Income Taxes - Additional Information (Detail) $ in Millions	12 Months Ended
Dec. 31, 2020 USD ($) State	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Income Tax Disclosure [Abstract]
Tax bad debt reserve	$ 137
Income taxes attributable to gains or losses on bank investment securities	$ 2	$ 5	$ (2)
Number of state and local jurisdictions where company files income tax returns \| State	40

Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from the allowance for doubtful accounts.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32632-109319

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32621-109319

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