Balance Sheet Offsetting - Financial Instruments Derivative Assets Liabilities and Resell Agreements Net of Amount Not Offset (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	$ 8,775	$ 3,810
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(5,340)	(3,308)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	(3,223)	(387)
Net Amount, Derivatives, Financial Assets	212	115
Net Amount Recognized, Resell agreements, Financial Assets	12,142	9,642
Gross Amounts Not Offset Financial Instruments, Resell Agreements	0	0
Gross Amounts Not Offset Collateral, Resell Agreements, Financial Assets	(12,142)	(9,642)
Net Amount, Financial Assets, Resell Agreements	0	0
Net Amount Recognized, Total Financial Assets	20,917	13,452
Gross Amounts Not Offset Collateral, Financial Assets	(15,365)	(10,029)
Net Amount, Financial Assets	212	115
Net Amount Recognized, Derivatives, Financial Liabilities	41,062	27,375
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(5,340)	(3,308)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(33,455)	(22,768)
Net Amount, Derivatives, Financial Liabilities	2,267	1,299
Net Amount Recognized, Repurchase Agreements, Financial Liabilities	1,074,576	1,117,199
Gross Amounts Not Offset Financial Instruments, Repurchase Agreements, Financial Liabilities	0	0
Gross Amounts Not Offset Collateral, Repurchase Agreements, Financial Liabilities	(1,074,576)	(1,117,199)
Net Amount, Repurchase Agreements, Financial Liabilities	0	0
Net Amount Recognized, Financial Liabilities	1,115,638	1,144,574
Gross Amounts Not Offset Collateral, Financial Liabilities	(1,108,031)	(1,139,967)
Net Amount, Financial Liabilities	2,267	1,299
Counterparty A [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	1,477	395
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(1,477)	(395)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	3,840	7,397
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(1,477)	(395)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(2,363)	(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	1,963	1,028
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(1,963)	(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	6,627	4,466
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(1,963)	(1,028)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(4,453)	(3,101)
Net Amount, Derivatives, Financial Liabilities	211	337
Counterparty C [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	75	55
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(75)	(55)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	806	1,520
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(75)	(55)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(731)	(1,450)
Net Amount, Derivatives, Financial Liabilities	0	15
Other Counterparties [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	5,260	2,332
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(1,825)	(1,830)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	(3,223)	(387)
Net Amount, Derivatives, Financial Assets	212	115
Net Amount Recognized, Derivatives, Financial Liabilities	29,789	13,992
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(1,825)	(1,830)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(25,908)	(11,215)
Net Amount, Derivatives, Financial Liabilities	$ 2,056	$ 947