Fair Value Measurement - 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	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Impaired Loans [Member] | Multifamily [Member] | Commercial Loans [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Fair Value	$ 247	$ 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%	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%	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%	15.00%
Impaired Loans [Member] | Nonresidential [Member] | Commercial Loans [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Fair Value		$ 2
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%
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%
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%
Impaired Loans [Member] | Auto [Member] | Consumer Loans [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Fair Value	$ 16
Impaired Loans [Member] | Auto [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%
Impaired Loans [Member] | Auto [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%
Impaired Loans [Member] | Auto [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%
Impaired Loans [Member] | Marine [Member] | Consumer Loans [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Fair Value	$ 38
Impaired Loans [Member] | Marine [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%
Impaired Loans [Member] | Marine [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%
Impaired Loans [Member] | Marine [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%
Impaired Loans [Member] | Secured [Member] | Commercial Loans [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Fair Value		$ 40
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%
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%
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%
Impaired Loans [Member] | Home Equity [Member] | Consumer Loans [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Fair Value		$ 89
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%
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%
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%
Impaired Loans [Member] | Residential Mortgage Loans [Member] | One-to Four-Family [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Fair Value	$ 131	$ 1,010
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	$ 227	$ 354
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] | Maximum [Member] | Sales Comparison Approach [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Adjustment for differences between comparable sales	50.00%	52.90%
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	45.85%	52.41%
Other Real Estate Owned [Member] | Residential Mortgage Loans [Member] | One-to Four-Family [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Fair Value	$ 59	$ 82
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	14.22%	13.43%
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	14.22%	13.43%