Schedule IV - Mortgage Loans on Real Estate - Schedule of Mortgage Loans on Real Estate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Face Amount of Mortgages	$ 790,094
Carrying Amount of Mortgages	730,515	$ 752,129	$ 915,905	$ 934,964
Reserves on impaired loans	40,400
Borrower A
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Prior Liens	0
Face Amount of Mortgages	107,196
Carrying Amount of Mortgages	$ 107,427
Borrower A | LIBOR | Contractual Interest Accrual Rates
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Basis spread on variable rate (as a percent)	5.15%
Borrower A | LIBOR | Contractual Interest Payment Rates
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Basis spread on variable rate (as a percent)	5.15%
Borrower B
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Prior Liens	$ 0
Face Amount of Mortgages	88,612
Carrying Amount of Mortgages	$ 88,114
Borrower B | LIBOR | Contractual Interest Accrual Rates
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Basis spread on variable rate (as a percent)	5.25%
Borrower B | LIBOR | Contractual Interest Payment Rates
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Basis spread on variable rate (as a percent)	5.25%
Borrower C
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Prior Liens	$ 0
Face Amount of Mortgages	84,000
Carrying Amount of Mortgages	$ 84,959
Borrower C | LIBOR | Contractual Interest Accrual Rates
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Basis spread on variable rate (as a percent)	6.00%
Borrower C | LIBOR | Contractual Interest Payment Rates
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Basis spread on variable rate (as a percent)	6.00%
Borrower D
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Prior Liens	$ 0
Face Amount of Mortgages	71,934
Carrying Amount of Mortgages	$ 71,120
Borrower D | LIBOR | Contractual Interest Accrual Rates
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Basis spread on variable rate (as a percent)	6.00%
Borrower D | LIBOR | Contractual Interest Payment Rates
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Basis spread on variable rate (as a percent)	6.00%
Borrower E
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Prior Liens	$ 0
Face Amount of Mortgages	31,039
Carrying Amount of Mortgages	$ 31,017
Borrower E | LIBOR | Contractual Interest Accrual Rates
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Basis spread on variable rate (as a percent)	4.00%
Borrower E | LIBOR | Contractual Interest Payment Rates
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Basis spread on variable rate (as a percent)	4.00%
Borrower F
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Prior Liens	$ 0
Face Amount of Mortgages	30,833
Carrying Amount of Mortgages	$ 30,432
Borrower F | LIBOR | Contractual Interest Accrual Rates
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Basis spread on variable rate (as a percent)	4.75%
Borrower F | LIBOR | Contractual Interest Payment Rates
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Basis spread on variable rate (as a percent)	4.75%
Borrower G
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Prior Liens	$ 0
Face Amount of Mortgages	29,252
Carrying Amount of Mortgages	$ 29,085
Borrower G | LIBOR | Contractual Interest Accrual Rates
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Basis spread on variable rate (as a percent)	6.00%
Borrower G | LIBOR | Contractual Interest Payment Rates
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Basis spread on variable rate (as a percent)	6.00%
Borrower H
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Prior Liens	$ 0
Face Amount of Mortgages	27,274
Carrying Amount of Mortgages	$ 27,019
Borrower H | LIBOR | Contractual Interest Accrual Rates
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Basis spread on variable rate (as a percent)	5.75%
Borrower H | LIBOR | Contractual Interest Payment Rates
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Basis spread on variable rate (as a percent)	5.75%
Borrower I
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Prior Liens	$ 0
Face Amount of Mortgages	27,000
Carrying Amount of Mortgages	$ 26,828
Borrower I | Contractual Interest Accrual Rates
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Fixed interest rate (as a percent)	7.50%
Borrower I | Contractual Interest Payment Rates
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Fixed interest rate (as a percent)	7.50%
Senior Mortgages Individually Less than 3 Percent
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Face Amount of Mortgages	$ 282,469
Carrying Amount of Mortgages	$ 224,353
Senior Mortgages Individually Less than 3 Percent | Minimum | Contractual Interest Accrual Rates
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Fixed interest rate (as a percent)	5.00%
Senior Mortgages Individually Less than 3 Percent | Minimum | Contractual Interest Payment Rates
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Fixed interest rate (as a percent)	6.00%
Senior Mortgages Individually Less than 3 Percent | Maximum | Contractual Interest Accrual Rates
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Fixed interest rate (as a percent)	9.68%
Senior Mortgages Individually Less than 3 Percent | Maximum | Contractual Interest Payment Rates
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Fixed interest rate (as a percent)	9.68%
Senior Mortgages Individually Less than 3 Percent | LIBOR | Minimum | Contractual Interest Accrual Rates
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Basis spread on variable rate (as a percent)	3.00%
Senior Mortgages Individually Less than 3 Percent | LIBOR | Minimum | Contractual Interest Payment Rates
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Basis spread on variable rate (as a percent)	3.00%
Senior Mortgages Individually Less than 3 Percent | LIBOR | Maximum | Contractual Interest Accrual Rates
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Basis spread on variable rate (as a percent)	7.50%
Senior Mortgages Individually Less than 3 Percent | LIBOR | Maximum | Contractual Interest Payment Rates
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Basis spread on variable rate (as a percent)	7.50%
Senior Mortgages
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Face Amount of Mortgages	$ 779,609
Carrying Amount of Mortgages	720,354
Subordinate Mortgages Individually Less than 3 Percent
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Face Amount of Mortgages	10,485
Carrying Amount of Mortgages	$ 10,161
Subordinate Mortgages Individually Less than 3 Percent | Minimum | Contractual Interest Accrual Rates
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Fixed interest rate (as a percent)	6.80%
Subordinate Mortgages Individually Less than 3 Percent | Minimum | Contractual Interest Payment Rates
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Fixed interest rate (as a percent)	6.80%
Subordinate Mortgages Individually Less than 3 Percent | Maximum | Contractual Interest Accrual Rates
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Fixed interest rate (as a percent)	14.00%
Subordinate Mortgages Individually Less than 3 Percent | Maximum | Contractual Interest Payment Rates
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Fixed interest rate (as a percent)	14.00%
Subordinate Mortgages
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Face Amount of Mortgages	$ 10,485
Carrying Amount of Mortgages	$ 10,161