Variable Interest Entities ("VIEs") (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Variable Interest Entity
Carrying Amount	$ 17,622,234	$ 18,190,754
Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity
Carrying Amount	4,412,401
Company's Maximum Risk of Loss	4,412,401
Variable Interest Entity, Not Primary Beneficiary | CMBS
Variable Interest Entity
Carrying Amount	3,189,281
Company's Maximum Risk of Loss	3,189,281
Variable Interest Entity, Not Primary Beneficiary | Non-Agency RMBS
Variable Interest Entity
Carrying Amount	1,194,952
Company's Maximum Risk of Loss	1,194,952
Variable Interest Entity, Not Primary Beneficiary | Investments in unconsolidated ventures
Variable Interest Entity
Carrying Amount	28,168
Company's Maximum Risk of Loss	$ 28,168