Effect of Master Netting Arrangements on Presentation of Derivative Assets and Liabilities in Consolidated Statements of Financial Condition (Detail) - USD ($) $ in Thousands	Sep. 30, 2017		Dec. 31, 2016
Derivative Financial Instruments, Liabilities
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Liabilities	$ 1,466		$ 23,728
Gross Amounts Offset in the Statements of Condition	1,248		16,588
Net Amounts of Liabilities Presented in the Statements of Condition	218		7,140
Financial Instruments	0		0
Cash Collateral Received	0		0
Net Amount	218		7,140
Derivative Financial Instruments, Assets
Derivatives, Fair Value [Line Items]
Gross Amounts Of Recognized Assets	1,262	[1]	20,422	[2]
Gross Amounts Offset in the Statements of Condition	674		17,861
Net Amounts of Assets Presented in the Statements of Condition	588		2,561
Financial Instruments	0		0
Cash Collateral Received	0		0
Net Amount	$ 588		$ 2,561

[1] Includes $144 thousand to purchase Treasury options.

[2] Includes $1.9 million to purchase Treasury options.