Balance Sheet Offsetting - Financial Instruments Derivative Assets Liabilities and Resell Agreements Net of Amount Not Offset (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	$ 8,068	$ 3,810
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(4,970)	(3,308)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	(2,873)	(387)
Net Amount, Derivatives, Financial Assets	225	115
Net Amount Recognized, Resell agreements, Financial Assets	9,642	9,642
Gross Amounts Not Offset Financial Instruments, Resell Agreements	0	0
Gross Amounts Not Offset Collateral, Resell Agreements, Financial Assets	(9,642)	(9,642)
Net Amount, Financial Assets, Resell Agreements	0	0
Net Amount Recognized, Total Financial Assets	17,710	13,452
Gross Amounts Not Offset Collateral, Financial Assets	(12,515)	(10,029)
Net Amount, Financial Assets	225	115
Net Amount Recognized, Derivatives, Financial Liabilities	38,345	27,375
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(4,970)	(3,308)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(31,356)	(22,768)
Net Amount, Derivatives, Financial Liabilities	2,019	1,299
Net Amount Recognized, Repurchase Agreements, Financial Liabilities	968,420	1,117,199
Gross Amounts Not Offset Financial Instruments, Repurchase Agreements, Financial Liabilities	0	0
Gross Amounts Not Offset Collateral, Repurchase Agreements, Financial Liabilities	(968,420)	(1,117,199)
Net Amount, Repurchase Agreements, Financial Liabilities	0	0
Net Amount Recognized, Financial Liabilities	1,006,765	1,144,574
Gross Amounts Not Offset Collateral, Financial Liabilities	(999,776)	(1,139,967)
Net Amount, Financial Liabilities	2,019	1,299
Counterparty A [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	1,234	395
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(1,234)	(395)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	4,660	7,397
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(1,234)	(395)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(3,426)	(7,002)
Net Amount, Derivatives, Financial Liabilities	0	0
Counterparty B [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	2,211	1,028
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(2,211)	(1,028)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	5,333	4,466
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(2,211)	(1,028)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(2,358)	(3,101)
Net Amount, Derivatives, Financial Liabilities	764	337
Counterparty C [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	77	55
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(77)	(55)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	1,195	1,520
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(77)	(55)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(1,040)	(1,450)
Net Amount, Derivatives, Financial Liabilities	78	15
Other Counterparties [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	4,546	2,332
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(1,448)	(1,830)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	(2,873)	(387)
Net Amount, Derivatives, Financial Assets	225	115
Net Amount Recognized, Derivatives, Financial Liabilities	27,157	13,992
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(1,448)	(1,830)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(24,532)	(11,215)
Net Amount, Derivatives, Financial Liabilities	$ 1,177	$ 947