RESIDENTIAL INVESTMENT SECURITIES - Weighted Average Life (Details) - Residential Investments - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Estimated Fair Value
Less than one year	$ 471,977	$ 63,510
Greater than one year through five years	13,838,890	12,626,932
Greater than five years through ten years	77,273,833	56,785,601
Greater than ten years	716,121	8,239,859
Total	92,300,821	77,715,902
Amortized Cost
Less than one year	476,538	61,775
Greater than one year through five years	13,925,749	12,666,394
Greater than five years through ten years	78,431,852	57,738,588
Greater than ten years	708,363	8,473,058
Total	$ 93,542,502	$ 78,939,815