Balance Sheet Offsetting - Financial Instruments Derivative Assets Liabilities and Resell Agreements Net of Amount Not Offset (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	$ 10,657	$ 27,849
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(10,526)	(6,143)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	(21,651)
Net Amount, Derivatives, Financial Assets	131	55
Net Amount Recognized, Resell agreements, Financial Assets	10,193	11,642
Gross Amounts Not Offset Financial Instruments, Resell Agreements	0	0
Gross Amounts Not Offset Collateral, Resell Agreements, Financial Assets	(10,193)	(11,642)
Net Amount, Financial Assets, Resell Agreements	0	0
Net Amount Recognized, Total Financial Assets	20,850	39,491
Gross Amounts Not Offset Collateral, Financial Assets	(10,193)	(33,293)
Net Amount, Financial Assets	131	55
Net Amount Recognized, Derivatives, Financial Liabilities	24,155	11,185
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(10,526)	(6,143)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(13,550)	(4,621)
Net Amount, Derivatives, Financial Liabilities	79	421
Net Amount Recognized, Repurchase Agreements, Financial Liabilities	1,308,257	1,360,298
Gross Amounts Not Offset Financial Instruments, Repurchase Agreements, Financial Liabilities	0	0
Gross Amounts Not Offset Collateral, Repurchase Agreements, Financial Liabilities	(1,308,257)	(1,360,298)
Net Amount, Repurchase Agreements, Financial Liabilities	0	0
Net Amount Recognized, Financial Liabilities	1,332,412	1,371,483
Gross Amounts Not Offset Collateral, Financial Liabilities	(1,321,807)	(1,364,919)
Net Amount, Financial Liabilities	79	421
Counterparty A [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	104	598
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(104)	(598)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	5,706	4,293
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(104)	(598)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(5,602)	(3,651)
Net Amount, Derivatives, Financial Liabilities	0	44
Counterparty B [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	3,937	7,255
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(3,937)	(3,380)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	(3,875)
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	7,224	3,380
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(3,937)	(3,380)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(3,287)	0
Net Amount, Derivatives, Financial Liabilities	0	0
Counterparty C [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	16	81
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(16)	(81)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	184	326
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(16)	(81)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(168)	(245)
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	6,600	19,915
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(6,469)	(2,084)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	(17,776)
Net Amount, Derivatives, Financial Assets	131	55
Net Amount Recognized, Derivatives, Financial Liabilities	11,041	3,186
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(6,469)	(2,084)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(4,493)	(725)
Net Amount, Derivatives, Financial Liabilities	$ 79	$ 377