Investments (Concentrations of Credit Risk - Narrative) (Details) - USD ($) $ in Billions	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Securities holdings exposure in single issuer greater than stated percentage of Company's equity	10.00%	10.00%
Foreign government \| Japan
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Government and agency fixed maturity securities	$ 28.6	$ 27.5
Foreign government \| KOREA, REPUBLIC OF
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Government and agency fixed maturity securities	$ 6.7	$ 6.5

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
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-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592

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