XML 54 R47.htm IDEA: XBRL DOCUMENT v3.25.2

Securities Available-for-Sale - Additional Information (Detail) - Security	6 Months Ended	12 Months Ended
	Jun. 30, 2025	Dec. 31, 2024
Debt Securities, Available-for-Sale [Abstract]
Number of holdings of securities aggregate carrying value exceeded ten percentage of stockholders' equity	0	0
Securities available for sale	111

X

- Definition

Number of holdings of securities aggregate carrying value exceeded ten percentage of equity.

+ References

No definition available.

+ Details

Name:	fsea_NumberOfHoldingsOfSecuritiesAggregateCarryingValueExceededTenPercentageOfEquity
Namespace Prefix:	fsea_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

X

- References

No definition available.

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (b)(3)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-6

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479106/326-30-50-4

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionNumberOfPositions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant