Investment in Securities (Unrealized Loss Positions Securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	$ 704,564	$ 153,786
Unrealized Losses, Less than 12 Months	(8,595)	(4,358)
Fair Value, Greater than 12 Months	36,491	4,627
Unrealized Losses, Greater than 12 Months	(2,178)	(1,523)
Fair Value, Total	741,055	158,413
Unrealized Loss, Total	(10,773)	(5,881)
Agency RMBS
Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	663,008	126,926
Unrealized Losses, Less than 12 Months	(7,783)	(981)
Fair Value, Greater than 12 Months	31,177	832
Unrealized Losses, Greater than 12 Months	(972)	(44)
Fair Value, Total	694,185	127,758
Unrealized Loss, Total	(8,755)	(1,025)
Non-Agency RMBS
Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	41,556	20,474
Unrealized Losses, Less than 12 Months	(812)	(2,616)
Fair Value, Greater than 12 Months	4,050	2,599
Unrealized Losses, Greater than 12 Months	(200)	(348)
Fair Value, Total	45,606	23,073
Unrealized Loss, Total	(1,012)	(2,964)
CLOs
Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	0	5,279
Unrealized Losses, Less than 12 Months	0	(753)
Fair Value, Greater than 12 Months	1,264	1,196
Unrealized Losses, Greater than 12 Months	(1,006)	(1,131)
Fair Value, Total	1,264	6,475
Unrealized Loss, Total	$ (1,006)	(1,884)
Other Securities
Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months		1,107
Unrealized Losses, Less than 12 Months		(8)
Fair Value, Greater than 12 Months		0
Unrealized Losses, Greater than 12 Months		0
Fair Value, Total		1,107
Unrealized Loss, Total		$ (8)