Loans - Presentation of Average Balance, Interest Income Recognized and Cash Basis Income Recognized for Loans Individually Evaluated for Impairment by Class (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Financing Receivable Impaired [Line Items]
With no specific allowance recorded, Unpaid Principal Balance	$ 14,637	$ 21,560	$ 14,637	$ 21,560	$ 14,098
With no specific allowance recorded, Recorded Investment	8,565	9,712	8,565	9,712	7,896
With no specific allowance recorded, Average Recorded Investment	8,111	9,151	8,038	9,581
With no specific allowance recorded, Interest Income Recognized	39	50	93	116
With no specific allowance recorded, Cash Basis Income Recognized	39	50	76	105
With a specific allowance recorded, Unpaid Principal Balance	15,820	16,806	15,820	16,806	19,083
With a specific allowance recorded, Recorded Investment	15,213	16,598	15,213	16,598	18,354
With a specific allowance recorded, Allowance for Loan Losses Allocated	1,661	1,616	1,661	1,616	2,059
With a specific allowance recorded, Average Recorded Investment	16,322	17,926	16,998	18,984
With a specific allowance recorded, Interest Income Recognized	183	193	421	453
With a specific allowance recorded, Cash Basis Income Recognized	180	189	361	390
Total Unpaid Principal Balance	30,457	38,366	30,457	38,366	33,181
Total Recorded Investment	23,778	26,310	23,778	26,310	26,250
Total Average Recorded Investment	24,433	27,077	25,036	28,565
Total Interest Income Recognized	222	243	514	569
Total Cash Basis Income Recognized	219	239	437	495
Commercial Loans [Member]
Financing Receivable Impaired [Line Items]
With no specific allowance recorded, Unpaid Principal Balance	5,120	10,757	5,120	10,757	5,104
With no specific allowance recorded, Recorded Investment	1,044	2,076	1,044	2,076	1,047
With no specific allowance recorded, Average Recorded Investment	988	1,430	1,007	1,524
With no specific allowance recorded, Interest Income Recognized	1	25	4	53
With no specific allowance recorded, Cash Basis Income Recognized	1	25	4	52
With a specific allowance recorded, Unpaid Principal Balance	2,476	501	2,476	501	2,770
With a specific allowance recorded, Recorded Investment	2,075	456	2,075	456	2,309
With a specific allowance recorded, Allowance for Loan Losses Allocated	514	5	514	5	516
With a specific allowance recorded, Average Recorded Investment	2,161	1,179	2,210	2,221
With a specific allowance recorded, Interest Income Recognized	22	5	52	12
With a specific allowance recorded, Cash Basis Income Recognized	22	5	44	11
Commercial Loans [Member] | Multifamily [Member]
Financing Receivable Impaired [Line Items]
With no specific allowance recorded, Unpaid Principal Balance	28	470	28	470	41
With no specific allowance recorded, Recorded Investment		413		413
With no specific allowance recorded, Average Recorded Investment		416		277
With no specific allowance recorded, Interest Income Recognized				3
With no specific allowance recorded, Cash Basis Income Recognized				3
With a specific allowance recorded, Unpaid Principal Balance	422		422		422
With a specific allowance recorded, Recorded Investment	275		275		275
With a specific allowance recorded, Allowance for Loan Losses Allocated	28		28		28
With a specific allowance recorded, Average Recorded Investment	275		275
Commercial Loans [Member] | Nonresidential [Member]
Financing Receivable Impaired [Line Items]
With no specific allowance recorded, Unpaid Principal Balance	630	2,030	630	2,030	651
With no specific allowance recorded, Recorded Investment	137	1,464	137	1,464	144
With no specific allowance recorded, Average Recorded Investment	78	815	100	1,040
With no specific allowance recorded, Interest Income Recognized	1	25	4	50
With no specific allowance recorded, Cash Basis Income Recognized	1	25	4	49
With a specific allowance recorded, Unpaid Principal Balance	1,235	501	1,235	501	1,455
With a specific allowance recorded, Recorded Investment	1,232	456	1,232	456	1,423
With a specific allowance recorded, Allowance for Loan Losses Allocated	13	5	13	5	16
