Schedule IV - Mortgage Loans on Real Estate - Schedule of Instruments (Details) $ in Thousands	12 Months Ended
Dec. 31, 2017 USD ($) loan	Dec. 31, 2016 loan
Mortgage Loans on Real Estate [Line Items]
Mortgage loans on real estate, interest rate	7.03%	6.73%
Prior Liens	$ 1,260,294
Face amount of Mortgages	3,533,236
Mortgage loans on real estate, carrying amount	3,512,642
Provision for Loan Losses	(4,000)
Total Mortgages after Provision for Loan Losses	3,508,642
Principal Amount of Mortgages Subject to Delinquent Principal or Interest	26,850
Aggregate cost for Federal income tax purposes	$ 3,500,000
Provision for loan losses
Mortgage Loans on Real Estate [Line Items]
Number or loans in default \| loan	2	2
Loans in default, carrying value	$ 26,900
First mortgage loan
Mortgage Loans on Real Estate [Line Items]
Prior Liens	0
Face amount of Mortgages	3,373,315
Mortgage loans on real estate, carrying amount	3,353,448
Principal Amount of Mortgages Subject to Delinquent Principal or Interest	26,850
Subordinated Mortgages
Mortgage Loans on Real Estate [Line Items]
Prior Liens	1,260,294
Face amount of Mortgages	159,921
Mortgage loans on real estate, carrying amount	159,194
Principal Amount of Mortgages Subject to Delinquent Principal or Interest	$ 0
Multi-family \| First Mortgage 5.15%
Mortgage Loans on Real Estate [Line Items]
Mortgage loans on real estate, interest rate	2.87%
Prior Liens	$ 0
Face amount of Mortgages	120,000
Mortgage loans on real estate, carrying amount	119,952
Principal Amount of Mortgages Subject to Delinquent Principal or Interest	$ 0
Hotel \| First Mortgage 5.75%
Mortgage Loans on Real Estate [Line Items]
Mortgage loans on real estate, interest rate	5.88%
Prior Liens	$ 0
Face amount of Mortgages	110,000
Mortgage loans on real estate, carrying amount	109,506
Principal Amount of Mortgages Subject to Delinquent Principal or Interest	0
Hotel, Industrial, Land, Mobile Home Park, Mixed Use, Multi-family, Office, Retail, Self Storage \| First mortgages individually less than 3%
Mortgage Loans on Real Estate [Line Items]
Prior Liens	0
Face amount of Mortgages	3,143,315
Mortgage loans on real estate, carrying amount	3,123,990
Principal Amount of Mortgages Subject to Delinquent Principal or Interest	$ 26,850
Hotel, Industrial, Land, Mobile Home Park, Mixed Use, Multi-family, Office, Retail, Self Storage \| First mortgages individually less than 3% \| Minimum
Mortgage Loans on Real Estate [Line Items]
Mortgage loans on real estate, interest rate	3.50%
Hotel, Industrial, Land, Mobile Home Park, Mixed Use, Multi-family, Office, Retail, Self Storage \| First mortgages individually less than 3% \| Maximum
Mortgage Loans on Real Estate [Line Items]
Mortgage loans on real estate, interest rate	14.50%
Hotel, Industrial, Land, Mobile Home Park, Mixed Use, Multi-family, Office, Retail, Self Storage \| Subordinated mortgages individually less than 3%
Mortgage Loans on Real Estate [Line Items]
Prior Liens	$ 1,260,294
Face amount of Mortgages	159,921
Mortgage loans on real estate, carrying amount	159,194
Principal Amount of Mortgages Subject to Delinquent Principal or Interest	$ 0
Hotel, Industrial, Land, Mobile Home Park, Mixed Use, Multi-family, Office, Retail, Self Storage \| Subordinated mortgages individually less than 3% \| Minimum
Mortgage Loans on Real Estate [Line Items]
Mortgage loans on real estate, interest rate	6.04%
Hotel, Industrial, Land, Mobile Home Park, Mixed Use, Multi-family, Office, Retail, Self Storage \| Subordinated mortgages individually less than 3% \| Maximum
Mortgage Loans on Real Estate [Line Items]
Mortgage loans on real estate, interest rate	15.00%

Financing Receivable Recorded Investment 90 Days Past Due And Still Accruing Number of Contracts

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Mortgage Loans on Real Estate, Carrying Amount of Mortgages, Net of Provision for Loan Losses

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Recorded investment in financing receivables that are 90 days or more past due and still accruing.

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The carrying amount of the outstanding balance due under the mortgage loan.

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Amount of the contractual principal due at the origination of the mortgage loan (face amount).

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Amount of the federal income tax basis of mortgage loans on real estate.

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The stated interest rate on the mortgage loan receivable or the weighted average interest rate on a group of loans.

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Amount of principal due on loans that have been delinquent, which may exclude loans having interest delinquent for three months or less, as of the balance sheet date.

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Amount of prior lien on the real estate that secures the mortgage loan.

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Amount of expense related to estimated loss from loan and lease transactions.

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