Investment Securities - Amortized Cost and Approximate Fair Value of Securities (Details) - USD ($)	Sep. 30, 2019	Dec. 31, 2018
Available for sale securities
Total Investment Securities	$ 1,593,317,000	$ 1,149,727,000
Gross Unrealized Gains	54,773,000	6,721,000
Gross Unrealized Losses	1,691,000	14,253,000
Fair Value	1,646,399,000	1,142,195,000
Held to maturity securities
Total Investment Securities	843,021,000	490,387,000
Gross Unrealized Gains	22,869,000	3,571,000
Gross Unrealized Losses	604,000	4,741,000
Fair Value	865,286,000	489,217,000
Amortized Cost	2,436,338,000	1,640,114,000
Gross Unrealized Gains	77,642,000	10,292,000
Gross Unrealized Losses	2,295,000	18,994,000
Fair Value	2,511,685,000	1,631,412,000
U.S. Government-sponsored agency securities
Available for sale securities
Total Investment Securities	48,553,000	13,493,000
Gross Unrealized Gains	450,000	92,000
Gross Unrealized Losses	26,000	3,000
Fair Value	48,977,000	13,582,000
Held to maturity securities
Total Investment Securities	42,434,000	22,618,000
Gross Unrealized Gains	4,000	0
Gross Unrealized Losses	95,000	545,000
Fair Value	42,343,000	22,073,000
State and municipal
Available for sale securities
Total Investment Securities	754,846,000	605,994,000
Gross Unrealized Gains	42,663,000	5,995,000
Gross Unrealized Losses	634,000	5,854,000
Fair Value	796,875,000	606,135,000
Held to maturity securities
Total Investment Securities	355,263,000	197,909,000
Gross Unrealized Gains	15,841,000	2,858,000
Gross Unrealized Losses	40,000	872,000
Fair Value	371,064,000	199,895,000
U.S. Government-sponsored mortgage-backed securities
Available for sale securities
Total Investment Securities	789,887,000	530,209,000
Gross Unrealized Gains	11,660,000	634,000
Gross Unrealized Losses	1,031,000	8,396,000
Fair Value	800,516,000	522,447,000
Held to maturity securities
Total Investment Securities	443,824,000	268,860,000
Gross Unrealized Gains	7,024,000	713,000
Gross Unrealized Losses	462,000	3,323,000
Fair Value	450,386,000	266,250,000
Corporate obligations
Available for sale securities
Total Investment Securities	31,000	31,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	31,000	31,000
Foreign investment
Available for sale securities
Gross Unrealized Losses	0
Held to maturity securities
Total Investment Securities	1,500,000	1,000,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	7,000	1,000
Fair Value	$ 1,493,000	$ 999,000

Debt Securities, Available-for-sale and Held-to-maturity, Accumulated Gross Unrealized Gain, before Tax

No definition available.

Name:	frme_DebtSecuritiesAvailableforsaleandHeldtomaturityAccumulatedGrossUnrealizedGainbeforeTax
Namespace Prefix:	frme_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Debt Securities, Available-for-sale and Held-to-maturity, Amortized Cost

+ References

No definition available.

+ Details

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

- Definition

Debt Securities, Available-for-sale and Held-to-maturity, Amortized Cost, Accumulated Gross Unrealized Loss, before Tax

+ References

No definition available.

+ Details

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

- Definition

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

+ Details

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

- Definition

Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

+ Details

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

- Definition

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

+ Details

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

- References

No definition available.

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26610-111562

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

+ Details

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

- Definition

Amount of investment in debt security measured at amortized cost (held-to-maturity) and investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

No definition available.

+ Details

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

- Definition

Amount of investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26610-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27232-111563

+ Details

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

- References

No definition available.

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27232-111563

+ Details

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

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27232-111563

+ Details

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

- Definition

Fair value of investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27232-111563

+ Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=frme_ForeignInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: