Loans and Allowance - Allowance for Credit Losses and Loan Portfolio by Loan Segment (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance balances - Individually evaluated for impairment	$ 4,076	$ 2,119
Allowance balances - Collectively evaluated for impairment	58,245	61,195
Allowance balances - Loans acquired with deteriorated credit quality	480	650
Total Allowance for Loan Losses	62,801	63,964	69,583	67,870
Loan balances - Individually evaluated for impairment	37,398	44,093
Loan balances - Collectively evaluated for impairment	3,860,289	3,807,707
Loan balances - Loans acquired with deteriorated credit quality	67,845	73,065
Loans	3,965,532frme_LoansAndLeasesReceivable	3,924,865frme_LoansAndLeasesReceivable
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance balances - Individually evaluated for impairment	2,700	1,455
Allowance balances - Collectively evaluated for impairment	27,307	27,369
Allowance balances - Loans acquired with deteriorated credit quality
Total Allowance for Loan Losses	30,007	28,824	30,907	27,176
Loan balances - Individually evaluated for impairment	11,432	16,108
Loan balances - Collectively evaluated for impairment	1,063,822	1,011,122
Loan balances - Loans acquired with deteriorated credit quality	8,383	9,403
Loans	1,083,637	1,036,633
Commercial Real Estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance balances - Individually evaluated for impairment	1,017	470
Allowance balances - Collectively evaluated for impairment	15,081	18,207
Allowance balances - Loans acquired with deteriorated credit quality	285	650
Total Allowance for Loan Losses	16,383	19,327	22,358	23,102
Loan balances - Individually evaluated for impairment	21,668	23,963
Loan balances - Collectively evaluated for impairment	1,806,864	1,796,797
Loan balances - Loans acquired with deteriorated credit quality	54,922	59,122
Loans	1,883,454	1,879,882
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance balances - Collectively evaluated for impairment	3,138	2,658
Total Allowance for Loan Losses	3,138	2,658	2,410	2,515
Loan balances - Collectively evaluated for impairment	70,223	73,400
Loans	70,223	73,400
Residential
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance balances - Individually evaluated for impairment	359	194
Allowance balances - Collectively evaluated for impairment	12,715	12,958
Allowance balances - Loans acquired with deteriorated credit quality	195
Total Allowance for Loan Losses	13,269	13,152	13,908	15,077
Loan balances - Individually evaluated for impairment	4,298	4,022
Loan balances - Collectively evaluated for impairment	918,527	925,282
Loan balances - Loans acquired with deteriorated credit quality	4,540	4,540
Loans	927,365	933,844
Finance Leases
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance balances - Collectively evaluated for impairment	4	3
Total Allowance for Loan Losses	4	3
Loan balances - Collectively evaluated for impairment	853	1,106
Loans	$ 853	$ 1,106

Loans and Leases Receivable

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The balance of financing receivables that were acquired with deteriorated credit quality.

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The valuation allowance for financing receivables acquired with deteriorated credit quality.

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The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

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Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

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Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

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