Derivative Financial Instruments (Derivative Asset and Liability Balance Sheet Offsetting) (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Offsetting Assets [Line Items]
Net amount presented in the Statements of Financial Condition	$ 96,153	$ 52,069	$ 48,198
Derivative Liabilities
Net amount presented in the Statements of Financial Condition	71,381	35,671	35,773
Interest Rate Contract
Offsetting Assets [Line Items]
Gross Amounts Recognized	91,262	48,985	45,168
Less: Amounts offset in the Statements of Financial Condition	0	0	0
Net amount presented in the Statements of Financial Condition	91,262	48,985	45,168
Offsetting Derivative Positions	(7,887)	(14,878)	(16,213)
Collateral Posted	(76,530)	(18,060)	(2,950)
Net Credit Exposure	6,845	16,047	26,005
Derivative Liabilities
Gross Amounts Recognized	69,342	33,716	34,035
Less: Amounts offset in the Statements of Financial Condition	0	0	0
Net amount presented in the Statements of Financial Condition	69,342	33,716	34,035
Offsetting Derivative Positions	(7,887)	(14,878)	(16,213)
Collateral Posted	(340)	(2,220)	(17,130)
Net Credit Exposure	$ 61,115	$ 16,618	$ 692