Balance Sheet Offsetting - Financial Instruments Derivative Assets Liabilities and Resell Agreements Net of Amount Not Offset (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	$ 3,457	$ 3,798
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(3,098)	(3,565)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	(130)	(64)
Net Amount, Derivatives, Financial Assets	229	169
Net Amount Recognized, Resell agreements, Financial Assets	17,642	9,642
Gross Amounts Not Offset Financial Instruments, Resell Agreements	0	0
Gross Amounts Not Offset Collateral, Resell Agreements, Financial Assets	(17,642)	(9,642)
Net Amount, Financial Assets, Resell Agreements	0	0
Net Amount Recognized, Total Financial Assets	21,099	13,440
Gross Amounts Not Offset Collateral, Financial Assets	(17,772)	(9,706)
Net Amount, Financial Assets	229	169
Net Amount Recognized, Derivatives, Financial Liabilities	16,883	32,534
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(3,098)	(3,565)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(13,691)	(27,347)
Net Amount, Derivatives, Financial Liabilities	94	1,622
Net Amount Recognized, Repurchase Agreements, Financial Liabilities	987,869	963,317
Gross Amounts Not Offset Financial Instruments, Repurchase Agreements, Financial Liabilities	0	0
Gross Amounts Not Offset Collateral, Repurchase Agreements, Financial Liabilities	(987,869)	(963,317)
Net Amount, Repurchase Agreements, Financial Liabilities	0	0
Net Amount Recognized, Financial Liabilities	1,004,752	995,851
Gross Amounts Not Offset Collateral, Financial Liabilities	(1,001,560)	(990,664)
Net Amount, Financial Liabilities	94	1,622
Counterparty A [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	397	687
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(397)	(687)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	8,984	11,233
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(397)	(687)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(8,587)	(10,026)
Net Amount, Derivatives, Financial Liabilities	0	520
Counterparty B [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	866	223
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(866)	(223)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	3,535	6,867
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(866)	(223)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(2,669)	(6,344)
Net Amount, Derivatives, Financial Liabilities	0	300
Counterparty C [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	204	158
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(204)	(158)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	1,128	4,578
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(204)	(158)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(830)	(4,415)
Net Amount, Derivatives, Financial Liabilities	94	5
Counterparty D [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	0	1,820
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	0	(1,820)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	0	7,706
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	0	(1,820)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	0	(5,886)
Net Amount, Derivatives, Financial Liabilities	0	0
Other Counterparties [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	1,990	910
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(1,631)	(677)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	(130)	(64)
Net Amount, Derivatives, Financial Assets	229	169
Net Amount Recognized, Derivatives, Financial Liabilities	3,236	2,150
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(1,631)	(677)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(1,605)	(676)
Net Amount, Derivatives, Financial Liabilities	$ 0	$ 797