Change in Net Unrealized Gains (Losses) on Available-for-Sale Securities Reported in Accumulated Other Comprehensive Income (Loss) (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Components of Net Unrealized Investment Gains Losses Included in Accumulated Other Comprehensive Income Loss [Line Items]
Recognition and measurement of financial assets and liabilities,tax amount	$ 0	$ 18
Reclassification adjustments to net investment (gains) losses, taxes	$ 13	$ (1)

Amount of tax expense (benefit) of reclassification adjustment from accumulated other comprehensive income for unrealized gain (loss) realized upon the sale of available-for-sale securities.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=114867106&loc=d3e640-108580

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration