Derivative Instruments (Collateral Support Agreements) (Details) - Credit Default Swap, Seller [Member] - USD ($) $ in Millions	Mar. 31, 2020	Mar. 31, 2019
Credit risk related contingent features collateral
Maximum potential payout	$ 394	$ 55
Maximum collateral potentially required to post	$ 394	$ 55

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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4K
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5708775-113959

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