Income Taxes - Additional Information (Detail)	3 Months Ended	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2019 USD ($) State	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Income Tax Disclosure [Abstract]
Tax bad debt reserve		$ 137,000,000
Income taxes attributable to gains or losses on bank investment securities		$ 5,000,000	$ (2,000,000)	$ 7,000,000
Alternative minimum tax expense recognized for gains or losses on bank investment securities				$ 0
Federal income tax rate		21.00%	21.00%	35.00%
Incremental income tax expense related to Tax Act	$ 85,000,000
Reduction in income tax expense due to reduction of federal tax rate			$ 15,000,000
Number of state and local jurisdictions where company files income tax returns | State		40