1st Source Corporation (Parent Company Only) Financial Information (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income:
Other	$ 11,949	$ 13,019	$ 10,362
Expenses:
Interest on subordinated notes	3,367	3,677	3,625
Interest on long-term debt and mandatorily redeemable securities	2,868	2,905	2,316
Interest on commercial paper and other short-term borrowings	517	1,934	2,838
Occupancy	10,276	10,528	10,041
Other	7,902	6,534	6,478
Income before income taxes	106,341	120,154	105,027
Income tax benefit	(24,880)	(28,139)	(22,613)
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	94,636	107,808	75,788
Parent
Income:
Rental (reimbursements to) income from subsidiaries	(908)	2,505	2,613
Other	293	366	367
Investment securities and other investment (losses) gains	(44)	109	(180)
Total income	45,548	49,715	47,880
Expenses:
Interest on subordinated notes	3,367	3,677	3,625
Interest on long-term debt and mandatorily redeemable securities	2,151	2,228	1,624
Interest on commercial paper and other short-term borrowings	11	13	14
Occupancy	1,816	1,861	1,774
Other	667	586	642
Total expenses	8,012	8,365	7,679
Income before income taxes	37,536	41,350	40,201
Income tax benefit	1,747	987	1,009
Income before equity in undistributed income of subsidiaries	39,283	42,337	41,210
Equity in undistributed income of subsidiaries	42,178	49,678	41,204
Net income	81,461	92,015	82,414
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	94,660	107,863	75,788
Parent | Bank subsidiaries
Income:
Dividends from bank subsidiary	46,207	46,735	45,080
Expenses:
Equity in undistributed income of subsidiaries	$ 42,178	$ 49,678	$ 41,204