Financial Derivatives (Schedule of Derivative Activity) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Interest rate swaps
Derivative [Line Items]
Average Monthly Notional	$ 912,934	$ 1,059,756
TBAs
Derivative [Line Items]
Average Monthly Notional	984,292
Credit default swaps
Derivative [Line Items]
Average Monthly Notional	403,254	566,805
Total return swaps
Derivative [Line Items]
Average Monthly Notional	38,400	53,603
Futures
Derivative [Line Items]
Average Monthly Notional	280,947	201,295
Options
Derivative [Line Items]
Average Monthly Notional	51,545	99,891
Forwards
Derivative [Line Items]
Average Monthly Notional	29,078	$ 45,522
Warrants
Derivative [Line Items]
Average Monthly Notional	$ 2,281