Certain Long-Duration Contracts with Guarantees (Narrative) (Details) - USD ($) $ in Billions	Dec. 31, 2017	Dec. 31, 2016
Annuity Contracts \| Market Value Adjusted
Net Amount at Risk by Product and Guarantee [Line Items]
General Account Investment Option	$ 3.1	$ 2.9

Value of the general accounts corresponding to the specified product, guarantee type and subset of the guarantee.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 80
-Section 50
-Paragraph 1
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=6484115&loc=d3e19393-158473

Name:	us-gaap_NetAmountAtRiskByProductAndGuaranteeGeneralAccountValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant