Counterparty Risk (Exposure to Counterparty Risk) (Details) - Counterparty Risk [Member] $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2021 USD ($) counterparty	Dec. 31, 2021 USD ($) counterparty
Cash and Cash Equivalents [Member]
Concentration Risk [Line Items]
Amount of Exposure | $	$ 111,647	$ 92,661
Number of Counterparties with Exposure | counterparty	8	9
Percentage of Total Outstanding Unpaid Principal Balance	40.10%	46.90%
Collateral On Repurchase Agreements Held By Dealers [Member]
Concentration Risk [Line Items]
Amount of Exposure | $	$ 1,860,059	$ 2,822,847
Number of Counterparties with Exposure | counterparty	24	23
Percentage of Total Outstanding Unpaid Principal Balance	15.30%	33.50%
Due From Broker [Member]
Concentration Risk [Line Items]
Amount of Exposure | $	$ 63,147	$ 93,549
Number of Counterparties with Exposure | counterparty	22	19
Percentage of Total Outstanding Unpaid Principal Balance	28.90%	29.10%
Receivable For Securities Sold [Member]
Concentration Risk [Line Items]
Amount of Exposure | $	$ 1,416	$ 11,014
Number of Counterparties with Exposure | counterparty	2	3
Percentage of Total Outstanding Unpaid Principal Balance	94.50%	50.70%