Schedule of Federal Reserve Bank Stock and Federal Home Loan Bank Stock and Other Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Other Securities [Line Items]
Fair Value	$ 65,252	$ 65,897
Federal Reserve Bank Stock and Other Securities [Member]
Schedule of Other Securities [Line Items]
Amortized Cost	57,249	59,871
Gross Unrealized Gains	8,005	6,026
Gross Unrealized Losses	(2)	0
Fair Value	65,252	65,897
FRB and FHLB stock [Member] | Federal Reserve Bank Stock and Other Securities [Member]
Schedule of Other Securities [Line Items]
Amortized Cost	33,262	33,262
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	33,262	33,262
Other securities - marketable [Member] | Federal Reserve Bank Stock and Other Securities [Member]
Schedule of Other Securities [Line Items]
Amortized Cost	0	3
Gross Unrealized Gains	5,091	4,637
Gross Unrealized Losses	0	0
Fair Value	5,091	4,640
Other securities - non-marketable [Member] | Federal Reserve Bank Stock and Other Securities [Member]
Schedule of Other Securities [Line Items]
Amortized Cost	23,987	26,606
Gross Unrealized Gains	2,914	1,389
Gross Unrealized Losses	(2)	0
Fair Value	$ 26,899	$ 27,995