With a specific allowance recorded, Average Recorded Investment	1,323	1,179	1,356	2,164
With a specific allowance recorded, Interest Income Recognized	22	5	52	12
With a specific allowance recorded, Cash Basis Income Recognized	22	5	44	11
Commercial Loans [Member] | Land [Member]
Financing Receivable Impaired [Line Items]
With no specific allowance recorded, Unpaid Principal Balance	706	3,922	706	3,922	716
With no specific allowance recorded, Recorded Investment		9		9	9
With no specific allowance recorded, Average Recorded Investment	5	9	6	17
Commercial Loans [Member] | Construction [Member]
Financing Receivable Impaired [Line Items]
With no specific allowance recorded, Unpaid Principal Balance	2,457	3,593	2,457	3,593	2,467
Commercial Loans [Member] | Secured [Member]
Financing Receivable Impaired [Line Items]
With no specific allowance recorded, Unpaid Principal Balance	1,115	242	1,115	242	1,042
With no specific allowance recorded, Recorded Investment	907	190	907	190	894
With no specific allowance recorded, Average Recorded Investment	905	190	901	190
With a specific allowance recorded, Unpaid Principal Balance	819		819		893
With a specific allowance recorded, Recorded Investment	568		568		611
With a specific allowance recorded, Allowance for Loan Losses Allocated	473		473		472
With a specific allowance recorded, Average Recorded Investment	563		579	57
Commercial Loans [Member] | Unsecured [Member]
Financing Receivable Impaired [Line Items]
With no specific allowance recorded, Unpaid Principal Balance	184	500	184	500	187
Residential Mortgage Loans [Member]
Financing Receivable Impaired [Line Items]
With no specific allowance recorded, Unpaid Principal Balance	6,918	7,704	6,918	7,704	6,432
With no specific allowance recorded, Recorded Investment	5,994	5,762	5,994	5,762	5,441
With no specific allowance recorded, Average Recorded Investment	5,683	5,763	5,602	6,095
With no specific allowance recorded, Interest Income Recognized	30	20	74	50
With no specific allowance recorded, Cash Basis Income Recognized	30	20	58	41
With a specific allowance recorded, Unpaid Principal Balance	8,574	10,526	8,574	10,526	10,874
With a specific allowance recorded, Recorded Investment	8,461	10,428	8,461	10,428	10,699
With a specific allowance recorded, Allowance for Loan Losses Allocated	834	1,171	834	1,171	1,145
With a specific allowance recorded, Average Recorded Investment	9,340	10,783	9,793	10,764
With a specific allowance recorded, Interest Income Recognized	98	114	233	273
With a specific allowance recorded, Cash Basis Income Recognized	96	111	195	229
Residential Mortgage Loans [Member] | One-to Four-Family [Member]
Financing Receivable Impaired [Line Items]
With no specific allowance recorded, Unpaid Principal Balance	6,918	7,704	6,918	7,704	6,432
With no specific allowance recorded, Recorded Investment	5,994	5,762	5,994	5,762	5,441
With no specific allowance recorded, Average Recorded Investment	5,683	5,763	5,602	6,095
With no specific allowance recorded, Interest Income Recognized	30	20	74	50
With no specific allowance recorded, Cash Basis Income Recognized	30	20	58	41
With a specific allowance recorded, Unpaid Principal Balance	8,574	10,526	8,574	10,526	10,874
With a specific allowance recorded, Recorded Investment	8,461	10,428	8,461	10,428	10,699
With a specific allowance recorded, Allowance for Loan Losses Allocated	834	1,171	834	1,171	1,145
With a specific allowance recorded, Average Recorded Investment	9,340	10,783	9,793	10,764
With a specific allowance recorded, Interest Income Recognized	98	114	233	273
With a specific allowance recorded, Cash Basis Income Recognized	96	111	195	229
Consumer Loans [Member]
Financing Receivable Impaired [Line Items]
With no specific allowance recorded, Unpaid Principal Balance	2,599	3,099	2,599	3,099	2,562
With no specific allowance recorded, Recorded Investment	1,527	1,874	1,527	1,874	1,408
With no specific allowance recorded, Average Recorded Investment	1,440	1,958	1,429	1,962
