BUSINESS COMBINATIONS - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions				1 Months Ended	3 Months Ended	9 Months Ended
	Nov. 03, 2017	Apr. 07, 2017	Mar. 11, 2016	Oct. 20, 2017	Jun. 30, 2017	Sep. 30, 2017	Jun. 03, 2016
Business Acquisition [Line Items]
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Deposit Premium Percentage							3.00%
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Loan Held For Sale							$ 314.0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Accounts Payable							$ 63.0
Business Acquisition Intangible Assets Expected Tax Deductible Amount						$ 13.0
Business Acquisition Intangible Assets Tax Deductible Period						15 years
Business Loan Premium							0.50%
Payments for Merger Related Costs					$ 4.3
Subsequent Event [Member]
Business Acquisition [Line Items]
Share Price	$ 0
Floridian Financial Group [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets			$ 417.0
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Noncurrent Liabilities Deposits			337.0
Business Combination Recognized Identifiable Assets Acquired and Liabilities Assumed Noncurrent Assets Loans			266.0
Business Acquisition, Equity Interest Issued or Issuable, Description						Under the terms of the definitive agreement, Floridian shareholders received, at their election, (i) the combination of $4.29 in cash and 0.5291 shares of Seacoast common stock, (ii) $12.25 in cash, or (iii) 0.8140 shares of Seacoast common stock, subject to a customary proration mechanism so that the aggregate consideration mix equaled 35% cash and 65% Seacoast shares (based on Seacoast’s closing price of $15.47 per share on March 11, 2016).
Business Acquisition, Percentage of Voting Interests Acquired						100.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Other, Amount			$ 32.0
Gulf Shore Banc shares Inc [Member]
Business Acquisition [Line Items]
Business Acquisition, Equity Interest Issued or Issuable, Description						Under the terms of the definitive agreement, GulfShore shareholders received, for each share of Gulfshore common stock, the combination of $1.47 in cash and 0.4807 shares of Seacoast common stock (based on Seacoast’s closing price of $23.94 per share on April 7, 2017).
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Other, Amount		$ 37.0
Gulf Shore Banc shares Inc [Member] | Common Stock [Member]
Business Acquisition [Line Items]
Business Acquisition, Percentage of Voting Interests Acquired						100.00%
Palm Beach Community Bank [Member] | Subsequent Event [Member]
Business Acquisition [Line Items]
Business Acquisition, Equity Interest Issued or Issuable, Description	Under the terms of the definitive agreement, PBCB shareholders received, for each share of PBCB stock, the combination of $6.26 in cash and 0.9240 shares of Seacoast common stock.
Business Acquisition, Percentage of Voting Interests Acquired	100.00%
NorthStar Bank [Member] | Subsequent Event [Member]
Business Acquisition [Line Items]
Business Acquisition, Equity Interest Issued or Issuable, Description				Under the terms of the definitive agreement, NSBC shareholders received, for each share of NSBC common stock, the combination of $2.40 in cash and 0.5605 shares of Seacoast common stock
Business Acquisition, Percentage of Voting Interests Acquired				100.00%