Leases - Additional information (Detail) - Branch Office and Certain Equipment [Member]	3 Months Ended
Mar. 31, 2022
Lessee Lease Description [Line Items]
Operating leases expiration beginning year	2022
Operating leases expiration ending year	2025
Operating lease option to extend	true
Operating lease, renewal term	4 years

Lessee operating lease expiration beginning year.

No definition available.

Name:	fsea_LesseeOperatingLeaseExpirationBeginningYear
Namespace Prefix:	fsea_
Data Type:	xbrli:gYearItemType
Balance Type:	na
Period Type:	duration

- Definition

Lessee operating lease expiration ending year.

+ References

No definition available.

+ Details

Name:	fsea_LesseeOperatingLeaseExpirationEndingYear
Namespace Prefix:	fsea_
Data Type:	xbrli:gYearItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Definition

Indicates (true false) whether lessee has option to extend operating lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=123408670&loc=SL77918673-209980

+ Details

Name:	us-gaap_LesseeOperatingLeaseExistenceOfOptionToExtend
Namespace Prefix:	us-gaap_
Data Type:	xbrli:booleanItemType
Balance Type:	na
Period Type:	duration

- Definition

Term of lessee's operating lease renewal, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=123408670&loc=SL77918673-209980

+ Details

Name:	us-gaap_LesseeOperatingLeaseRenewalTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=fsea_BranchOfficeAndCertainEquipmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: