Other Borrowed Funds and Subordinated Debentures - Schedule of the Maturity Distribution of FHLBB Advances with the Weighted Average Interest Rates (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Federal Home Loan Bank, Advances, Fiscal Year Maturity [Abstract]
Within one year	$ 169,500	$ 53,000	$ 46,000
Over one year to two years	55,000	19,500	17,500
Over two years to three years	45,000	55,000	19,500
Over three years to five years	70,000	77,000	90,000
Over five years	56,000	50,500	22,000
Federal Home Loan Bank, Advances, Total	$ 395,500	$ 255,000	$ 195,000
Within one year, Weighted Average Rate	0.51%	0.40%	1.86%
Over one year to two years, Weighted Average Rate	3.07%	2.42%	3.01%
Over two years to three years, Weighted Average Rate	3.18%	3.07%	2.42%
Over three years to five years, Weighted Average Rate	2.43%	3.05%	3.33%
Over five years, Weighted Average Rate	3.16%	3.39%	4.19%
Weighted Average Rate, Total	1.89%	2.52%	2.96%