Summary of Fair Value and Related Notional Amounts of Derivative Instruments (Parenthetical) (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Summary of fair value of derivative instruments
Embedded derivatives, Fair value	$ 627	$ 984

Net fair value as of the balance sheet date of the embedded derivative or group of embedded derivatives classified as assets, net of those classified as liabilities.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

Name:	us-gaap_EmbeddedDerivativeFairValueOfEmbeddedDerivativeNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant