Changes in the Carrying Amount and Accretable Yield for Acquired Impaired Loans (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield Movement [Line Items]
Balance at beginning of period, Accretable Yield	$ 1,417	$ 0
Additions due to BCB acquisition on July 1, 2014, Accretable Yield	0	1,603
Accretion, Accretable Yield	(141)	(186)
Payments received, net, Accretable Yield	0	0
Balance at end of period, Accretable Yield	1,276	1,417
Balance at beginning of period, Carrying Amount of Loans	2,063	0
Additions due to BCB acquisition on July 1, 2014, Carrying Amount of Loans	0	2,325
Accretion, Carrying Amount of Loans	141	186
Payments received, net, Carrying Amount of Loans	(346)	(448)
Balance at end of period, Carrying Amount of Loans	$ 1,858	$ 2,063