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Fair Value Disclosures (Fair Value, Investments, Entities That Calculate Net Asset Value Per Share) (Details) - USD ($) $ in Thousands | Sep. 30, 2018 | Aug. 06, 2018 | Dec. 31, 2017
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Other Investment Vehicles [Member] |  |  |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |  |  |
Other investments, at fair value |  | $ 602 |  | $ 1,143
U.S. Insurance Fund [Member] |  |  |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |  |  |
Unfunded commitments | [1] | 2,000 |  |
U.S. Insurance Fund [Member] | Other Investment Vehicles [Member] |  |  |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |  |  |
Other investments, at fair value | [1] | 1,609 |  |
SPAC Fund [Member] |  |  |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |  |  |
Other investments, at fair value |  |  | $ 600 |
SPAC Fund [Member] | Other Investment Vehicles [Member] |  |  |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |  |  |
Other investments, at fair value | [2] | $ 1,006 |  |
[1] | The U.S. Insurance JV invests in debt issued by small and medium sized U.S. and Bermuda insurance and reinsurance companies.
[2] | The SPAC Fund invests in equity interests of SPACs.

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- Definition Fair value of investment other than investment in equity security, investment in debt security and equity method investment. Includes, but is not limited to, investment in certain entities that calculate net asset value per share. Example includes, but is not limited to, investment in hedge fund, venture capital fund, private equity fund, and real estate partnership or fund. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 6A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=99377789&loc=SL6742756-110258 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 35 -Paragraph 54B -URI http://asc.fasb.org/extlink&oid=99392525&loc=SL7495116-110257 + 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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- 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 6A -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=99377789&loc=SL6742756-110258 + Details
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