Securitization Transactions Schedule of Assets and Liabilities attributable to Consolidated VIEs (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Variable Interest Entity [Line Items]
Cash and cash equivalents	$ 47,233	$ 123,274	$ 183,909	$ 114,140
Investments Fair Value	2,071,707	1,505,026
Interest and dividends payable	5,904	3,460
Other secured borrowings, at fair value	125,105	0
Consolidated Entities [Member]
Variable Interest Entity [Line Items]
Cash and cash equivalents	333	0
Investments Fair Value	132,424	0
Interest and dividends payable	333	0
Other secured borrowings, at fair value	$ 125,105	$ 0