Income Taxes - Expiration of Tax Credit Carryforwards (Detail) $ in Millions	Dec. 31, 2020 USD ($)
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	$ 468
2020 Tax Loss Carryforwards
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	468
2021 Tax Loss Carryforwards
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	1
2022 Tax Loss Carryforwards
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	11
2023 Tax Loss Carryforwards
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	10
2024 Tax Loss Carryforwards
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	11
2025 Tax Loss Carryforwards
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	18
2026-2030 Tax Loss Carryforwards
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	71
Later Tax Loss Carryforwards
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	294
Unlimited Tax Loss Carryforwards
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	$ 52

The amount of the tax credit carryforward, before tax effects, available to reduce future taxable income under enacted tax laws.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32559-109319

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