MORTGAGE SERVICING RIGHTS - Presentation of Activity Related to MSR (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Servicing Asset at Fair Value, Amount [Roll Forward]
Fair value, beginning of period	$ 652,216	$ 652,216	$ 0
Sales		(33)	0
Other	(27)		0
Change in fair value due to:
Changes in valuation inputs or assumptions	(4,629)		178,463
Other changes, including realization of expected cash flows	(66,667)		(48,652)
Fair value, end of period	580,860	$ 580,860	652,216
Purchased Servicing Rights
Servicing Asset at Fair Value, Amount [Roll Forward]
Purchases	0		166,585
Hatteras
Servicing Asset at Fair Value, Amount [Roll Forward]
Obtained through Hatteras Acquisition	$ 0		$ 355,820