Advances Collateralized by Blanket Lien on Residential Real Estate Loans and Certain Mortgage-Backed Securities (Parenthetical) (Detail) (Callable Securities, USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Year 2013
Schedule of Federal Home Loan Bank Advances and Other Borrowings [Line Items]
Callable amount at option of FHLBB	$ 13.0
Weighted average interest rate on advances, if advances are not called	1.40%
Year 2014
Schedule of Federal Home Loan Bank Advances and Other Borrowings [Line Items]
Callable amount at option of FHLBB	30.0
Weighted average interest rate on advances, if advances are not called	2.20%
Year 2015
Schedule of Federal Home Loan Bank Advances and Other Borrowings [Line Items]
Callable amount at option of FHLBB	$ 23.0
Weighted average interest rate on advances, if advances are not called	2.50%