1st Source Corporation (Parent Company Only) Financial Information (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income:
Rental income from subsidiaries	$ 30,381	$ 25,863	$ 22,302
Other	10,947	12,081	12,319
Expenses:
Interest on subordinated notes	4,002	4,220	4,220
Interest on long-term debt and mandatorily redeemable securities	2,435	2,089	1,970
Interest on commercial paper and other short-term borrowings	1,115	525	484
Other	5,574	5,856	6,976
Income before income taxes	101,360	89,126	88,563
Income tax benefit	(33,309)	(31,340)	(31,077)
Net income	68,051	57,786	57,486
Comprehensive income	63,375	52,575	54,634
Parent
Income:
Rental income from subsidiaries	2,354	2,363	2,342
Other	422	444	426
Investment securities and other investment gains	6,431	3,901	26
Total income	48,482	42,772	38,858
Expenses:
Interest on subordinated notes	4,002	4,220	4,220
Interest on long-term debt and mandatorily redeemable securities	1,685	1,454	1,375
Interest on commercial paper and other short-term borrowings	17	20	30
Rent	2,070	1,739	1,737
Other	1,733	1,179	351
Total expenses	9,507	8,612	7,713
Income before income taxes	38,975	34,160	31,145
Income tax benefit	204	741	1,721
Income before equity in undistributed income of subsidiaries	39,179	34,901	32,866
Equity in undistributed income of subsidiaries	28,872	22,885	24,620
Net income	68,051	57,786	57,486
Comprehensive income	63,375	52,575	54,634
Parent | Bank subsidiaries
Income:
Dividends from bank subsidiary	38,317	36,064	36,064
Expenses:
Equity in undistributed income of subsidiaries	28,872	22,569	24,289
Parent | Non-bank subsidiaries
Income:
Dividends from bank subsidiary	958	0	0
Expenses:
Equity in undistributed income of subsidiaries	$ 0	$ 316	$ 331