Derivative Financial Instruments (Derivative Asset and Liability Balance Sheet Offsetting) (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Offsetting Assets [Line Items]
Net amount presented in the Statements of Financial Condition	$ 78,960	$ 52,069	$ 54,273
Derivative Liabilities
Net amount presented in the Statements of Financial Condition	55,699	35,671	41,531
Interest Rate Contract
Offsetting Assets [Line Items]
Gross Amounts Recognized	72,970	48,985	48,168
Less: Amounts offset in the Statements of Financial Condition	0	0	0
Net amount presented in the Statements of Financial Condition	72,970	48,985	48,168
Offsetting Derivative Positions	(9,627)	(14,878)	(16,529)
Collateral Posted	(54,490)	(18,060)	(6,990)
Net Credit Exposure	8,853	16,047	24,649
Derivative Liabilities
Gross Amounts Recognized	52,443	33,716	35,459
Less: Amounts offset in the Statements of Financial Condition	0	0	0
Net amount presented in the Statements of Financial Condition	52,443	33,716	35,459
Offsetting Derivative Positions	(9,627)	(14,878)	(16,529)
Collateral Posted	(340)	(2,220)	(12,300)
Net Credit Exposure	$ 42,476	$ 16,618	$ 6,630