SEGMENTS - Product Concentration (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Product concentration
Servicing portfolio loans unpaid principal balance	$ 85,689,262	$ 80,485,634	$ 77,820,741	$ 75,836,280	$ 74,309,991	$ 70,138,557	$ 66,290,754	$ 64,384,024	$ 63,081,154
Fannie Mae
Product concentration
Servicing portfolio loans unpaid principal balance	35,983,178				32,075,617				27,728,164
Freddie Mac
Product concentration
Servicing portfolio loans unpaid principal balance	30,350,724				26,782,581				20,688,410
Ginnie Mae-HUD
Product concentration
Servicing portfolio loans unpaid principal balance	9,944,222				9,640,312				9,155,794
Life insurance companies and other
Product concentration
Servicing portfolio loans unpaid principal balance	$ 9,411,138				$ 5,811,481				$ 5,508,786