Other Borrowed Funds and Subordinated Debentures - Schedule of the Maturity Distribution of FHLBB Advances with the Weighted Average Interest Rates (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Federal Home Loan Bank, Advances, Fiscal Year Maturity [Abstract]
Within one year	$ 218,000	$ 63,000	$ 164,500
Over one year to two years	42,500	28,000	63,000
Over two years to three years	3,500	25,000	28,000
Over three years to five years	70,000	33,500	28,500
Over five years	36,955	52,878	63,778
Federal Home Loan Bank, Advances, Total	$ 370,955	$ 202,378	$ 347,778
Within one year, Weighted Average Rate	1.86%	2.17%	1.82%
Over one year to two years, Weighted Average Rate	2.58%	2.29%	2.17%
Over two years to three years, Weighted Average Rate	2.15%	3.34%	2.29%
Over three years to five years, Weighted Average Rate	2.85%	2.23%	3.19%
Over five years, Weighted Average Rate	2.88%	2.47%	2.38%
Weighted Average Rate, Total	2.23%	2.42%	2.13%