Borrowed Funds - Additional Information (Detail) - USD ($)	9 Months Ended	12 Months Ended
Sep. 30, 2015	Dec. 31, 2014
Borrowings [Line Items]
Minimum lease obligation term under capital leases	3 years
Federal Home Loan Bank Advances
Borrowings [Line Items]
Unpaid principal balance of pledged secure FHLB of Chicago borrowings	$ 603,700,000	$ 652,300,000
FHLB of Chicago borrowings collateralized by mortgage-related securities	60,700,000	$ 88,900,000
Federal Home Loan Bank Advances \| Call Features
Borrowings [Line Items]
FHLB of Chicago borrowings collateralized by mortgage-related securities	$ 0

The carrying amount, as of the date of the latest financial statement presented, of loans receivable which are owned but transferred to serve as collateral for the payment of FHLB debt obligations, and for which the transferee is not permitted to sell or re-pledge them to an unrelated party.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 860

-SubTopic 30

-Section 50

-Paragraph 1A

-Subparagraph (b)(1)(i)

-URI http://asc.fasb.org/extlink&oid=55761124&loc=SL6224234-111729

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