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Employee Benefit Plans (Details) - Employee Stock Ownership and Profit Sharing Plan $ in Thousands | 12 Months Ended
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Dec. 31, 2020 USD ($) h shares | Dec. 31, 2019 USD ($) shares | Dec. 31, 2018 USD ($)
Employee benefit plans |  |  |
Minimum service period required to participate in Plan | 90 days |  |
Percentage of employee's deferred compensation up to which the employer matches (as a percent) | 4.00% |  |
Employer's matching contribution on the next 2% of employee contribution (as a percent) | 50.00% |  |
Employee contribution after matching contribution of employer (as a percent) | 2.00% |  |
Employee's eligible compensation on which employer pays fixed contribution (as a percent) | 2.00% |  |
Common stock held in relation to employee benefit plans (in shares) | shares | 824,466 | 852,128 |
Minimum service period required to be eligible to get employer's contribution | 5 years |  |
Minimum working hours required per year to be eligible to get employer's contribution | h | 1,000 |  |
Employee deferral that is always vested (as a percent) | 100.00% |  |
Contribution expense | $ | $ 5,700 | $ 5,480 | $ 4,870

X
- Definition The minimum credited years of service that an individual is required to perform in order to be fully vested under the deferred compensation arrangement. + References No definition available. + Details
Name: | source_DeferredCompensationArrangementWithIndividualsRequisiteServicePeriod
Namespace Prefix: | source_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition -- None. No documentation exists for this element. -- + References No definition available. + Details
Name: | source_DefinedBenefitAndContributionPlanDisclosureLineItems
Namespace Prefix: | source_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the first level of employee contributions (percentage of compensation) which are matched by the employer at its discretion. + References No definition available. + Details
Name: | source_DefinedContributionPlanDiscretionaryEmployerMatchEmployeeContributionLevelOne
Namespace Prefix: | source_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Represents the second level of employee contributions (percentage of compensation) which are matched by the employer at its discretion. + References No definition available. + Details
Name: | source_DefinedContributionPlanDiscretionaryEmployerMatchEmployeeContributionLevelTwo
Namespace Prefix: | source_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the employer's discretionary matching contribution of the second level of employee contributions. + References No definition available. + Details
Name: | source_DefinedContributionPlanDiscretionaryEmployerMatchLevelTwo
Namespace Prefix: | source_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the percentage of the employee deferral that is always vested. + References No definition available. + Details
Name: | source_DefinedContributionPlanEmployeeDeferralPercentageVested
Namespace Prefix: | source_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the fixed percentage of eligible earnings that can be contributed by the employer. + References No definition available. + Details
Name: | source_DefinedContributionPlanEmployerDiscretionaryContributionPercentage
Namespace Prefix: | source_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition The number of employer shares or related party shares included in plan assets. + References No definition available. + Details
Name: | source_DefinedContributionPlanNumberofSharesofEquitySecuritiesIssuedbyEmployerandRelatedPartiesIncludedinPlanAssets
Namespace Prefix: | source_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Definition Represents the requisite service period of employees to become eligible to participate in the plan. + References No definition available. + Details
Name: | source_DefinedContributionPlanRequisitePeriodOfServiceToBecomeEligible
Namespace Prefix: | source_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Represents the minimum number of working hours required during a fiscal year per employee to be considered a credited year of service. + References No definition available. + Details
Name: | source_DefinedContributionPlanWorkingHoursRequiredPerYear
Namespace Prefix: | source_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of cost for defined contribution plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942 + Details
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Balance Type: | debit
Period Type: | duration

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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |