Balance Sheet Offsetting - Financial Instruments Derivative Assets Liabilities and Resell Agreements Net of Amount Not Offset (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	$ 64,616	$ 5,266
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(19,221)	(5,236)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	(45,090)	0
Net Amount, Derivatives, Financial Assets	305	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	84,616	36,565
Gross Amounts Not Offset Collateral, Financial Assets	(65,090)	(31,299)
Net Amount, Financial Assets	305	30
Net Amount Recognized, Derivatives, Financial Liabilities	38,618	26,640
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(19,221)	(5,236)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(19,333)	(21,404)
Net Amount, Derivatives, Financial Liabilities	64	0
Net Amount Recognized, Repurchase Agreements, Financial Liabilities	1,146,478	1,668,142
Gross Amounts Not Offset Financial Instruments, Repurchase Agreements, Financial Liabilities	0	0
Gross Amounts Not Offset Collateral, Repurchase Agreements, Financial Liabilities	(1,146,478)	(1,668,142)
Net Amount, Repurchase Agreements, Financial Liabilities	0	0
Net Amount Recognized, Financial Liabilities	1,185,096	1,694,782
Gross Amounts Not Offset Collateral, Financial Liabilities	(1,165,811)	(1,689,546)
Net Amount, Financial Liabilities	64	0
Counterparty A [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	10	39
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(10)	(39)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	7,606	5,192
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(10)	(39)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(7,580)	(5,153)
Net Amount, Derivatives, Financial Liabilities	16	0
Counterparty B [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	10,483	1,650
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(10,483)	(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	11,111	7,424
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(10,483)	(1,650)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(580)	(5,774)
Net Amount, Derivatives, Financial Liabilities	48	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	146	135
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	0	(1)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(146)	(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	54,123	3,576
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(8,728)	(3,546)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	(45,090)	0
Net Amount, Derivatives, Financial Assets	305	30
Net Amount Recognized, Derivatives, Financial Liabilities	19,755	13,889
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(8,728)	(3,546)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(11,027)	(10,343)
Net Amount, Derivatives, Financial Liabilities	$ 0	$ 0