Balance Sheet Offsetting - Financial Instruments Derivative Assets Liabilities and Resell Agreements Net of Amount Not Offset (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	$ 20,847	$ 5,266
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(20,514)	(5,236)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	(327)	0
Net Amount, Derivatives, Financial Assets	6	30
Net Amount Recognized, Resell agreements, Financial Assets	20,000	31,299
Gross Amounts Not Offset Financial Instruments, Resell Agreements	0	0
Gross Amounts Not Offset Collateral, Resell Agreements, Financial Assets	(20,000)	(31,299)
Net Amount, Financial Assets, Resell Agreements	0	0
Net Amount Recognized, Total Financial Assets	40,847	36,565
Gross Amounts Not Offset Collateral, Financial Assets	(20,327)	(31,299)
Net Amount, Financial Assets	6	30
Net Amount Recognized, Derivatives, Financial Liabilities	52,153	26,640
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(20,514)	(5,236)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(29,445)	(21,404)
Net Amount, Derivatives, Financial Liabilities	2,194	0
Net Amount Recognized, Repurchase Agreements, Financial Liabilities	1,574,367	1,668,142
Gross Amounts Not Offset Financial Instruments, Repurchase Agreements, Financial Liabilities	0	0
Gross Amounts Not Offset Collateral, Repurchase Agreements, Financial Liabilities	(1,574,367)	(1,668,142)
Net Amount, Repurchase Agreements, Financial Liabilities	0	0
Net Amount Recognized, Financial Liabilities	1,626,520	1,694,782
Gross Amounts Not Offset Collateral, Financial Liabilities	(1,603,812)	(1,689,546)
Net Amount, Financial Liabilities	2,194	0
Counterparty A [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	8	39
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(8)	(39)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	7,178	5,192
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(8)	(39)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(7,170)	(5,153)
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	5,996	1,650
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(5,996)	(1,650)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	14,386	7,424
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(5,996)	(1,650)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(8,210)	(5,774)
Net Amount, Derivatives, Financial Liabilities	180	0
Counterparty C [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets		1
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets		(1)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets		0
Net Amount, Derivatives, Financial Assets		0
Net Amount Recognized, Derivatives, Financial Liabilities	94	135
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	0	(1)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(94)	(134)
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	14,843	3,576
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(14,510)	(3,546)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	(327)	0
Net Amount, Derivatives, Financial Assets	6	30
Net Amount Recognized, Derivatives, Financial Liabilities	30,495	13,889
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(14,510)	(3,546)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(13,971)	(10,343)
Net Amount, Derivatives, Financial Liabilities	$ 2,014	$ 0