CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands		Dec. 31, 2017	May 01, 2017	Apr. 30, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Investment securities held to maturity		$ 568,208			$ 611,933
Preferred Stock, par value (in dollars per share)		$ 1,000			$ 1,000
Preferred Stock, liquidation value (in dollars per share)		$ 1,000			$ 1,000
Preferred Stock, shares authorized		600			600
Preferred Stock, shares issued		125			125
Preferred Stock, shares outstanding		125			125
Common Stock, stated value (in dollars per share)		$ 0.125			$ 0.125
Common Stock, shares authorized	[1]	100,000,000	100,000,000	50,000,000	50,000,000
Common Stock, shares issued		49,158,238			40,912,697
Common Stock, shares outstanding		49,158,238			40,912,697

[1] On May 1, 2017, the shareholders of First Merchants Corporation approved an amendment to the Articles of Incorporation to increase the number of shares of common stock authorized to issue from 50,000,000 to 100,000,000 shares.