Balance Sheet Offsetting - Financial Instruments Derivative Assets Liabilities and Resell Agreements Net of Amount Not Offset (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	$ 8,105	$ 27,849
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(7,345)	(6,143)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	(235)	(21,651)
Net Amount, Derivatives, Financial Assets	525	55
Net Amount Recognized, Resell agreements, Financial Assets	9,642	11,642
Gross Amounts Not Offset Financial Instruments, Resell Agreements	0	0
Gross Amounts Not Offset Collateral, Resell Agreements, Financial Assets	(9,642)	(11,642)
Net Amount, Financial Assets, Resell Agreements	0	0
Net Amount Recognized, Total Financial Assets	17,747	39,491
Gross Amounts Not Offset Collateral, Financial Assets	(9,877)	(33,293)
Net Amount, Financial Assets	525	55
Net Amount Recognized, Derivatives, Financial Liabilities	20,893	11,185
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(7,345)	(6,143)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(13,263)	(4,621)
Net Amount, Derivatives, Financial Liabilities	285	421
Net Amount Recognized, Repurchase Agreements, Financial Liabilities	1,210,224	1,360,298
Gross Amounts Not Offset Financial Instruments, Repurchase Agreements, Financial Liabilities	0	0
Gross Amounts Not Offset Collateral, Repurchase Agreements, Financial Liabilities	(1,210,224)	(1,360,298)
Net Amount, Repurchase Agreements, Financial Liabilities	0	0
Net Amount Recognized, Financial Liabilities	1,231,117	1,371,483
Gross Amounts Not Offset Collateral, Financial Liabilities	(1,223,487)	(1,364,919)
Net Amount, Financial Liabilities	285	421
Counterparty A [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	320	598
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(320)	(598)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	4,753	4,293
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(320)	(598)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(4,433)	(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	3,022	7,255
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(3,022)	(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	6,033	3,380
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(3,022)	(3,380)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(3,011)	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	34	81
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(34)	(81)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	183	326
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(34)	(81)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(149)	(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	4,729	19,915
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(3,969)	(2,084)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	(235)	(17,776)
Net Amount, Derivatives, Financial Assets	525	55
Net Amount Recognized, Derivatives, Financial Liabilities	9,924	3,186
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(3,969)	(2,084)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(5,670)	(725)
Net Amount, Derivatives, Financial Liabilities	$ 285	$ 377