Financial Derivatives Financial Derivatives (Schedule of Derivative Activity) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Interest Rate Swaps [Member]
Derivative [Line Items]
Average Monthly Notional Amount	$ 1,226,140	$ 1,306,853
Credit Default Swap [Member]
Derivative [Line Items]
Average Monthly Notional Amount	620,059	531,008
Total Return Swaps [Member]
Derivative [Line Items]
Average Monthly Notional Amount	14,910	19,760
Futures [Member]
Derivative [Line Items]
Average Monthly Notional Amount	80,413	48,244
Options [Member]
Derivative [Line Items]
Average Monthly Notional Amount	122,707	94,415
Forwards [Member]
Derivative [Line Items]
Average Monthly Notional Amount	50,263	76,784
Warrant [Member]
Derivative [Line Items]
Average Monthly Notional Amount	0	378
Mortgage Loan Purchase Commitments [Member]
Derivative [Line Items]
Average Monthly Notional Amount	$ 0	$ 1,585