1st Source Corporation (Parent Company Only) Financial Information (Details 3) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities:
Net income	$ 68,051	$ 57,786	$ 57,486
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation of premises and equipment	5,658	5,245	4,780
Stock-based compensation	2,963	2,884	3,843
Change in trading account securities	0	0	205
Other	4,550	450	3,168
Net change in operating activities	117,261	98,060	100,608
Investing activities:
Net change in investing activities	(462,206)	(309,847)	(381,613)
Financing activities:
Proceeds from issuance of long-term debt and mandatorily redeemable securities	19,999	20,837	0
Payments on long-term debt and mandatorily redeemable securities	(26,628)	(6,429)	(1,250)
Stock issued under stock purchase plans	153	120	149
Acquisition of treasury stock	(41)	(8,030)	(9,970)
Cash dividends paid on common stock	(20,431)	(19,416)	(18,126)
Net change in financing activities	314,674	240,370	294,536
Net change in cash and cash equivalents	(30,271)	28,583	13,531
Cash and cash equivalents, beginning of year	108,304	79,721	66,190
Cash and cash equivalents, end of year	78,033	108,304	79,721
Parent Company
Operating activities:
Net income	68,051	57,786	57,486
Adjustments to reconcile net income to net cash provided by operating activities:
Equity (undistributed) distributed in excess of income of subsidiaries	(28,872)	(22,885)	(24,620)
Depreciation of premises and equipment	2	4	9
Stock-based compensation	48	52	64
Realized/unrealized investment securities and other investment gains	(6,431)	(3,901)	(26)
Change in trading account securities	0	0	205
Other	4,122	3,132	2,585
Net change in operating activities	36,920	34,188	35,703
Investing activities:
Proceeds from sales and maturities of investment securities	6,327	1,795	1,470
Net change in partnership investments	(62)	2,903	423
Return of capital from subsidiaries	854	0	0
Net change in investing activities	7,119	4,698	1,893
Financing activities:
Net change in commercial paper	354	(2,281)	(4,126)
Proceeds from issuance of long-term debt and mandatorily redeemable securities	1,248	1,607	1,520
Payments on long-term debt and mandatorily redeemable securities	(667)	(627)	(712)
Stock issued under stock purchase plans	153	120	149
Net proceeds from issuance of treasury stock	2,176	2,636	2,373
Acquisition of treasury stock	(41)	(8,030)	(9,970)
Cash dividends paid on common stock	(20,431)	(19,416)	(18,126)
Net change in financing activities	(17,208)	(25,991)	(28,892)
Net change in cash and cash equivalents	26,831	12,895	8,704
Cash and cash equivalents, beginning of year	73,324	60,429	51,725
Cash and cash equivalents, end of year	$ 100,155	$ 73,324	$ 60,429