Investments - Revenue and General Obligation Bonds (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2022 USD ($) item state	Dec. 31, 2021 USD ($)
Debt Securities, Available-for-sale [Line Items]
Investment securities available for sale	$ 1,025,032	$ 973,314
Number of municipal bond | item	400
States and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities available for sale	$ 286,380	304,268
Investment Portfolio, Securities Issued, Municipalities and Agencies, Number | item	334
Investment Portfolio, Securities Issued, States, Number | state	33
General Obligation Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities available for sale	$ 249,517	$ 259,682
General Obligation Bonds [Member] | Alvin Independent School District of Brazoria [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities available for sale	4,000
Subordinate Debenture [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities available for sale	$ 43,400
Number of subordinated debentures owned | item	36