Allowance Balances [Abstract]

No definition available.

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- Definition

Loan Balance [Abstract]

+ References

No definition available.

+ Details

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Namespace Prefix:	frme_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Loans and Leases Receivable

+ References

No definition available.

+ Details

Name:	frme_LoansAndLeasesReceivable
Namespace Prefix:	frme_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

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

+ References

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The balance of financing receivables that were collectively evaluated for impairment.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The balance of financing receivables that were individually evaluated for impairment.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953423-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (b)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=121614798&loc=d3e15032-111544

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953423-111524

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 6.L.1)
-URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74567-122707

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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- Details

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Loans and Allowance - Allowance for Credit Losses and Loan Portfolio by Loan Segment (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Allowance Balances:
Individually evaluated for impairment	$ 9,139		$ 689
Collectively evaluated for impairment	117,587		79,595
Total Allowance for Loan Losses	126,726	$ 121,119	80,284	$ 80,571	$ 81,274	$ 80,552
Loan Balance [Abstract]
Individually evaluated for impairment	52,484		11,709
Collectively evaluated for impairment	9,183,892		8,434,292
Loans	9,243,833		8,459,310
Loans acquired with deteriorated credit quality
Loan Balance [Abstract]
Loans	7,457		13,309
Commercial
Allowance Balances:
Individually evaluated for impairment	601		0
Collectively evaluated for impairment	44,746		32,902
Total Allowance for Loan Losses	45,347	44,678	32,902	33,001	33,202	32,657
Loan Balance [Abstract]
Individually evaluated for impairment	4,837		457
Collectively evaluated for impairment	3,568,432		2,748,681
Loans	3,573,968		2,750,854
Commercial \| Loans acquired with deteriorated credit quality
Loan Balance [Abstract]
Loans	699		1,716
Commercial Real Estate
Allowance Balances:
Individually evaluated for impairment	7,981		231
Collectively evaluated for impairment	41,973		28,547
Total Allowance for Loan Losses	49,954	46,918	28,778	28,966	29,531	29,609
Loan Balance [Abstract]
Individually evaluated for impairment	44,203		8,728
Collectively evaluated for impairment	3,819,629		3,821,660
Loans	3,870,590		3,840,266
Commercial Real Estate \| Loans acquired with deteriorated credit quality
Loan Balance [Abstract]
Loans	6,758		9,878
Consumer
Allowance Balances:
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	9,378		4,035
Total Allowance for Loan Losses	9,378	8,445	4,035	4,106	4,070	3,964
Loan Balance [Abstract]
Individually evaluated for impairment	3		4
Collectively evaluated for impairment	125,408		135,985
Loans	125,411		135,989
Residential
Allowance Balances:
Individually evaluated for impairment	557		458
Collectively evaluated for impairment	21,490		14,111
Total Allowance for Loan Losses	22,047	$ 21,078	14,569	$ 14,498	$ 14,471	$ 14,322
Loan Balance [Abstract]
Individually evaluated for impairment	3,441		2,520
Collectively evaluated for impairment	1,670,423		1,727,966
Loans	1,673,864		1,732,201
Residential \| Loans acquired with deteriorated credit quality
Loan Balance [Abstract]
Loans	$ 0		$ 1,715