Contracts Accounted for as Credit Derivatives - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Credit Derivatives
Estimated remaining weighted average life of credit derivatives (in years)	12 years	11 years 8 months 4 days
Net par outstanding	$ 5,615	$ 6,207

Average period covered by credit risk derivative contracts, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4K
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5708775-113959

Name:	us-gaap_AverageTermOfCreditRiskDerivatives1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration