E I T |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Other long-term investments | 19.1 | 19.9
Unfunded Commitments | 0.0 | 0.0
Hedge funds | Hedge funds |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Other long-term investments | 73.8 | 41.4
Unfunded Commitments | 0.0 | 0.0
Private equity funds | Industrial |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Other long-term investments | 42.7 | 19.4
Unfunded Commitments | 12.1 | 22.9
Private equity funds | Aerospace Defense Government |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Other long-term investments | 28.7 | 19.4
Unfunded Commitments | 15.6 | 25.9
Private equity funds | Direct Lending |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Other long-term investments | 6.7 | 1.4
Unfunded Commitments | 23.3 | 28.6
Private equity funds | Financial Services Sector |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Other long-term investments | 3.0 | 1.0
Unfunded Commitments | 13.0 | 5.0
Private equity funds | Private Equity Insurance |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Other long-term investments | 0.0 | 0.0
Unfunded Commitments | 41.2 | 41.2
Private equity funds | Private equity funds |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Other long-term investments | 81.1 | 41.2
Unfunded Commitments | $ 105.2 | $ 123.6

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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=66048111&loc=SL6742756-110258 + Details
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