Regulatory Matters (Details) $ in Thousands	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Total Capital (to Risk-Weighted Assets):
Actual Amount	$ 966,092	$ 882,453
Actual Ratio (as a percent)	0.1599	0.1490
Minimum Capital Adequacy Amount	$ 483,350	$ 473,782
Minimum Capital Adequacy Ratio (as a percent)	0.0800	0.0800
Minimum Capital Adequacy with Capital Buffer Amount	$ 634,397	$ 621,839
Minimum Capital Adequacy with Capital Buffer Ratio (as a percent)	0.1050	0.1050
To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	$ 604,188	$ 592,227
To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio (as a percent)	0.1000	0.1000
Tier I Capital (to Risk-Weighted Assets):
Actual Amount	$ 889,709	$ 807,926
Actual Ratio (as a percent)	0.1473	0.1364
Minimum Capital Adequacy Amount	$ 362,513	$ 355,336
Minimum Capital Adequacy Ratio (as a percent)	0.0600	0.0600
Minimum Capital Adequacy with Capital Buffer Amount	$ 513,560	$ 503,393
Minimum Capital Adequacy with Capital Buffer Ratio (as a percent)	0.0850	0.0850
To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	$ 483,350	$ 473,782
To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio (as a percent)	0.0800	0.0800
Common Equity Tier 1 Capital (to Risk-Weighted Assets):
Actual Amount	$ 788,884	$ 743,467
Actual Ratio (as a percent)	0.1306	0.1255
Minimum Capital Adequacy Amount	$ 271,884	$ 266,502
Minimum Capital Adequacy Ratio (as a percent)	0.0450	0.0450
Minimum Capital Adequacy with Capital Buffer Amount	$ 422,931	$ 414,559
Minimum Capital Adequacy with Capital Buffer Ratio (as a percent)	0.0700	0.0700
To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	$ 392,722	$ 384,948
To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio (as a percent)	0.0650	0.0650
Tier I Capital (to Average Assets):
Actual Amount	$ 889,709	$ 807,926
Actual Ratio (as a percent)	0.1215	0.1219
Minimum Capital Adequacy Amount	$ 292,977	$ 265,122
Minimum Capital Adequacy Ratio (as a percent)	0.0400	0.0400
To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	$ 366,221	$ 331,402
To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio (as a percent)	0.0500	0.0500
1st Source Bank
Total Capital (to Risk-Weighted Assets):
Actual Amount	$ 881,983	$ 804,131
Actual Ratio (as a percent)	0.1459	0.1357
Minimum Capital Adequacy Amount	$ 483,637	$ 474,189
Minimum Capital Adequacy Ratio (as a percent)	0.0800	0.0800
Minimum Capital Adequacy with Capital Buffer Amount	$ 634,774	$ 622,373
Minimum Capital Adequacy with Capital Buffer Ratio (as a percent)	0.1050	0.1050
To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	$ 604,546	$ 592,736
To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio (as a percent)	0.1000	0.1000
Tier I Capital (to Risk-Weighted Assets):
Actual Amount	$ 805,556	$ 729,541
Actual Ratio (as a percent)	0.1332	0.1231
Minimum Capital Adequacy Amount	$ 362,728	$ 355,642
Minimum Capital Adequacy Ratio (as a percent)	0.0600	0.0600
Minimum Capital Adequacy with Capital Buffer Amount	$ 513,864	$ 503,826
Minimum Capital Adequacy with Capital Buffer Ratio (as a percent)	0.0850	0.0850
To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	$ 483,637	$ 474,189
To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio (as a percent)	0.0800	0.0800
Common Equity Tier 1 Capital (to Risk-Weighted Assets):
Actual Amount	$ 761,731	$ 722,082
Actual Ratio (as a percent)	0.1260	0.1218
Minimum Capital Adequacy Amount	$ 272,046	$ 266,731
Minimum Capital Adequacy Ratio (as a percent)	0.0450	0.0450
Minimum Capital Adequacy with Capital Buffer Amount	$ 423,182	$ 414,915
Minimum Capital Adequacy with Capital Buffer Ratio (as a percent)	0.0700	0.0700
To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	$ 392,955	$ 385,279
To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio (as a percent)	0.0650	0.0650
Tier I Capital (to Average Assets):
Actual Amount	$ 805,556	$ 729,541
Actual Ratio (as a percent)	0.1100	0.1103
Minimum Capital Adequacy Amount	$ 292,959	$ 264,500
Minimum Capital Adequacy Ratio (as a percent)	0.0400	0.0400
To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	$ 366,199	$ 330,625
To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio (as a percent)	0.0500	0.0500