Consolidated K-Series - Condensed Statement of Operations of Consolidated K-Series (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Condensed Income Statements, Captions [Line Items]
Interest expense	$ 90,646	$ 78,062	$ 270,009	$ 220,801
NET INTEREST INCOME	19,603	13,320	56,854	42,947
Unrealized gain on multi-family loans and debt held in securitization trusts, net	12,303	2,353	31,867	5,184
NET INCOME ATTRIBUTABLE TO COMPANY	33,973	27,845	93,285	61,364
Consolidated K-Series
Condensed Income Statements, Captions [Line Items]
Interest income	86,458	76,186	257,179	213,242
Interest expense	75,145	67,030	224,310	187,835
NET INTEREST INCOME	11,313	9,156	32,869	25,407
Unrealized gain on multi-family loans and debt held in securitization trusts, net	12,303	2,353	31,867	5,184
NET INCOME ATTRIBUTABLE TO COMPANY	$ 23,616	$ 11,509	$ 64,736	$ 30,591