Offsetting of Derivative Assets And Liabilities (Parenthetical) (Detail) - USD ($)	Sep. 30, 2017	Dec. 31, 2016
Offsetting Derivatives Financial Assets And Financial Liabilities [Line Items]
Excess cash collateral received on derivatives	$ 2,088,000	$ 0
Excess cash collateral provided on derivatives	0	0
OTC - Bilateral
Offsetting Derivatives Financial Assets And Financial Liabilities [Line Items]
Excess securities collateral received on derivatives	2,040,000	395,000
Excess securities collateral provided on derivatives	1,629,000	1,878,000
OTC - Cleared
Offsetting Derivatives Financial Assets And Financial Liabilities [Line Items]
Excess securities collateral provided on derivatives	$ 11,391,000	$ 11,291,000