Repurchase Agreements And Reverse Repurchase Agreements (Narrative) (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2020 USD ($) days	Dec. 31, 2019 USD ($) days
Assets Sold under Agreements to Repurchase [Line Items]
Receivable Under Reverse Repurchase Agreements	$ 7,944,000	$ 10,181,000
Securities Sold under Agreements to Repurchase	$ 69,685,000	$ 89,182,000
Short-term Debt, Weighted Average Interest Rate, at Point in Time	0.41%	2.17%
Debt of consolidated variable interest entities, at fair value	$ (204,000)	$ (228,000)
Repurchase Agreements Extinguished - Extinguishment of Debt	$ 3,700,000
Weighted Average Remaining Maturity of Repurchase Agreements Extinguished - Extinguished Debt	2 years 2 months 12 days
Gain (Loss) on Extinguishment of Debt	$ (146,000)
Weighted Average Interest Rate of Repurchase Agreements Extinguished - Extinguished Debt	2.11%
Short-term Debt, Weighted Average Interest Rate, over Time	2.11%
TBA securities Fifteen Year and Thirty Year Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Derivative, Fair Value, Net	$ 130,000	25,000
TBA and Forward Settling Agency Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Derivative, Forward Settlement Value	20,413,000	7,404,000
Derivative, Fair Value, Net	130,000	25,000
Maturity Overnight [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase	13,000,000	17,000,000
Forward Contracts [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase	$ 1,500,000	$ 4,500,000
Weighted Average Forward Start Days | days	1	12
Short-term Debt, Weighted Average Interest Rate, at Point in Time	0.30%	1.60%
Bethesda Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Percentage of Repurchase Agreement Funding	48.00%	40.00%
FICC [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Percentage of Repurchase Agreement Funding	47.00%	38.00%
Obligation to Return Securities Borrowed Under Reverse Repurchase Agreements at Fair Value	$ 4,300,000	$ 5,400,000