Variable Interest Entities (Details) $ in Thousands	Mar. 31, 2020 USD ($) item	Jan. 31, 2020	Dec. 31, 2019 USD ($)
Variable interest entities
Total Assets	$ 77,577,189		$ 78,042,336
Total Liabilities	$ 72,759,838		72,905,322
Number of collateralized debt obligation | item	0
Investment in unconsolidated entities	$ 87,334		84,329
Primary beneficiary
Variable interest entities
Total Assets	61,157,805		62,187,175
Total Liabilities	59,807,306		$ 60,743,494
Equity interest		51.00%
Primary beneficiary | ASU 2015-02
Variable interest entities
Total Assets	1,100,000
Total Liabilities	586,700
Primary beneficiary | CMBS Venture Holdings
Variable interest entities
Total Assets	323,900
Total Liabilities	88,300
Equity interest			51.00%
Primary beneficiary | SPT Dolphin | ASU 2015-02
Variable interest entities
VIE assets	678,000
VIE liabilities	$ 445,300
Not primary beneficiary
Variable interest entities
Number of CDO structures currently in default | item	4
Maximum risk of loss related to VIEs, on fair value basis	$ 32,900
Not primary beneficiary | ASU 2015-02 | Measurement Period Adjustments
Variable interest entities
Investment in unconsolidated entities	21,000
Not primary beneficiary | Securitization SPEs
Variable interest entities
Debt obligations to beneficial interest holders, unpaid principal balances	$ 4,400,000