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Derivatives (Forward Contracts) (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Derivative [Line Items] |  |  |
Notional amount | $ 206.3 |  |
Fair value investments | 3,208.3 | $ 2,547.3 | $ 1,543.5
Forward Contracts |  |  |
Derivative [Line Items] |  |  |
Realized and unrealized gains (losses) | (23.8) | (1.2) |
Other Investments |  |  |
Derivative [Line Items] |  |  |
Net amount recognized in other log term investments | (3.7) | (1.2) |
GBP |  |  |
Derivative [Line Items] |  |  |
Notional amount | 152.0 |  |
Barclays Bank PLC | A | Forward Contracts |  |  |
Derivative [Line Items] |  |  |
Notional amount | 206.3 |  |
Fair value investments | $ (3.7) | (1.2) |
Barclays Bank PLC | A- | Forward Contracts |  |  |
Derivative [Line Items] |  |  |
Notional amount |  | 184.6 |
Carrying value |  | $ (1.2) |

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- Definition Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226008-175313 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13495-108611 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13433-108611 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 + Details
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