Investments (Related Party Investment Transactions) (Details) - Affiliated Entity - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Invested Assets Transferred To and From Affiliates
Estimated fair value of invested assets transferred to affiliates	$ 0	$ 292
Amortized cost of invested assets transferred to affiliates	0	294
Net investment gains (losses) recognized on transfers	$ 0	$ (2)

Reflects, by transferred financial asset type, the amount of sales proceeds in excess of or deficient from the sum of the carrying amounts of transferred financial assets plus transaction costs, including those transactions in which the seller/transferor has continuing involvement with the financial assets that have been transferred.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82924980&loc=d3e107207-111719

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 10
-Section 40
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=65892426&loc=d3e96813-111708

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration