Derivative Instruments (Collateral Support Agreements) (Details) - Open Credit Default Swap Liabilities [Member] - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Credit risk related contingent features collateral
Maximum potential payout		$ 52
Less: Counterparty thresholds
Maximum collateral potentially required to post		$ 52

Maximum potential amount of future undiscounted payments that could be required under the credit derivative, before reduction for potential recoveries under recourse or collateralization provisions.

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

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