Financial Derivatives Financial Derivatives (Schedule of Derivative Activity) (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Interest Rate Swaps [Member]
Derivative [Line Items]
Average Monthly Notional Amount	$ 1,113,420	$ 1,306,853
Credit Default Swap [Member]
Derivative [Line Items]
Average Monthly Notional Amount	601,794	531,008
Total Return Swaps [Member]
Derivative [Line Items]
Average Monthly Notional Amount	36,059	19,760
Futures [Member]
Derivative [Line Items]
Average Monthly Notional Amount	101,664	48,244
Options [Member]
Derivative [Line Items]
Average Monthly Notional Amount	124,741	94,415
Forwards [Member]
Derivative [Line Items]
Average Monthly Notional Amount	44,541	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