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		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Financing Receivable Impaired [Line Items]
With no specific allowance recorded, Unpaid Principal Balance	$ 13,831	$ 14,485	$ 13,831	$ 14,485	$ 14,098
With no specific allowance recorded, Recorded Investment	7,962	8,325	7,962	8,325	7,896
With no specific allowance recorded, Average Recorded Investment	8,265	9,021	8,019	9,268
With no specific allowance recorded, Interest Income Recognized	67	46	165	114
With no specific allowance recorded, Cash Basis Income Recognized	51	44	132	106
With a specific allowance recorded, Unpaid Principal Balance	15,448	18,290	15,448	18,290	19,083
With a specific allowance recorded, Recorded Investment	14,835	18,029	14,835	18,029	18,354
With a specific allowance recorded, Allowance for Loan Losses Allocated	1,637	1,596	1,637	1,596	2,059
With a specific allowance recorded, Average Recorded Investment	15,025	17,314	16,459	18,746
With a specific allowance recorded, Interest Income Recognized	262	209	680	703
With a specific allowance recorded, Cash Basis Income Recognized	172	209	538	645
Total Unpaid Principal Balance	29,279	32,775	29,279	32,775	33,181
Total Recorded Investment	22,797	26,354	22,797	26,354	26,250
Total Average Recorded Investment	23,290	26,335	24,478	28,014
Total Interest Income Recognized	329	255	845	817
Total Cash Basis Income Recognized	223	253	670	751
Commercial Loans [Member]
Financing Receivable Impaired [Line Items]
With no specific allowance recorded, Unpaid Principal Balance	4,739	4,778	4,739	4,778	5,104
With no specific allowance recorded, Recorded Investment	1,055	774	1,055	774	1,047
With no specific allowance recorded, Average Recorded Investment	1,050	1,426	1,019	1,337
With no specific allowance recorded, Interest Income Recognized	9	4	14	10
With no specific allowance recorded, Cash Basis Income Recognized	9	4	14	10
With a specific allowance recorded, Unpaid Principal Balance	2,457	1,582	2,457	1,582	2,770
With a specific allowance recorded, Recorded Investment	2,053	1,548	2,053	1,548	2,309
With a specific allowance recorded, Allowance for Loan Losses Allocated	514	16	514	16	516
With a specific allowance recorded, Average Recorded Investment	2,064	1,002	2,171	2,053
With a specific allowance recorded, Interest Income Recognized	13	26	65	86
With a specific allowance recorded, Cash Basis Income Recognized	13	26	65	85
Commercial Loans [Member] | Multifamily [Member]
Financing Receivable Impaired [Line Items]
With no specific allowance recorded, Unpaid Principal Balance	25	466	25	466	41
With no specific allowance recorded, Recorded Investment		402		402
With no specific allowance recorded, Average Recorded Investment		408		309
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	624	673	624	673	651
With no specific allowance recorded, Recorded Investment	134	153	134	153	144
With no specific allowance recorded, Average Recorded Investment	136	809	108	818
With no specific allowance recorded, Interest Income Recognized	1	3	5	6
With no specific allowance recorded, Cash Basis Income Recognized	1	3	5	6
With a specific allowance recorded, Unpaid Principal Balance	1,216	1,582	1,216	1,582	1,455
With a specific allowance recorded, Recorded Investment	1,210	1,548	1,210	1,548	1,423
With a specific allowance recorded, Allowance for Loan Losses Allocated	13	16	13	16	16
With a specific allowance recorded, Average Recorded Investment	1,221	1,002	1,320	2,010
With a specific allowance recorded, Interest Income Recognized	13	26	65	86
With a specific allowance recorded, Cash Basis Income Recognized	13	26	65	85
Commercial Loans [Member] | Land [Member]
Financing Receivable Impaired [Line Items]
With no specific allowance recorded, Unpaid Principal Balance		715		715	716
With no specific allowance recorded, Recorded Investment		9		9	9
With no specific allowance recorded, Average Recorded Investment		9	5	15
Commercial Loans [Member] | Construction [Member]
Financing Receivable Impaired [Line Items]
With no specific allowance recorded, Unpaid Principal Balance	2,447	2,467	2,447	2,467	2,467
Commercial Loans [Member] | Secured [Member]
Financing Receivable Impaired [Line Items]
With no specific allowance recorded, Unpaid Principal Balance	1,169	263	1,169	263	1,042
With no specific allowance recorded, Recorded Investment	921	210	921	210	894
With no specific allowance recorded, Average Recorded Investment	914	200	906	195
With no specific allowance recorded, Interest Income Recognized		1	1	1
With no specific allowance recorded, Cash Basis Income Recognized		1	1	1
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	568		576	43
Commercial Loans [Member] | Unsecured [Member]
Financing Receivable Impaired [Line Items]
With no specific allowance recorded, Unpaid Principal Balance	474	194	474	194	187
With no specific allowance recorded, Interest Income Recognized	8		8
With no specific allowance recorded, Cash Basis Income Recognized	8		8
Residential Mortgage Loans [Member]
Financing Receivable Impaired [Line Items]
With no specific allowance recorded, Unpaid Principal Balance	6,578	6,793	6,578	6,793	6,432
With no specific allowance recorded, Recorded Investment	5,424	5,814	5,424	5,814	5,441
With no specific allowance recorded, Average Recorded Investment	5,709	5,788	5,558	6,025
With no specific allowance recorded, Interest Income Recognized	41	35	117	81
With no specific allowance recorded, Cash Basis Income Recognized	32	33	92	73
With a specific allowance recorded, Unpaid Principal Balance	8,455	11,042	8,455	11,042	10,874
With a specific allowance recorded, Recorded Investment	8,340	10,904	8,340	10,904	10,699
With a specific allowance recorded, Allowance for Loan Losses Allocated	817	1,169	817	1,169	1,145
With a specific allowance recorded, Average Recorded Investment	8,401	10,666	9,430	10,799
With a specific allowance recorded, Interest Income Recognized	147	112	378	385
With a specific allowance recorded, Cash Basis Income Recognized	98	112	291	343
Residential Mortgage Loans [Member] | One-to Four-Family [Member]
Financing Receivable Impaired [Line Items]
With no specific allowance recorded, Unpaid Principal Balance	6,578	6,793	6,578	6,793	6,432
With no specific allowance recorded, Recorded Investment	5,424	5,814	5,424	5,814	5,441
With no specific allowance recorded, Average Recorded Investment	5,709	5,788	5,558	6,025
With no specific allowance recorded, Interest Income Recognized	41	35	117	81
With no specific allowance recorded, Cash Basis Income Recognized	32	33	92	73
With a specific allowance recorded, Unpaid Principal Balance	8,455	11,042	8,455	11,042	10,874
With a specific allowance recorded, Recorded Investment	8,340	10,904	8,340	10,904	10,699
With a specific allowance recorded, Allowance for Loan Losses Allocated	817	1,169	817	1,169	1,145
With a specific allowance recorded, Average Recorded Investment	8,401	10,666	9,430	10,799
With a specific allowance recorded, Interest Income Recognized	147	112	378	385
With a specific allowance recorded, Cash Basis Income Recognized	98	112	291	343
Consumer Loans [Member]
Financing Receivable Impaired [Line Items]
With no specific allowance recorded, Unpaid Principal Balance	2,514	2,914	2,514	2,914	2,562
With no specific allowance recorded, Recorded Investment	1,483	1,737	1,483	1,737	1,408
With no specific allowance recorded, Average Recorded Investment	1,506	1,807	1,442	1,906
With no specific allowance recorded, Interest Income Recognized	17	7	34	23
With no specific allowance recorded, Cash Basis Income Recognized	10	7	26	23
With a specific allowance recorded, Unpaid Principal Balance	4,536	5,666	4,536	5,666	5,439
With a specific allowance recorded, Recorded Investment	4,442	5,577	4,442	5,577	5,346
With a specific allowance recorded, Allowance for Loan Losses Allocated	306	411	306	411	398
With a specific allowance recorded, Average Recorded Investment	4,560	5,646	4,858	5,894
With a specific allowance recorded, Interest Income Recognized	102	71	237	232
With a specific allowance recorded, Cash Basis Income Recognized	61	71	182	217
Consumer Loans [Member] | Home Equity [Member]
Financing Receivable Impaired [Line Items]
With no specific allowance recorded, Unpaid Principal Balance	1,262	1,556	1,262	1,556	1,399
With no specific allowance recorded, Recorded Investment	1,044	1,177	1,044	1,177	1,059
With no specific allowance recorded, Average Recorded Investment	1,134	1,356	1,076	1,477
With no specific allowance recorded, Interest Income Recognized	6	4	17	11
With no specific allowance recorded, Cash Basis Income Recognized	4	4	15	11
With a specific allowance recorded, Unpaid Principal Balance	4,156	5,132	4,156	5,132	4,921
With a specific allowance recorded, Recorded Investment	4,074	5,055	4,074	5,055	4,840
With a specific allowance recorded, Allowance for Loan Losses Allocated	289	387	289	387	377
With a specific allowance recorded, Average Recorded Investment	4,138	4,987	4,398	5,173
With a specific allowance recorded, Interest Income Recognized	92	65	215	214
With a specific allowance recorded, Cash Basis Income Recognized	56	65	165	199
Consumer Loans [Member] | Auto [Member]
Financing Receivable Impaired [Line Items]
With no specific allowance recorded, Unpaid Principal Balance	26	8	26	8	29
With no specific allowance recorded, Recorded Investment	19	1	19	1	14
With no specific allowance recorded, Average Recorded Investment	21	10	29	9
With no specific allowance recorded, Interest Income Recognized	1		1
With no specific allowance recorded, Cash Basis Income Recognized	1		1
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	169	181	200
With a specific allowance recorded, Unpaid Principal Balance	93	102	93	102	100
With a specific allowance recorded, Recorded Investment	93	102	93	102	100
With a specific allowance recorded, Allowance for Loan Losses Allocated	1	1	1	1	1
With a specific allowance recorded, Average Recorded Investment	94	103	96	105
With a specific allowance recorded, Interest Income Recognized	2	1	5	4
With a specific allowance recorded, Cash Basis Income Recognized	1	1	4	4
Consumer Loans [Member] | Recreational Vehicle [Member]
Financing Receivable Impaired [Line Items]
With no specific allowance recorded, Unpaid Principal Balance	652	809	652	809	578
With no specific allowance recorded, Recorded Investment	218	389	218	389	151
With no specific allowance recorded, Average Recorded Investment	159	271	146	220
With no specific allowance recorded, Interest Income Recognized	9	3	15	12
With no specific allowance recorded, Cash Basis Income Recognized	4	3	9	12
With a specific allowance recorded, Unpaid Principal Balance	287	432	287	432	418
With a specific allowance recorded, Recorded Investment	275	420	275	420	406
With a specific allowance recorded, Allowance for Loan Losses Allocated	16	23	16	23	20
With a specific allowance recorded, Average Recorded Investment	328	556	364	616
With a specific allowance recorded, Interest Income Recognized	8	5	17	14
With a specific allowance recorded, Cash Basis Income Recognized	4	5	13	14
Consumer Loans [Member] | Other [Member]
Financing Receivable Impaired [Line Items]
With no specific allowance recorded, Unpaid Principal Balance	21	1	21	1	3
With no specific allowance recorded, Recorded Investment	21	1	21	$ 1	$ 3
With no specific allowance recorded, Average Recorded Investment	11	$ 1	10
With no specific allowance recorded, Interest Income Recognized	1		1
With no specific allowance recorded, Cash Basis Income Recognized	$ 1		$ 1