Derivative Financial Instruments (Derivative Asset and Liability Balance Sheet Offsetting) (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Offsetting Assets [Line Items]
Net amount presented in the Statements of Financial Condition	$ 92,156	$ 52,069	$ 49,703
Derivative Liabilities
Net amount presented in the Statements of Financial Condition	69,364	35,671	37,651
Interest Rate Contract
Offsetting Assets [Line Items]
Gross Amounts Recognized	86,435	48,985	46,647
Less: Amounts offset in the Statements of Financial Condition	0	0	0
Net amount presented in the Statements of Financial Condition	86,435	48,985	46,647
Offsetting Derivative Positions	(10,719)	(14,878)	(15,828)
Collateral Posted	(61,880)	(18,060)	(3,150)
Net Credit Exposure	13,836	16,047	27,669
Derivative Liabilities
Gross Amounts Recognized	64,368	33,716	35,343
Less: Amounts offset in the Statements of Financial Condition	0	0	0
Net amount presented in the Statements of Financial Condition	64,368	33,716	35,343
Offsetting Derivative Positions	(10,719)	(14,878)	(15,828)
Collateral Posted (1)	(340)	(2,220)	(19,515)
Net Credit Exposure	$ 53,309	$ 16,618	0
Collateral posted			$ 21,000