Counterparty Risk (Exposure to Counterparty Risk) (Details) - Counterparty Risk [Member] $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021 USD ($) counterparty	Dec. 31, 2020 USD ($) counterparty
Cash and Cash Equivalents [Member]
Concentration Risk [Line Items]
Amount of Exposure | $	$ 103,617	$ 111,647
Number of Counterparties with Exposure | counterparty	8	8
Percentage of Total Outstanding Unpaid Principal Balance	29.20%	40.10%
Collateral On Repurchase Agreements Held By Dealers [Member]
Concentration Risk [Line Items]
Amount of Exposure | $	$ 2,443,429	$ 1,860,059
Number of Counterparties with Exposure | counterparty	24	24
Percentage of Total Outstanding Unpaid Principal Balance	28.20%	15.30%
Due From Broker [Member]
Concentration Risk [Line Items]
Amount of Exposure | $	$ 89,983	$ 63,147
Number of Counterparties with Exposure | counterparty	22	22
Percentage of Total Outstanding Unpaid Principal Balance	30.60%	28.90%
Receivable For Securities Sold [Member]
Concentration Risk [Line Items]
Amount of Exposure | $	$ 26,992	$ 1,416
Number of Counterparties with Exposure | counterparty	3	2
Percentage of Total Outstanding Unpaid Principal Balance	49.30%	94.50%