Loan and Lease Financings (Details)	6 Months Ended
Jun. 30, 2018 USD ($) item	Dec. 31, 2017 USD ($)
Receivables [Abstract]
Number of methods to assess credit risk \| item	2
Loan and Lease Financings
Recorded investment in loans and leases	$ 4,839,823,000	$ 4,527,678,000
Loan and lease financings excluding residential real estate and consumer loans
Loan and Lease Financings
Recorded investment in loans and leases	4,179,812,000	3,873,410,000
Commercial and agricultural
Loan and Lease Financings
Recorded investment in loans and leases	1,047,705,000	929,997,000
Auto and light truck
Loan and Lease Financings
Recorded investment in loans and leases	580,045,000	496,816,000
Medium and heavy duty truck
Loan and Lease Financings
Recorded investment in loans and leases	276,273,000	296,935,000
Aircraft
Loan and Lease Financings
Recorded investment in loans and leases	863,496,000	844,657,000
Construction equipment
Loan and Lease Financings
Recorded investment in loans and leases	642,634,000	563,437,000
Commercial real estate
Loan and Lease Financings
Recorded investment in loans and leases	769,659,000	741,568,000
Credit Quality Grades 1-6 \| Loan and lease financings excluding residential real estate and consumer loans
Loan and Lease Financings
Recorded investment in loans and leases	4,040,629,000	3,776,621,000
Credit Quality Grades 1-6 \| Commercial and agricultural
Loan and Lease Financings
Recorded investment in loans and leases	1,014,535,000	906,074,000
Credit Quality Grades 1-6 \| Auto and light truck
Loan and Lease Financings
Recorded investment in loans and leases	559,686,000	482,455,000
Credit Quality Grades 1-6 \| Medium and heavy duty truck
Loan and Lease Financings
Recorded investment in loans and leases	274,035,000	293,318,000
Credit Quality Grades 1-6 \| Aircraft
Loan and Lease Financings
Recorded investment in loans and leases	812,726,000	815,956,000
Credit Quality Grades 1-6 \| Construction equipment
Loan and Lease Financings
Recorded investment in loans and leases	624,034,000	552,684,000
Credit Quality Grades 1-6 \| Commercial real estate
Loan and Lease Financings
Recorded investment in loans and leases	755,613,000	726,134,000
Credit Quality Grades 7-12
Loan and Lease Financings
Relationships reviewed quarterly as part of management's evaluation of the appropriateness of the reserve for loan and lease losses	100,000
Credit Quality Grades 7-12 \| Loan and lease financings excluding residential real estate and consumer loans
Loan and Lease Financings
Recorded investment in loans and leases	139,183,000	96,789,000
Credit Quality Grades 7-12 \| Commercial and agricultural
Loan and Lease Financings
Recorded investment in loans and leases	33,170,000	23,923,000
Credit Quality Grades 7-12 \| Auto and light truck
Loan and Lease Financings
Recorded investment in loans and leases	20,359,000	14,361,000
Credit Quality Grades 7-12 \| Medium and heavy duty truck
Loan and Lease Financings
Recorded investment in loans and leases	2,238,000	3,617,000
Credit Quality Grades 7-12 \| Aircraft
Loan and Lease Financings
Recorded investment in loans and leases	50,770,000	28,701,000
Credit Quality Grades 7-12 \| Construction equipment
Loan and Lease Financings
Recorded investment in loans and leases	18,600,000	10,753,000
Credit Quality Grades 7-12 \| Commercial real estate
Loan and Lease Financings
Recorded investment in loans and leases	$ 14,046,000	$ 15,434,000

Represents the amount of loans and leases where the internal credit quality grade is at or below a predetermined classification which are included in comprehensive analysis.

No definition available.

+ Details

Name:	srce_LoansAndLeasesReceivableEvaluatedForImpairment
Namespace Prefix:	srce_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the number of methods to assess credit risk of loans and leases receivables.

+ References

No definition available.

+ Details

Name:	srce_LoansAndLeasesReceivableNumberOfMethodsToAssessCreditRisk
Namespace Prefix:	srce_
Data Type:	xbrli:integerItemType
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_FinancingReceivableRecordedInvestmentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5074-111524

+ Details

Name:	us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

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

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=srce_GroupOfLoansOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=srce_CommercialAndAgriculturalLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=srce_AutoAndLightTruckMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=srce_MediumAndHeavyDutyTruckMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=srce_AircraftFinancingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=srce_ConstructionEquipmentFinancingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=us-gaap_CommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=srce_InternallyAssignedGrade1To6Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=srce_InternallyAssignedGrade7To12Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: