Schedule IV - Mortgage Loans on Real Estate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Mortgage Loans on Real Estate [Line Items]
Market spread	one-month LIBOR
Unpaid Principal Balance	$ 4,313,591
Carrying Amount of Loans	$ 4,293,787	$ 3,175,672	$ 2,449,990	$ 1,933,398
Senior Loans in Excess of 3% of the Carrying Amount of Total Loans | GA | Borrower A | Office
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	3.40%
Extended Maturity Date	2024
Periodic Payment Terms	I/O
Unpaid Principal Balance	$ 167,084
Carrying Amount of Loans	$ 165,074
Senior Loans in Excess of 3% of the Carrying Amount of Total Loans | MI | Borrower B | Office
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	3.60%
Extended Maturity Date	2024
Periodic Payment Terms	I/O
Unpaid Principal Balance	$ 131,916
Carrying Amount of Loans	$ 129,851
Senior Loans in Excess of 3% of the Carrying Amount of Total Loans | FL | Borrower C | Multifamily
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	2.90%
Extended Maturity Date	2024
Periodic Payment Terms	I/O
Unpaid Principal Balance	$ 169,134
Carrying Amount of Loans	$ 169,134
Senior Loans in Excess of 3% of the Carrying Amount of Total Loans | PA | Borrower D | Office
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	2.70%
Extended Maturity Date	2023
Periodic Payment Terms	I/O
Unpaid Principal Balance	$ 178,458
Carrying Amount of Loans	$ 178,458
Senior Loans in Excess of 3% of the Carrying Amount of Total Loans | PA | Borrower G | Office
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	4.30%
Extended Maturity Date	2022
Periodic Payment Terms	I/O
Unpaid Principal Balance	$ 158,947
Carrying Amount of Loans	$ 157,926
Senior Loans in Excess of 3% of the Carrying Amount of Total Loans | TN | Borrower E | Mixed Use
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	4.10%
Extended Maturity Date	2021
Periodic Payment Terms	I/O
Unpaid Principal Balance	$ 142,000
Carrying Amount of Loans	$ 141,401
Senior Loans in Excess of 3% of the Carrying Amount of Total Loans | NC | Borrower F | Hotel
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	3.80%
Extended Maturity Date	2022
Periodic Payment Terms	I/O
Unpaid Principal Balance	$ 171,604
Carrying Amount of Loans	$ 170,854
Senior Loans in Excess of 3% of the Carrying Amount of Total Loans | NJ | Borrower H | Multifamily
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	3.80%
Extended Maturity Date	2023
Periodic Payment Terms	I/O
Unpaid Principal Balance	$ 156,012
Carrying Amount of Loans	$ 155,421
Senior Loans in Excess of 3% of the Carrying Amount of Total Loans | TX | Borrower I | Mixed Use
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	2.80%
Extended Maturity Date	2023
Periodic Payment Terms	I/O
Unpaid Principal Balance	$ 134,750
Carrying Amount of Loans	$ 134,750
Senior Loans in Excess of 3% of the Carrying Amount of Total Loans | CA | Borrower J | Office
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	3.30%
Extended Maturity Date	2023
Periodic Payment Terms	I/O
Unpaid Principal Balance	$ 132,245
Carrying Amount of Loans	131,405
Senior Loans Less than 3% of the Carrying Amount of Total Loans
Mortgage Loans on Real Estate [Line Items]
Unpaid Principal Balance	4,313,591
Carrying Amount of Loans	$ 4,293,787
Senior Loans Less than 3% of the Carrying Amount of Total Loans | Senior Loan | Office
Mortgage Loans on Real Estate [Line Items]
Market spread	Floating:LIBOR
Periodic Payment Terms	I/O
Unpaid Principal Balance	$ 813,350
Carrying Amount of Loans	$ 809,865
Senior Loans Less than 3% of the Carrying Amount of Total Loans | Senior Loan | Multifamily
Mortgage Loans on Real Estate [Line Items]
Market spread	Floating:LIBOR
Periodic Payment Terms	I/O
Unpaid Principal Balance	$ 791,537
Carrying Amount of Loans	$ 787,657
Senior Loans Less than 3% of the Carrying Amount of Total Loans | Senior Loan | Mixed Use
Mortgage Loans on Real Estate [Line Items]
Market spread	Floating:LIBOR
Periodic Payment Terms	I/O
Unpaid Principal Balance	$ 446,702
Carrying Amount of Loans	$ 444,848
Senior Loans Less than 3% of the Carrying Amount of Total Loans | Senior Loan | Hotel
Mortgage Loans on Real Estate [Line Items]
Market spread	Floating:LIBOR
Periodic Payment Terms	I/O
Unpaid Principal Balance	$ 325,950
Carrying Amount of Loans	$ 324,922
Senior Loans Less than 3% of the Carrying Amount of Total Loans | Senior Loan | Condominium
Mortgage Loans on Real Estate [Line Items]
Market spread	Floating:LIBOR
Periodic Payment Terms	I/O
Unpaid Principal Balance	$ 144,093
Carrying Amount of Loans	$ 143,545
Senior Loans Less than 3% of the Carrying Amount of Total Loans | Senior Loan | Retail
Mortgage Loans on Real Estate [Line Items]
Market spread	Floating:LIBOR
Periodic Payment Terms	I/O
Unpaid Principal Balance	$ 183,309
Carrying Amount of Loans	$ 182,270
Senior Loans Less than 3% of the Carrying Amount of Total Loans | Senior Loan | Industrial
Mortgage Loans on Real Estate [Line Items]
Market spread	Floating:LIBOR
Periodic Payment Terms	I/O
Unpaid Principal Balance	$ 66,500
Carrying Amount of Loans	$ 66,406
Senior Loans Less than 3% of the Carrying Amount of Total Loans | Minimum | Senior Loan | Office
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	3.00%
Extended Maturity Date	2021
Senior Loans Less than 3% of the Carrying Amount of Total Loans | Minimum | Senior Loan | Multifamily
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	3.30%
Extended Maturity Date	2020
Senior Loans Less than 3% of the Carrying Amount of Total Loans | Minimum | Senior Loan | Mixed Use
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	3.20%
Extended Maturity Date	2019
Senior Loans Less than 3% of the Carrying Amount of Total Loans | Minimum | Senior Loan | Hotel
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	3.70%
Extended Maturity Date	2020
Senior Loans Less than 3% of the Carrying Amount of Total Loans | Minimum | Senior Loan | Condominium
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	4.70%
Extended Maturity Date	2020
Senior Loans Less than 3% of the Carrying Amount of Total Loans | Minimum | Senior Loan | Retail
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	3.70%
Extended Maturity Date	2022
Senior Loans Less than 3% of the Carrying Amount of Total Loans | Minimum | Senior Loan | Industrial
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	4.00%
Senior Loans Less than 3% of the Carrying Amount of Total Loans | Maximum | Senior Loan | Office
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	5.10%
Extended Maturity Date	2023
Senior Loans Less than 3% of the Carrying Amount of Total Loans | Maximum | Senior Loan | Multifamily
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	5.30%
Extended Maturity Date	2024
Senior Loans Less than 3% of the Carrying Amount of Total Loans | Maximum | Senior Loan | Mixed Use
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	6.00%
Extended Maturity Date	2024
Senior Loans Less than 3% of the Carrying Amount of Total Loans | Maximum | Senior Loan | Hotel
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	5.90%
Extended Maturity Date	2023
Senior Loans Less than 3% of the Carrying Amount of Total Loans | Maximum | Senior Loan | Condominium
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	4.80%
Extended Maturity Date	2021
Senior Loans Less than 3% of the Carrying Amount of Total Loans | Maximum | Senior Loan | Retail
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	4.50%
Extended Maturity Date	2023
Senior Loans Less than 3% of the Carrying Amount of Total Loans | Maximum | Senior Loan | Industrial
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	5.00%
Extended Maturity Date	2021