Summary of Financial Effects of Troubled Debt Restructurings (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Investment	Dec. 31, 2016 USD ($) Investment	Dec. 31, 2015 USD ($) Investment
Financing Receivable, Modifications [Line Items]
Number of loans classified as a non-accrual TDRs | Investment	71	13	8,000
Pre-Modification Recorded Investment	$ 61,641	$ 14,937	$ 2,106
Post-Modification Recorded Investment	36,047	13,100	2,144
Trouble debt restructuring, charge-off amount	14,273	170	33
Capitalized interest	$ 12	$ 11	$ 8
Multi-Family
Financing Receivable, Modifications [Line Items]
Number of loans classified as a non-accrual TDRs | Investment		1
Pre-Modification Recorded Investment		$ 9,340
Post-Modification Recorded Investment		$ 8,129
Weighted Average Interest Rate, Pre-Modification		4.63%
Weighted Average Interest Rate, Post-Modification		4.00%
One-to-four family
Financing Receivable, Modifications [Line Items]
Number of loans classified as a non-accrual TDRs | Investment	4	5	4,000
Pre-Modification Recorded Investment	$ 810	$ 900	$ 568
Post-Modification Recorded Investment	$ 986	$ 1,036	$ 619
Weighted Average Interest Rate, Pre-Modification	5.93%	4.26%	4.02%
Weighted Average Interest Rate, Post-Modification	2.21%	2.65%	2.72%
Capitalized interest	$ 12	$ 11	$ 6
Commercial and Industrial
Financing Receivable, Modifications [Line Items]
Number of loans classified as a non-accrual TDRs | Investment	65	7	2,000
Pre-Modification Recorded Investment	$ 52,179	$ 4,697	$ 1,345
Post-Modification Recorded Investment	$ 26,409	$ 3,935	$ 1,312
Weighted Average Interest Rate, Pre-Modification	3.36%	3.22%	3.40%
Weighted Average Interest Rate, Post-Modification	3.26%	3.19%	3.52%
Trouble debt restructuring, charge-off amount	$ 14,273	$ 170	$ 33
Other loans
Financing Receivable, Modifications [Line Items]
Number of loans classified as a non-accrual TDRs | Investment			2,000
Pre-Modification Recorded Investment			$ 193
Post-Modification Recorded Investment			$ 213
Weighted Average Interest Rate, Pre-Modification			4.58%
Weighted Average Interest Rate, Post-Modification			2.00%
Capitalized interest			$ 2
Acquisition, Development and Construction
Financing Receivable, Modifications [Line Items]
Number of loans classified as a non-accrual TDRs | Investment	2
Pre-Modification Recorded Investment	$ 8,652
Post-Modification Recorded Investment	$ 8,652
Weighted Average Interest Rate, Pre-Modification	5.50%
Weighted Average Interest Rate, Post-Modification	5.50%