SEGMENTS - Product Concentration (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015
Product concentration
Servicing portfolio loans unpaid principal balance	$ 74,492,166	$ 70,284,682	$ 66,290,754	$ 64,384,024	$ 63,081,154	$ 59,121,989	$ 57,321,824	$ 51,040,752	$ 50,212,264
Fannie Mae
Product concentration
Servicing portfolio loans unpaid principal balance	32,075,617				27,728,164				22,915,088
Freddie Mac
Product concentration
Servicing portfolio loans unpaid principal balance	26,782,581				20,688,410				17,810,007
Ginnie Mae-HUD
Product concentration
Servicing portfolio loans unpaid principal balance	9,640,312				9,155,794				5,657,809
Life insurance companies and other
Product concentration
Servicing portfolio loans unpaid principal balance	$ 5,993,656				$ 5,508,786				$ 3,829,360