With no specific allowance recorded, Interest Income Recognized	8	5	15	13
With no specific allowance recorded, Cash Basis Income Recognized	8	5	14	12
With a specific allowance recorded, Unpaid Principal Balance	4,770	5,779	4,770	5,779	5,439
With a specific allowance recorded, Recorded Investment	4,677	5,714	4,677	5,714	5,346
With a specific allowance recorded, Allowance for Loan Losses Allocated	313	440	313	440	398
With a specific allowance recorded, Average Recorded Investment	4,821	5,964	4,995	5,999
With a specific allowance recorded, Interest Income Recognized	63	74	136	168
With a specific allowance recorded, Cash Basis Income Recognized	62	73	122	150
Consumer Loans [Member] | Home Equity [Member]
Financing Receivable Impaired [Line Items]
With no specific allowance recorded, Unpaid Principal Balance	1,496	1,965	1,496	1,965	1,399
With no specific allowance recorded, Recorded Investment	1,224	1,535	1,224	1,535	1,059
With no specific allowance recorded, Average Recorded Investment	1,101	1,574	1,087	1,577
With no specific allowance recorded, Interest Income Recognized	6	3	11	8
With no specific allowance recorded, Cash Basis Income Recognized	6	3	10	7
With a specific allowance recorded, Unpaid Principal Balance	4,282	4,973	4,282	4,973	4,921
With a specific allowance recorded, Recorded Investment	4,201	4,919	4,201	4,919	4,840
With a specific allowance recorded, Allowance for Loan Losses Allocated	295	377	295	377	377
With a specific allowance recorded, Average Recorded Investment	4,338	5,151	4,505	5,212
With a specific allowance recorded, Interest Income Recognized	57	66	123	151
With a specific allowance recorded, Cash Basis Income Recognized	56	66	110	134
Consumer Loans [Member] | Auto [Member]
Financing Receivable Impaired [Line Items]
With no specific allowance recorded, Unpaid Principal Balance	29	34	29	34	29
With no specific allowance recorded, Recorded Investment	22	18	22	18	14
With no specific allowance recorded, Average Recorded Investment	42	16	33	11
Consumer Loans [Member] | Marine [Member]
Financing Receivable Impaired [Line Items]
With no specific allowance recorded, Unpaid Principal Balance	553	540	553	540	553
With no specific allowance recorded, Recorded Investment	181	169	181	169	181
With no specific allowance recorded, Average Recorded Investment	181	182	181	210
With a specific allowance recorded, Unpaid Principal Balance	95	104	95	104	100
With a specific allowance recorded, Recorded Investment	95	104	95	104	100
With a specific allowance recorded, Allowance for Loan Losses Allocated	1	1	1	1	1
With a specific allowance recorded, Average Recorded Investment	96	105	97	106
With a specific allowance recorded, Interest Income Recognized	1	1	2	3
With a specific allowance recorded, Cash Basis Income Recognized	1	1	2	3
Consumer Loans [Member] | Recreational Vehicle [Member]
Financing Receivable Impaired [Line Items]
With no specific allowance recorded, Unpaid Principal Balance	520	560	520	560	578
With no specific allowance recorded, Recorded Investment	100	152	100	152	151
With no specific allowance recorded, Average Recorded Investment	108	186	122	164
With no specific allowance recorded, Interest Income Recognized	2	2	4	5
With no specific allowance recorded, Cash Basis Income Recognized	2	2	4	5
With a specific allowance recorded, Unpaid Principal Balance	393	702	393	702	418
With a specific allowance recorded, Recorded Investment	381	691	381	691	406
With a specific allowance recorded, Allowance for Loan Losses Allocated	17	62	17	62	20
With a specific allowance recorded, Average Recorded Investment	387	708	393	681
With a specific allowance recorded, Interest Income Recognized	5	7	11	14
With a specific allowance recorded, Cash Basis Income Recognized	5	$ 6	10	$ 13
Consumer Loans [Member] | Other [Member]
Financing Receivable Impaired [Line Items]
With no specific allowance recorded, Unpaid Principal Balance	1		1		3
With no specific allowance recorded, Recorded Investment					$ 3
With no specific allowance recorded, Average Recorded Investment	$ 8		$ 6