DEBT - Disclosures (Details)		3 Months Ended				6 Months Ended		12 Months Ended
	Dec. 14, 2015	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018 USD ($) extension	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) extension	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 21, 2017
Funding agreements
Line of credit facility, maximum borrowing capacity				$ 1,540,989,000		$ 1,540,989,000		$ 1,540,989,000
Amount of debt discount on the initial draw down amount								2,600,000
Maximum
Funding agreements
Extension period of maturity date						12 months
Secured term loan
Funding agreements
Line of credit facility, maximum borrowing capacity				$ 110,000,000		$ 110,000,000		110,000,000
Number of extension periods available for maturity date | extension				1		1
Extension period of maturity date						12 months
Aggregate principal amount				$ 110,000,000		$ 110,000,000
Debt discount on initial draw down (as a percent)					7.40%	8.50%	7.60%
Secured term loan | LIBOR
Funding agreements
Debt Instrument, Interest Rate, Increase (Decrease)				12.50%
Interest rate margin (as a percent)						500.00%			600.00%
Secured term loan | LIBOR | Scenario, Forecast
Funding agreements
Debt Instrument, Interest Rate, Increase (Decrease)		75.00%	37.50%
Secured term loan | Minimum | LIBOR
Funding agreements
Interest rate margin (as a percent)									100.00%
Wells Fargo Facility
Funding agreements
Line of credit facility, maximum borrowing capacity				$ 500,000,000		$ 500,000,000		500,000,000
Wells Fargo Facility | Secured revolving funding facility
Funding agreements
Line of credit facility, maximum borrowing capacity				$ 500,000,000		$ 500,000,000
Number of extension periods available for maturity date | extension				2		2
Extension period of maturity date						12 months
Non-utilization fee on average available balance (as a percent)						0.25%
Non-utilization threshold percentage (less than) (as a percent)				75.00%		75.00%
Non-utilization/commitment fee				$ 6,000	$ 83,000		$ 92,000
Wells Fargo Facility | Secured revolving funding facility | Minimum | 30 day LIBOR
Funding agreements
Interest rate margin (as a percent)	1.75%
Wells Fargo Facility | Secured revolving funding facility | Maximum | 30 day LIBOR
Funding agreements
Interest rate margin (as a percent)	2.35%
Wells Fargo Facility | Revolving master repurchase facility | 30 day LIBOR
Funding agreements
Non-utilization/commitment fee						$ 0
Citibank Facility
Funding agreements
Line of credit facility, maximum borrowing capacity				250,000,000		250,000,000		250,000,000
Citibank Facility | Secured revolving funding facility
Funding agreements
Line of credit facility, maximum borrowing capacity				$ 250,000,000		$ 250,000,000
Number of extension periods available for maturity date | extension				3		3
Extension period of maturity date						12 months
Non-utilization fee on average available balance (as a percent)						0.25%
Non-utilization/commitment fee				$ 28,000	79,000	$ 62,000	82,000
Citibank Facility | Secured revolving funding facility | Minimum | 30 day LIBOR
Funding agreements
Interest rate margin (as a percent)						2.25%
Citibank Facility | Secured revolving funding facility | Maximum | 30 day LIBOR
Funding agreements
Interest rate margin (as a percent)						2.50%
BAML Facility
Funding agreements
Line of credit facility, maximum borrowing capacity				125,000,000		$ 125,000,000		125,000,000
BAML Facility | Secured revolving funding facility
Funding agreements
Non-utilization fee on average available balance (as a percent)						0.125%
BAML Facility | Secured funding facility
Funding agreements
Line of credit facility, maximum borrowing capacity				125,000,000		$ 125,000,000
Non-utilization/commitment fee				6,000	17,000	$ 22,000	31,000
Term of debt						2 years
Facility used on average (at least) (as a percent)						50.00%
BAML Facility | Secured funding facility | One-month LIBOR
Funding agreements
