Securitization Transactions Schedule of Assets and Liabilities attributable to Consolidated VIEs (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Variable Interest Entity [Line Items]
Cash and cash equivalents	$ 25,715	$ 47,233
Investments Fair Value	2,343,738	2,071,707
Interest and dividends payable	5,168	5,904
Other secured borrowings, at fair value	113,775	125,105
Consolidated Entities [Member]
Variable Interest Entity [Line Items]
Cash and cash equivalents	0	333
Investments Fair Value	120,333	132,424
Interest and Dividends Receivable	361	0
Interest and dividends payable	361	333
Other secured borrowings, at fair value	$ 113,775	$ 125,105