REPURCHASE AGREEMENTS AND REVERSE REPURCHASE AGREEMENTS - Remaining Maturities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreement liabilities associated with these liabilities	$ 5,213,869	$ 3,595,586
SecuritiesSoldUnderAgreementsToRepurchaseMember
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	5,430,319	3,729,543
Repurchase agreement liabilities associated with these liabilities	$ 5,213,869	$ 3,595,586
Net weighted average borrowing rate	0.13%	0.23%
Overnight (1 Day or Less) [Member] \| SecuritiesSoldUnderAgreementsToRepurchaseMember
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	$ 3,501	$ 0
Repurchase agreement liabilities associated with these liabilities	$ 2,500	$ 0
Net weighted average borrowing rate	0.63%	0.00%
Between 2 and 30 Days [Member] \| SecuritiesSoldUnderAgreementsToRepurchaseMember
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	$ 3,393,762	$ 2,112,969
Repurchase agreement liabilities associated with these liabilities	$ 3,250,133	$ 2,047,897
Net weighted average borrowing rate	0.13%	0.23%
Between 31 and 90 Days [Member] \| SecuritiesSoldUnderAgreementsToRepurchaseMember
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	$ 1,979,011	$ 1,560,798
Repurchase agreement liabilities associated with these liabilities	$ 1,909,639	$ 1,494,500
Net weighted average borrowing rate	0.12%	0.22%
Greater Than 90 days [Member] \| SecuritiesSoldUnderAgreementsToRepurchaseMember
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	$ 54,045	$ 55,776
Repurchase agreement liabilities associated with these liabilities	$ 51,597	$ 53,189
Net weighted average borrowing rate	0.15%	0.30%

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- Definition

Amount, after the effects of master netting arrangements, of funds outstanding borrowed in the form of a security repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same securities at a date certain for a specified price. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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Fair value of securities pledged as collateral against securities sold under agreement to repurchase.

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Weighted average interest rate of short-term debt outstanding calculated at point in time.

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