Interest rate margin (as a percent)						2.00%
BAML Facility | Secured funding facility | Minimum | One-month LIBOR
Funding agreements
Interest rate margin (as a percent)						2.25%
BAML Facility | Secured funding facility | Maximum | One-month LIBOR
Funding agreements
Interest rate margin (as a percent)						2.75%
City National Bank Facility | March 2014 CNB Facility
Funding agreements
Line of credit facility, maximum borrowing capacity				50,000,000		$ 50,000,000		50,000,000
Non-utilization fee on average available balance (as a percent)						0.375%
Non-utilization/commitment fee				$ 29,000	76,000	$ 47,000	94,000
City National Bank Facility | March 2014 CNB Facility | LIBOR for a one, two, three, six or 12-month
Funding agreements
Interest rate margin (as a percent)						3.00%
City National Bank Facility | March 2014 CNB Facility | One-month LIBOR
Funding agreements
Interest rate margin (as a percent)						1.00%
City National Bank Facility | March 2014 CNB Facility | Federal funds rate
Funding agreements
Interest rate margin (as a percent)						0.50%
City National Bank Facility | March 2014 CNB Facility | Base rate
Funding agreements
Interest rate margin (as a percent)						1.25%
City National Bank Facility | March 2014 CNB Facility | Minimum
Funding agreements
Interest rate (as a percent)				3.00%		3.00%
Facility used on average (at least) (as a percent)						75.00%
April 2014 UBS Facility
Funding agreements
Line of credit facility, maximum borrowing capacity				$ 140,000,000		$ 140,000,000		140,000,000
April 2014 UBS Facility | Revolving master repurchase facility
Funding agreements
Line of credit facility, maximum borrowing capacity				140,000,000		$ 140,000,000
April 2014 UBS Facility | Assets subject to an advance for one year or less | Revolving master repurchase facility | One-month LIBOR
Funding agreements
Interest rate margin (as a percent)						1.88%
April 2014 UBS Facility | Assets subject to an advance in excess of one year but less than two years | Revolving master repurchase facility | One-month LIBOR
Funding agreements
Interest rate margin (as a percent)						2.08%
April 2014 UBS Facility | Assets subject to an advance for greater than two years | Revolving master repurchase facility | One-month LIBOR
Funding agreements
Interest rate margin (as a percent)						2.28%
U.S. Bank Facility
Funding agreements
Line of credit facility, maximum borrowing capacity				$ 185,989,000		$ 185,989,000		185,989,000
U.S. Bank Facility | Revolving master repurchase facility
Funding agreements
Number of extension periods available for maturity date | extension				2		2
Extension period of maturity date						12 months
U.S. Bank Facility | Revolving master repurchase facility | 30 day LIBOR
Funding agreements
Line of credit facility, maximum borrowing capacity				$ 186,000,000		$ 186,000,000
Interest rate margin (as a percent)						50.00%
Non-utilization/commitment fee					$ 42,000	$ 0	83,000
U.S. Bank Facility | Revolving master repurchase facility | One-month LIBOR
Funding agreements
Interest rate margin (as a percent)						2.25%
MetLife Facility
Funding agreements
Line of credit facility, maximum borrowing capacity				$ 180,000,000		$ 180,000,000		$ 180,000,000
MetLife Facility | Secured revolving funding facility
Funding agreements
Non-utilization threshold percentage (less than) (as a percent)				65.00%		65.00%
Non-utilization/commitment fee				$ 1,000			$ 7,000
MetLife Facility | Revolving master repurchase facility
Funding agreements
Line of credit facility, maximum borrowing capacity				$ 180,000,000		$ 180,000,000
Number of extension periods available for maturity date | extension				2		2
Extension period of maturity date						12 months
MetLife Facility | Revolving master repurchase facility | 30 day LIBOR
Funding agreements
Interest rate margin (as a percent)						2.35%	2.30%
Non-utilization fee on average available balance (as a percent)							0.25%