Offsetting of Derivative Assets And Liabilities (Parenthetical) (Detail) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Offsetting Derivatives Financial Assets And Financial Liabilities [Line Items]
Excess cash collateral received on derivatives	$ 2,004,000	$ 2,213,000
Excess cash collateral provided on derivatives	294,000	0
OTC - Bilateral
Offsetting Derivatives Financial Assets And Financial Liabilities [Line Items]
Excess securities collateral received on derivatives	2,995,000	4,214,000
Excess securities collateral provided on derivatives	1,052,000	146,000
OTC - Cleared
Offsetting Derivatives Financial Assets And Financial Liabilities [Line Items]
Excess securities collateral provided on derivatives	$ 10,846,000	$ 11,310,000