SCHEDULE IV MORTGAGE LOANS ON REAL ESTATE - Number of Loans (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) loan
Mortgage loans on real estate
Original loan amounts	$ 227,289
First-lien mortgage loans
Mortgage loans on real estate
Number of Loans | loan	5
First-lien mortgage loans | Mortgage Loans between 500,000 and 2,000,000
Mortgage loans on real estate
Number of Loans | loan	1
First-lien mortgage loans | Mortgage Loans between 500,000 and 2,000,000 | Minimum
Mortgage loans on real estate
Original loan amounts	$ 500
First-lien mortgage loans | Mortgage Loans between 500,000 and 2,000,000 | Maximum
Mortgage loans on real estate
Original loan amounts	$ 2,000
First-lien mortgage loans | Mortgage Loans between 2,001,000 and 3,000,000
Mortgage loans on real estate
Number of Loans | loan	0
First-lien mortgage loans | Mortgage Loans between 2,001,000 and 3,000,000 | Minimum
Mortgage loans on real estate
Original loan amounts	$ 2,001
First-lien mortgage loans | Mortgage Loans between 2,001,000 and 3,000,000 | Maximum
Mortgage loans on real estate
Original loan amounts	$ 3,000
First-lien mortgage loans | Mortgage Loans between 3,001,000 and 4,000,000
Mortgage loans on real estate
Number of Loans | loan	0
First-lien mortgage loans | Mortgage Loans between 3,001,000 and 4,000,000 | Minimum
Mortgage loans on real estate
Original loan amounts	$ 3,001
First-lien mortgage loans | Mortgage Loans between 3,001,000 and 4,000,000 | Maximum
Mortgage loans on real estate
Original loan amounts	$ 4,000
First-lien mortgage loans | Mortgage Loans between 4,001,000 and 5,000,000
Mortgage loans on real estate
Number of Loans | loan	0
First-lien mortgage loans | Mortgage Loans between 4,001,000 and 5,000,000 | Minimum
Mortgage loans on real estate
Original loan amounts	$ 4,001
First-lien mortgage loans | Mortgage Loans between 4,001,000 and 5,000,000 | Maximum
Mortgage loans on real estate
Original loan amounts	$ 5,000
First-lien mortgage loans | Mortgage Loans between 5,001,000 and 6,000,000
Mortgage loans on real estate
Number of Loans | loan	0
First-lien mortgage loans | Mortgage Loans between 5,001,000 and 6,000,000 | Minimum
Mortgage loans on real estate
Original loan amounts	$ 5,001
First-lien mortgage loans | Mortgage Loans between 5,001,000 and 6,000,000 | Maximum
Mortgage loans on real estate
Original loan amounts	$ 6,000
First-lien mortgage loans | Mortgage Loans between 6,001,000 and 7,000,000
Mortgage loans on real estate
Number of Loans | loan	0
First-lien mortgage loans | Mortgage Loans between 6,001,000 and 7,000,000 | Minimum
Mortgage loans on real estate
Original loan amounts	$ 6,001
First-lien mortgage loans | Mortgage Loans between 6,001,000 and 7,000,000 | Maximum
Mortgage loans on real estate
Original loan amounts	$ 7,000
First-lien mortgage loans | Mortgage Loans over 7,000,000
Mortgage loans on real estate
Number of Loans | loan	4
First-lien mortgage loans | Mortgage Loans over 7,000,000 | Minimum
Mortgage loans on real estate
Original loan amounts	$ 7,001