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Fair Value (Quantitative information about significant unobservable inputs in the fair value measurement of level 3 assets) (Details) - Fixed maturity securities - Income Approach Valuation Technique - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
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Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Estimated fair value | $ 136 | $ 106
Minimum |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Credit spread | 1.00% | 2.00%
Maximum |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Credit spread | 12.00% | 40.00%
Weighted Average |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Credit spread | 3.00% | 4.00%

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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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