1st Source Corporation (Parent Company Only) Financial Information (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
ASSETS
Cash and cash equivalents	$ 78,033	$ 108,304	$ 79,721	$ 66,190
Investment securities, available-for-sale (amortized cost of $0 at December 31, 2017 and $884 at December 31, 2016)	904,033	850,467
Other assets	155,412	130,059
Total assets	5,887,284	5,486,268
LIABILITIES AND SHAREHOLDERS' EQUITY
Long-term debt and mandatorily redeemable securities	70,060	74,308
Subordinated notes	58,764	58,764
Other liabilities	72,598	54,843
Total liabilities	5,168,747	4,813,618
Total shareholders’ equity	718,537	672,650	644,053	614,473
Total liabilities and shareholders’ equity	5,887,284	5,486,268
Available-for-sale Securities, Amortized Cost	909,368	848,316
Parent
ASSETS
Cash and cash equivalents	100,155	73,324	$ 60,429	$ 51,725
Investment securities, available-for-sale (amortized cost of $0 at December 31, 2017 and $884 at December 31, 2016)	0	7,369
Other assets	2,696	4,013
Total assets	809,471	763,933
LIABILITIES AND SHAREHOLDERS' EQUITY
Commercial paper	6,115	5,761
Long-term debt and mandatorily redeemable securities	22,942	21,228
Subordinated notes	58,764	58,764
Other liabilities	3,113	5,530
Total liabilities	90,934	91,283
Total shareholders’ equity	718,537	672,650
Total liabilities and shareholders’ equity	809,471	763,933
Available-for-sale Securities, Amortized Cost	0	884
Parent | Bank subsidiaries
ASSETS
Short-term investments with bank subsidiary	500	500
Investments in subsidiary	706,119	676,915
Parent | Non-bank subsidiaries
ASSETS
Investments in subsidiary	$ 1	$ 1,812