Investment Securities - Amortized Cost and Approximate Fair Value of Securities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Available for sale securities
Amortized Cost	$ 517,276	$ 529,212
Gross Unrealized Gains	21,151	20,581
Gross Unrealized Losses	372	250
Investment securities available for sale	538,055us-gaap_AvailableForSaleSecurities	549,543us-gaap_AvailableForSaleSecurities
Held to maturity securities
Amortized Cost	651,418us-gaap_HeldToMaturitySecurities	631,088us-gaap_HeldToMaturitySecurities
Gross Unrealized Gains	19,417	17,243
Gross Unrealized Losses	224	608
Investment securities held to maturity - fair value	670,611	647,723
Amortized Cost	1,168,694frme_SecuritiesAmortizedCost	1,160,300frme_SecuritiesAmortizedCost
Gross Unrealized Gains	40,568frme_SecuritiesGrossUnrealizedGains	37,824frme_SecuritiesGrossUnrealizedGains
Gross Unrealized Losses	596frme_SecuritiesGrossUnrealizedLosses	858frme_SecuritiesGrossUnrealizedLosses
Fair Value	1,208,666frme_SecuritiesFairValue	1,197,266frme_SecuritiesFairValue
U.S. Government-sponsored agency securities
Available for sale securities
Amortized Cost	100	100
Gross Unrealized Gains	7	9
Gross Unrealized Losses
Investment securities available for sale	107	109
State and municipal
Available for sale securities
Amortized Cost	249,699	216,915
Gross Unrealized Gains	12,195	11,801
Gross Unrealized Losses	310	123
Investment securities available for sale	261,584	228,593
Held to maturity securities
Amortized Cost	225,225	204,443
Gross Unrealized Gains	6,429	5,716
Gross Unrealized Losses	76	96
Investment securities held to maturity - fair value	231,578	210,063
U.S. Government-sponsored mortgage-backed securities
Available for sale securities
Amortized Cost	265,740	310,460
Gross Unrealized Gains	8,949	8,771
Gross Unrealized Losses	62	127
Investment securities available for sale	274,627	319,104
Held to maturity securities
Amortized Cost	426,193	426,645
Gross Unrealized Gains	12,988	11,527
Gross Unrealized Losses	148	512
Investment securities held to maturity - fair value	439,033	437,660
Corporate obligations
Available for sale securities
Amortized Cost	31	31
Gross Unrealized Gains
Gross Unrealized Losses
Investment securities available for sale	31	31
Equity securities
Available for sale securities
Amortized Cost	1,706	1,706
Investment securities available for sale	$ 1,706	$ 1,706

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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Amount of accumulated unrealized gain on investments in debt securities classified as held-to-maturity.

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Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity.

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Fair value of investments in debt securities classified as held-to-maturity.

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