Income Taxes - Additional Information (Detail)	3 Months Ended	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) State	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Income Tax Disclosure [Line Items]
Tax bad debt reserve		$ 137,121,000
Income taxes attributable to gains or losses on bank investment securities		$ (1,628,000)	$ 7,195,000	$ 11,925,000
Alternative minimum tax expense recognized for gains or losses on bank investment securities			$ 0	$ 0
Federal income tax rate		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
ASU 2016-09 [Member]
Income Tax Disclosure [Line Items]
New accounting pronouncement or change in accounting principle effect of adoption change in income tax expense (benefit)		$ (9,000,000)	$ (22,000,000)