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New Business (Narrative) (Details) - USD ($) $ in Thousands | May 16, 2018 | Sep. 30, 2018 | Aug. 06, 2018 | Dec. 31, 2017
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Investments Measured At NAV [Member] |  |  |  |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |  |  |  |
Other investments, at fair value | [1] |  | $ 3,217 |  | $ 1,143
U.S. Insurance Fund [Member] |  |  |  |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |  |  |  |
Required investment percentage of total equity |  | 4.50% |  |  |
Origination fee, percentage |  | 1.00% |  |  |
U.S. Insurance Fund [Member] | Maximum [Member] |  |  |  |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |  |  |  |
Other investments, at fair value |  | $ 3,000 |  |  |
U.S. Insurance Fund [Member] | Investor [Member] |  |  |  |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |  |  |  |
Other investments, at fair value |  | $ 63,000 |  |  |
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] | Investments Measured At NAV [Member] |  |  |  |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |  |  |  |
Other investments, at fair value |  |  | $ 15,331 |  |
[1] | As a practical expedient, the Company uses NAV or its equivalent to measure the fair value of its investments in EuroDekania, the U.S. Insurance JV, and the SPAC Fund. EuroDekania invests in hybrid capital securities of European companies. The U.S. Insurance JV invests in debt issued by small and medium sized U.S. and Bermuda insurance and reinsurance companies. The SPAC Fund invests in equity securities of SPACs. According to ASC 820, these investments are not categorized within the fair value hierarchy.

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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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