Debt Securities Held-to-maturity -Additional Information (Details)	Sep. 30, 2021 USD ($) score	Jun. 30, 2021 USD ($)	Dec. 31, 2020 USD ($) score	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Debt Securities, Held-to-maturity
Amortized cost	$ 85,655,000		$ 92,621,000
Allowance for credit losses	9,222,000		10,261,000
Securities in non-performing status	0		0
Obligations of Puerto Rico, States and political subdivisions
Debt Securities, Held-to-maturity
Amortized cost	74,064,000		81,029,000
Allowance for credit losses	$ 9,222,000	$ 10,214,000	$ 10,261,000	$ 12,421,000	$ 12,735,000	$ 0
Average Credit Score Fico | score	702		697
Percentage of nominal value	66.00%		66.00%
Obligations of Puerto Rico, States and political subdivisions | Munis Payable From Real and Personal Property Taxes
Debt Securities, Held-to-maturity
Amortized cost	$ 30,145,000		$ 35,315,000
Allowance for credit losses	1,000,000.0		1,400,000
Obligations of Puerto Rico, States and political subdivisions | Munis Not Guaranteed By Puerto Rico Central Government
Debt Securities, Held-to-maturity
Amortized cost	44,000,000		46,000,000
Allowance for credit losses	$ 8,200,000		$ 8,900,000