Balance Sheet Offsetting - Financial Instruments Derivative Assets Liabilities and Resell Agreements Net of Amount Not Offset (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	$ 28,141	$ 11,111
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(28,141)	(11,111)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Resell agreements, Financial Assets	7,903	0
Gross Amounts Not Offset Financial Instruments, Resell Agreements	0	0
Gross Amounts Not Offset Collateral, Resell Agreements, Financial Assets	(7,903)	0
Net Amount, Financial Assets, Resell Agreements	0	0
Net Amount Recognized, Total Financial Assets	36,044	11,111
Gross Amounts Not Offset Collateral, Financial Assets	(7,903)	0
Net Amount, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	224,432	69,323
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(28,141)	(11,111)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(188,436)	(57,031)
Net Amount, Derivatives, Financial Liabilities	7,855	1,181
Net Amount Recognized, Repurchase Agreements, Financial Liabilities	2,200,029	2,068,147
Gross Amounts Not Offset Financial Instruments, Repurchase Agreements, Financial Liabilities	0	0
Gross Amounts Not Offset Collateral, Repurchase Agreements, Financial Liabilities	(2,200,029)	(2,068,147)
Net Amount, Repurchase Agreements, Financial Liabilities	0	0
Net Amount Recognized, Financial Liabilities	2,424,461	2,137,470
Gross Amounts Not Offset Collateral, Financial Liabilities	(2,388,465)	(2,125,178)
Net Amount, Financial Liabilities	7,855	1,181
Counterparty A [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	4	2
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(4)	(2)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	4,577	6,430
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(4)	(2)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(4,573)	(6,428)
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	12,537	5,838
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(12,537)	(5,838)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	77,110	20,722
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(12,537)	(5,838)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(60,860)	(14,700)
Net Amount, Derivatives, Financial Liabilities	3,713	184
Counterparty C [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Liabilities	19	71
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	0	0
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(19)	(71)
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	15,600	5,271
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(15,600)	(5,271)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	142,726	42,100
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(15,600)	(5,271)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(122,984)	(35,832)
Net Amount, Derivatives, Financial Liabilities	$ 4,142	$ 997