Securitization Transactions Schedule of Assets and Liabilities attributable to Consolidated VIEs (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Variable Interest Entity [Line Items]
Cash and cash equivalents	$ 53,598	$ 47,233
Investments Fair Value	2,670,069	2,071,707
Interest and dividends payable	6,451	5,904
Other secured borrowings, at fair value	89,569	125,105
Consolidated Entities [Member]
Variable Interest Entity [Line Items]
Cash and cash equivalents	0	333
Investments Fair Value	94,965	132,424
Interest and Dividends Receivable	37	0
Interest and dividends payable	37	333
Other secured borrowings, at fair value	$ 89,569	$ 125,105