SEGMENTS - Geographic Concentration (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Geographic concentration
Percentage of unpaid principal balance of the loans serviced for others	100.00%	100.00%	100.00%
California
Geographic concentration
Percentage of unpaid principal balance of the loans serviced for others	18.40%	17.20%	16.10%
Florida
Geographic concentration
Percentage of unpaid principal balance of the loans serviced for others	9.40%	8.20%	8.40%
Texas
Geographic concentration
Percentage of unpaid principal balance of the loans serviced for others	9.20%	8.50%	7.90%
Wisconsin
Geographic concentration
Percentage of unpaid principal balance of the loans serviced for others	4.50%	5.00%	4.90%
All other states
Geographic concentration
Percentage of unpaid principal balance of the loans serviced for others	58.50%	61.10%	62.70%
Maximum
Geographic concentration
Threshold percentage, unpaid principal balance and related servicing revenues by geographical area	5.00%	5.00%	5.00%