Derivative Financial Instruments (Derivative Asset and Liability Balance Sheet Offsetting) (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Offsetting Assets [Line Items]
Gross Amounts Recognized	$ 108,784	$ 73,172	$ 92,156
Derivative Liabilities
Gross Amounts Recognized	136,312	68,088	69,364
Interest Rate Contract
Offsetting Assets [Line Items]
Gross Amounts Recognized	104,562	68,425	86,435
Less: Amounts offset in the Statements of Financial Condition	0	0	0
Net amount presented in the Statements of Financial Condition	104,562	68,425	86,435
Gross amounts not offset in the Statements of Financial Condition
Offsetting Derivative Positions	(3,934)	(28,124)	(10,719)
Collateral Posted	0	(23,810)	(61,880)
Net Credit Exposure	100,628	16,491	13,836
Derivative Liabilities
Gross Amounts Recognized	131,035	62,571	64,368
Less: Amounts offset in the Statements of Financial Condition	0	0	0
Net amount presented in the Statements of Financial Condition	131,035	62,571	64,368
Gross amounts not offset in the Statements of Financial Condition
Offsetting Derivative Positions	(3,934)	(28,124)	(10,719)
Collateral Posted	(127,101)	(2,640)	(340)
Net Credit Exposure	$ 0	$ 31,807	$ 53,309