SEGMENTS - Product Concentration (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Product concentration
Servicing portfolio loans unpaid principal balance	$ 93,225,169	$ 91,754,499	$ 89,897,025	$ 87,691,682	$ 85,689,262	$ 80,485,634	$ 77,820,741	$ 75,836,280	$ 74,309,991
Fannie Mae
Product concentration
Servicing portfolio loans unpaid principal balance	40,049,095				35,983,178				32,075,617
Freddie Mac
Product concentration
Servicing portfolio loans unpaid principal balance	32,583,842				30,350,724				26,782,581
Ginnie Mae-HUD
Product concentration
Servicing portfolio loans unpaid principal balance	9,972,989				9,944,222				9,640,312
Life insurance companies and other
Product concentration
Servicing portfolio loans unpaid principal balance	$ 10,619,243				$ 9,411,138				$ 5,811,481