Fair Value Measurements - Summary of Financial Assets and Liabilities Not Carried at Fair Value On Recurring Basis (Details) - Fair Value Measurements Nonrecurring - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Carrying Value | Loans Held for Investment
Financial Assets
Financial Assets, Nonrecurring	$ 4,712,134	$ 4,293,787
Carrying Value | Term Loan Facility
Financial Liabilities
Financial Liabilities, Nonrecurring	266,549	113,504
Carrying Value | Collateralized Loan Obligation
Financial Liabilities
Financial Liabilities, Nonrecurring	1,251,393	1,509,930
Carrying Value | Secured Financing Arrangements
Financial Liabilities
Financial Liabilities, Nonrecurring	2,097,844	1,526,449
Estimate of Fair Value Measurement | Level III | Loans Held for Investment
Financial Assets
Financial Assets, Nonrecurring	4,741,978	4,317,844
Estimate of Fair Value Measurement | Level III | Term Loan Facility
Financial Liabilities
Financial Liabilities, Nonrecurring	266,549	113,504
Estimate of Fair Value Measurement | Level III | Collateralized Loan Obligation
Financial Liabilities
Financial Liabilities, Nonrecurring	1,251,393	1,509,930
Estimate of Fair Value Measurement | Level III | