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Investments - Net Investment Income (Parenthetical) (Details) - Deposit asset underlying 10% coinsurance agreement [Member] - USD ($) | 3 Months Ended | 6 Months Ended
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Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017
Schedule of Net Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Losses recognized for change in fair value of deposit asset underlying the 10% coinsurance agreement | $ 400,000 | $ 0 | $ 1,700,000 | $ 0
Percentage of coinsurance costs | 10.00% | 10.00% | 10.00% | 10.00%

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- Definition Percentage of coinsurance covered costs. + References No definition available. + Details
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- Definition Amount of realized and unrealized gain (loss) included in earnings, which have arisen from the use of significant unobservable inputs (level 3) to measure fair value of assets, liabilities, and financial instruments classified in shareholders' equity. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258 + 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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- Details
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