Related Party Transactions (Details) $ in Thousands, £ in Millions	1 Months Ended						3 Months Ended		9 Months Ended
	May 31, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2018 GBP (£)	May 31, 2018 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Related Party Transaction [Line Items]
Base management fees incurred but not yet paid							$ 10,434		$ 10,434		$ 9,804
Commercial mortgage loans and subordinate loans and other lending assets, net							6,114,574		6,114,574		4,927,593
Related party expenses							10,434	$ 9,515	$ 30,306	$ 26,620
Arrangement Fees
Related Party Transaction [Line Items]
Related party expenses	$ 600
Limited Liability Company
Related Party Transaction [Line Items]
Rate of management fees									1.50%
Extension period									1 year
Period of termination									180 days
Termination fee calculation period									24 months
Limited Liability Company | Management Fees
Related Party Transaction [Line Items]
Base management fees incurred but not yet paid							10,400		$ 10,400		9,800
Affiliated Entity | Management Fees
Related Party Transaction [Line Items]
Related party expenses							10,400	9,500	30,300	26,600
Affiliated Entity | Reimbursements
Related Party Transaction [Line Items]
Related party expenses							500	$ 600	2,000	$ 1,800
Commercial Mortgage Portfolio Segment
Related Party Transaction [Line Items]
Commercial mortgage loans and subordinate loans and other lending assets, net							$ 4,779,501		$ 4,779,501		$ 3,878,981
Commercial Mortgage Portfolio Segment | Affiliated Entity
Related Party Transaction [Line Items]
Payments to acquire loans receivable		$ 6,400	£ 4.8	$ 28,200	$ 25,000	$ 25,000
Commercial mortgage loans and subordinate loans and other lending assets, net				$ 300,000	$ 125,000	$ 100,000