MORTGAGE LOAN RECEIVABLES - Activity in Loan Portfolio (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Activity in loan portfolio
Balance at the beginning of the period	$ 2,353,977		$ 2,310,409	$ 1,939,008
Balance at the beginning of the period, provision for loan losses	(4,000)
Origination of mortgage loan receivables	2,873,304		2,098,052	3,557,164
Purchases of mortgage loan receivables	94,079		73,421
Repayment of mortgage loan receivables	(386,852)		(722,360)	(754,760)
Proceeds from sales of mortgage loan receivables	(1,491,092)		(1,440,195)	(2,509,090)
Non-cash disposition of loan via foreclosure				(4,620)
Realized gain on sale of mortgage loan receivables	54,046		26,009	71,066
Transfer between held for investment and held for sale	0
Accretion/amortization of discount, premium and other fees	11,180		8,941	12,241
Loan loss provision	0		(300)	(600)
Balance at the end of the period, provision for loan losses	(4,000)	[1]	(4,000)
Balance at the end of the period	3,508,642		2,353,977	2,310,409
Proceeds from sales of mortgage loan receivables held for sale	115,359		0	0
Loans held for sale transferred loan held for investment, book value	120,000
Loans held for sale transferred to loans held for investments, fair value	$ 119,900
Loans from held for sale transferred to portfolio loans, remaining maturity	3 years
Loans held-for-sale transferred to held-for-investment, fair value	$ 33,800
Loans held-for-sale transferred to held-for-investment, remaining maturity	3 years 6 months 7 days
Origination of mortgage loans receivable held for investment	$ 0		50,378	0
Repayment of mortgage loans receivable held for investment	0		(70,678)	0
Mortgage loans held by consolidated subsidiaries
Activity in loan portfolio
Balance at the beginning of the period	2,000,095		1,742,345	1,524,153
Origination of mortgage loan receivables	1,407,669		969,401	963,023
Purchases of mortgage loan receivables	94,079		0
Repayment of mortgage loan receivables	(384,283)		(720,592)	(752,452)
Proceeds from sales of mortgage loan receivables	0		0	0
Non-cash disposition of loan via foreclosure				(4,620)
Realized gain on sale of mortgage loan receivables	0		0	0
Transfer between held for investment and held for sale	153,722
Transfer between held for investment and held for sale	153,722		0	0
Accretion/amortization of discount, premium and other fees	11,180		8,941	12,241
Loan loss provision	0		0	0
Balance at the end of the period	3,282,462		2,000,095	1,742,345
Origination of mortgage loans receivable held for investment			50,400
Repayment of mortgage loans receivable held for investment			(70,700)
Mortgage loan receivables held for investment, net, at amortized cost
Activity in loan portfolio
Balance at the beginning of the period, provision for loan losses	(4,000)		(3,700)	(3,100)
Loan loss provision	0		(300)	(600)
Balance at the end of the period, provision for loan losses	(4,000)		(4,000)	(3,700)
Mortgage loan receivables held for sale
Activity in loan portfolio
Balance at the beginning of the period	357,882		571,764	417,955
Origination of mortgage loan receivables	1,465,635		1,128,651	2,594,141
Purchases of mortgage loan receivables	0		73,421
Repayment of mortgage loan receivables	(2,569)		(1,768)	(2,308)
Proceeds from sales of mortgage loan receivables	(1,491,092)		(1,440,195)	(2,509,090)
Non-cash disposition of loan via foreclosure				0
Realized gain on sale of mortgage loan receivables	54,046		26,009	71,066
Transfer between held for investment and held for sale	(153,722)
Accretion/amortization of discount, premium and other fees	0		0	0
Loan loss provision	0		0	0
Balance at the end of the period	230,180		357,882	$ 571,764
Repayment of mortgage loans receivable held for sale	500
Proceeds from sales of mortgage loan receivables held for sale	115,400
Realized losses on loans recorded as other than temporary impairments	$ 1,800		$ 2,600

[1] Includes amounts relating to consolidated variable interest entities. See Note 3.