Investment in Securities (Unrealized Loss Positions Securities) (Details) $ in Thousands	Mar. 31, 2019 USD ($)
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	$ 254,754
Unrealized Losses, Less than 12 Months	(3,385)
Fair Value, Greater than 12 Months	584,702
Unrealized Losses, Greater than 12 Months	(16,067)
Fair Value, Total	839,456
Unrealized Loss, Total	(19,452)
Agency RMBS
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	123,567
Unrealized Losses, Less than 12 Months	(627)
Fair Value, Greater than 12 Months	521,597
Unrealized Losses, Greater than 12 Months	(11,904)
Fair Value, Total	645,164
Unrealized Loss, Total	(12,531)
Non-Agency RMBS
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	85,338
Unrealized Losses, Less than 12 Months	(1,585)
Fair Value, Greater than 12 Months	34,988
Unrealized Losses, Greater than 12 Months	(1,102)
Fair Value, Total	120,326
Unrealized Loss, Total	(2,687)
CLOs
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	28,953
Unrealized Losses, Less than 12 Months	(996)
Fair Value, Greater than 12 Months	25,154
Unrealized Losses, Greater than 12 Months	(2,661)
Fair Value, Total	54,107
Unrealized Loss, Total	(3,657)
Other Securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	16,896
Unrealized Losses, Less than 12 Months	(177)
Fair Value, Greater than 12 Months	2,963
Unrealized Losses, Greater than 12 Months	(400)
Fair Value, Total	19,859
Unrealized Loss, Total	$ (577)