Subsequent Events (Details) (Banco Popular North America, Subsequent Event, USD $) In Millions, unless otherwise specified	Apr. 22, 2014 retail_branch
Banco Popular North America \| Subsequent Event
Subsequent Event [Line Items]
Number of retail branches acquired	12
Deposits	$ 750
Loans	$ 525

This element represents the amount of acquired deposits.

No definition available.

Name:	fmbi_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedDeposits
Namespace Prefix:	fmbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant