Fair Value - Quantitative Information About Level 3 Fair Value Measurements for Financial Instruments Measured at Fair Value on Nonrecurring Basis (Detail) - Fair Value, Measurements, Nonrecurring [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Impaired Loans [Member] | Multifamily [Member] | Commercial Loans [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Fair Value	$ 247
Impaired Loans [Member] | Multifamily [Member] | Commercial Loans [Member] | Minimum [Member] | Sales Comparison Approach [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Adjustment for differences between comparable sales	0.00%
Impaired Loans [Member] | Multifamily [Member] | Commercial Loans [Member] | Maximum [Member] | Sales Comparison Approach [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Adjustment for differences between comparable sales	35.00%
Impaired Loans [Member] | Multifamily [Member] | Commercial Loans [Member] | Weighted Average [Member] | Sales Comparison Approach [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Adjustment for differences between comparable sales	15.00%
Impaired Loans [Member] | Nonresidential [Member] | Commercial Loans [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Fair Value	$ 2	$ 2,257
Impaired Loans [Member] | Nonresidential [Member] | Commercial Loans [Member] | Minimum [Member] | Sales Comparison Approach [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Adjustment for differences between comparable sales	0.00%	0.00%
Impaired Loans [Member] | Nonresidential [Member] | Commercial Loans [Member] | Maximum [Member] | Sales Comparison Approach [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Adjustment for differences between comparable sales	35.00%	35.00%
Impaired Loans [Member] | Nonresidential [Member] | Commercial Loans [Member] | Weighted Average [Member] | Sales Comparison Approach [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Adjustment for differences between comparable sales	15.00%	15.00%
Impaired Loans [Member] | Secured [Member] | Commercial Loans [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Fair Value	$ 40	$ 284
Impaired Loans [Member] | Secured [Member] | Commercial Loans [Member] | Minimum [Member] | Sales Comparison Approach [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Adjustment for differences between comparable sales	0.00%	0.00%
Impaired Loans [Member] | Secured [Member] | Commercial Loans [Member] | Maximum [Member] | Sales Comparison Approach [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Adjustment for differences between comparable sales	64.00%	64.00%
Impaired Loans [Member] | Secured [Member] | Commercial Loans [Member] | Weighted Average [Member] | Sales Comparison Approach [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Adjustment for differences between comparable sales	16.00%	16.00%
Impaired Loans [Member] | Home Equity [Member] | Consumer Loans [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Fair Value	$ 89	$ 228
Impaired Loans [Member] | Home Equity [Member] | Consumer Loans [Member] | Minimum [Member] | Sales Comparison Approach [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Adjustment for differences between comparable sales	0.00%	0.00%
Impaired Loans [Member] | Home Equity [Member] | Consumer Loans [Member] | Maximum [Member] | Sales Comparison Approach [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Adjustment for differences between comparable sales	17.85%	17.85%
Impaired Loans [Member] | Home Equity [Member] | Consumer Loans [Member] | Weighted Average [Member] | Sales Comparison Approach [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Adjustment for differences between comparable sales	8.93%	8.93%
Impaired Loans [Member] | Residential Mortgage Loans [Member] | One-to Four-Family [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Fair Value	$ 1,010	$ 919
Impaired Loans [Member] | Residential Mortgage Loans [Member] | One-to Four-Family [Member] | Minimum [Member] | Sales Comparison Approach [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Adjustment for differences between comparable sales	0.00%	0.00%
Impaired Loans [Member] | Residential Mortgage Loans [Member] | One-to Four-Family [Member] | Maximum [Member] | Sales Comparison Approach [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Adjustment for differences between comparable sales	10.77%	10.77%
Impaired Loans [Member] | Residential Mortgage Loans [Member] | One-to Four-Family [Member] | Weighted Average [Member] | Sales Comparison Approach [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Adjustment for differences between comparable sales	4.27%	4.27%
Other Real Estate Owned [Member] | Construction [Member] | Commercial Loans [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Fair Value	$ 354	$ 748
Other Real Estate Owned [Member] | Construction [Member] | Commercial Loans [Member] | Minimum [Member] | Sales Comparison Approach [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Adjustment for differences between comparable sales	0.00%	0.00%
Other Real Estate Owned [Member] | Construction [Member] | Commercial Loans [Member] | Minimum [Member] | Cost Approach [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Adjustment for differences between comparable sales		0.00%
Other Real Estate Owned [Member] | Construction [Member] | Commercial Loans [Member] | Maximum [Member] | Sales Comparison Approach [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Adjustment for differences between comparable sales	52.90%	90.40%
Other Real Estate Owned [Member] | Construction [Member] | Commercial Loans [Member] | Maximum [Member] | Cost Approach [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Adjustment for differences between comparable sales		33.33%
Other Real Estate Owned [Member] | Construction [Member] | Commercial Loans [Member] | Weighted Average [Member] | Sales Comparison Approach [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Adjustment for differences between comparable sales	52.41%	27.46%
Other Real Estate Owned [Member] | Construction [Member] | Commercial Loans [Member] | Weighted Average [Member] | Cost Approach [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Adjustment for differences between comparable sales		16.67%
Other Real Estate Owned [Member] | Residential Mortgage Loans [Member] | One-to Four-Family [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Fair Value	$ 82	$ 281
Other Real Estate Owned [Member] | Residential Mortgage Loans [Member] | One-to Four-Family [Member] | Minimum [Member] | Sales Comparison Approach [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Adjustment for differences between comparable sales	0.00%	0.00%
Other Real Estate Owned [Member] | Residential Mortgage Loans [Member] | One-to Four-Family [Member] | Maximum [Member] | Sales Comparison Approach [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Adjustment for differences between comparable sales	13.43%	27.00%
Other Real Estate Owned [Member] | Residential Mortgage Loans [Member] | One-to Four-Family [Member] | Weighted Average [Member] | Sales Comparison Approach [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Adjustment for differences between comparable sales	13.43%	7.74%