Investments - Securities Accounted for As Available-for-Sale Included in Investments (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Gross unrealized gain included in accumulated other comprehensive gain/(loss)	$ 746		$ 137
Available-for-sale securities sold	1,450	$ 4,960
Realized gains on sales of available-for-sale securities	$ 265	$ 36

Amount of unrealized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

Name:	us-gaap_AvailableForSaleDebtSecuritiesGrossUnrealizedGain
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration