Schedule IV - Mortgage Loans on Real Estate - Summary of Changes in Carrying Amounts of Mortgage Loans (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Balance at beginning of year		$ 4,927,593	$ 3,679,758
Loan fundings		3,435,457	2,350,865
Loan repayments		(2,037,322)	(1,066,843)
Gain (loss) on foreign currency translation		43,649	(51,013)
Realized loss on investment, net of provision for loan loss reversal		2,487	0
Provision for loan losses		(35,000)	(20,000)
Deferred Fees		(46,275)	(34,066)
PIK interest, amortization of fees and other items		84,504	68,892
Balance at close of the year		6,375,093	4,927,593
Provision for loan losses and impairment, net of reversals		35,000	(20,000)
Commercial Mortgage and Subordinated Portfolio Segment
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Gain (loss) on foreign currency translation		44,338
Realized loss on investment, net of provision for loan loss reversal	$ 12,500	(12,513)
Financing receivable, credit loss, expense (reversal)	$ 15,000	15,000
Provision for loan losses and impairment, net of reversals		57,000	37,000
Affiliated Entity
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Loan fundings			34,600
Sale Of Mezzanine Loan \| Commercial Mortgage and Subordinated Portfolio Segment
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Sale of mezzanine loan		(152,600)
Residential Condominium - Bethesda, MD \| Commercial Mortgage Portfolio Segment
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Provision for loan losses and impairment, net of reversals		3,000	5,000
Residential Condominium - Bethesda, MD \| Commercial Mortgage and Subordinated Portfolio Segment
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Provision for loan losses and impairment, net of reversals		13,000
Retail Center - Cincinnati, OH
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Loan and leases receivable, cost recovery		1,400
Retail Center - Cincinnati, OH \| Commercial Mortgage Portfolio Segment
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Provision for loan losses and impairment, net of reversals		$ 32,000	$ 15,000

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