Business Combinations - Narrative (Details) $ in Thousands	Aug. 23, 2021 branch	Aug. 06, 2021 USD ($) branch	Aug. 21, 2020 USD ($) branch	Mar. 13, 2020 USD ($) branch	Sep. 30, 2021 USD ($)	Jun. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Business Acquisition [Line Items]
Initial allowance for credit losses					$ 87,823	$ 81,127	$ 92,733	$ 94,013	$ 91,250	$ 35,154
Sabal Palm Bancorp, Inc.
Business Acquisition [Line Items]
Number of branches operated | branch	3
Loans to be acquired in acquisition					255,000
Deposits to be acquired in acquisition					389,000
Florida Business Bank, Corp.
Business Acquisition [Line Items]
Number of branches operated | branch	1
Loans to be acquired in acquisition					139,000
Deposits to be acquired in acquisition					$ 169,000
Legacy Bank of Florida
Business Acquisition [Line Items]
Number of branches operated | branch		5
Loans to be acquired in acquisition		$ 477,215
Deposits to be acquired in acquisition		$ 494,921
Percentage of common stock acquired		100.00%
Common stock portion, number of Seacoast stock for each share of stock converted (in shares)		0.1703
Goodwill recognized from acquisition, nondeductible		$ 31,000
Initial allowance for credit losses		11,200
Legacy Bank of Florida | PCD Loans
Business Acquisition [Line Items]
Loans to be acquired in acquisition		62,589
Initial allowance for credit losses		3,000
Legacy Bank of Florida | Acquired Non-PCD Loans
Business Acquisition [Line Items]
Initial allowance for credit losses		$ 8,200
Fourth Street Banking Company
Business Acquisition [Line Items]
Number of branches operated | branch			2
Loans to be acquired in acquisition			$ 303,434
Deposits to be acquired in acquisition			$ 329,662
Percentage of common stock acquired			100.00%
Common stock portion, number of Seacoast stock for each share of stock converted (in shares)			0.1275
Goodwill recognized from acquisition, nondeductible			$ 9,000
Initial allowance for credit losses			10,400
Fourth Street Banking Company | PCD Loans
Business Acquisition [Line Items]
Loans to be acquired in acquisition			49,373
Initial allowance for credit losses			5,800
Fourth Street Banking Company | Acquired Non-PCD Loans
Business Acquisition [Line Items]
Initial allowance for credit losses			$ 4,600
First Bank of the Palm Beaches
Business Acquisition [Line Items]
Number of branches operated | branch				2
Loans to be acquired in acquisition				$ 146,777
Deposits to be acquired in acquisition				$ 173,741
Percentage of common stock acquired				100.00%
Common stock portion, number of Seacoast stock for each share of stock converted (in shares)				0.2000
Goodwill recognized from acquisition, nondeductible				$ 6,900
Initial allowance for credit losses				2,300
First Bank of the Palm Beaches | PCD Loans
Business Acquisition [Line Items]
Loans to be acquired in acquisition				43,038
Initial allowance for credit losses				500
First Bank of the Palm Beaches | Acquired Non-PCD Loans
Business Acquisition [Line Items]
Initial allowance for credit losses				$ 1,800