1st Source Corporation (Parent Company Only) Financial Information (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
ASSETS
Operating lease right of use assets	$ 23,825	$ 24,147
Other assets	288,575	227,990
Total assets	7,316,411	6,622,776
LIABILITIES AND SHAREHOLDERS' EQUITY
Long-term debt and mandatorily redeemable securities	81,864	71,639
Subordinated notes	58,764	58,764
Operating lease liabilities	23,688	24,319
Other liabilities	148,444	140,518
Total liabilities	6,385,741	5,774,140
Total shareholders’ equity	886,845	828,277
Total liabilities and shareholders’ equity	7,316,411	6,622,776
Parent
ASSETS
Cash and cash equivalents	113,242	107,285	$ 106,647	$ 100,155
Operating lease right of use assets	17,452	17,106
Other assets	5,251	4,442
Total assets	995,439	935,526
LIABILITIES AND SHAREHOLDERS' EQUITY
Commercial paper	4,767	3,993
Long-term debt and mandatorily redeemable securities	26,681	25,819
Subordinated notes	58,764	58,764
Operating lease liabilities	17,369	17,329
Other liabilities	1,013	1,344
Total liabilities	108,594	107,249
Total shareholders’ equity	886,845	828,277
Total liabilities and shareholders’ equity	995,439	935,526
Parent | Bank subsidiaries
ASSETS
Short-term investments with bank subsidiary	500	500
Investments in subsidiary	858,993	806,192
Parent | Non-bank subsidiaries
ASSETS
Investments in subsidiary	$ 1	$ 1