Counterparty Risk (Exposure to Counterparty Risk) (Details) - Counterparty Risk [Member] $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) counterparty	Dec. 31, 2019 USD ($) counterparty
Cash and Cash Equivalents [Member]
Concentration Risk [Line Items]
Amount of Exposure | $	$ 111,647	$ 72,302
Number of Counterparties with Exposure | counterparty	8	11
Percentage of Total Outstanding Unpaid Principal Balance	40.10%	42.20%
Collateral On Repurchase Agreements Held By Dealers [Member]
Concentration Risk [Line Items]
Amount of Exposure | $	$ 1,860,059	$ 2,793,696
Number of Counterparties with Exposure | counterparty	24	28
Percentage of Total Outstanding Unpaid Principal Balance	15.30%	13.80%
Due From Broker [Member]
Concentration Risk [Line Items]
Amount of Exposure | $	$ 63,147	$ 79,829
Number of Counterparties with Exposure | counterparty	22	24
Percentage of Total Outstanding Unpaid Principal Balance	28.90%	30.90%
Receivable For Securities Sold [Member]
Concentration Risk [Line Items]
Amount of Exposure | $	$ 1,416	$ 69,995
Number of Counterparties with Exposure | counterparty	2	5
Percentage of Total Outstanding Unpaid Principal Balance	94.50%	62.30%