Financial Derivatives Financial Derivatives (Schedule of Derivative Activity) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Interest Rate Swaps [Member]
Derivative [Line Items]
Average Monthly Notional Amount	$ 1,306,853	$ 1,731,368
Credit Default Swap [Member]
Derivative [Line Items]
Average Monthly Notional Amount	531,008	1,586,923
Total Return Swaps [Member]
Derivative [Line Items]
Average Monthly Notional Amount	19,760	113,628
Futures [Member]
Derivative [Line Items]
Average Monthly Notional Amount	48,244	371,900
Options [Member]
Derivative [Line Items]
Average Monthly Notional Amount	94,415	357,260
Forwards [Member]
Derivative [Line Items]
Average Monthly Notional Amount	76,784	80,513
Warrant [Member]
Derivative [Line Items]
Average Monthly Notional Amount	378	1,640
Mortgage Loan Purchase Commitments [Member]
Derivative [Line Items]
Average Monthly Notional Amount	$ 1,585	$ 6,143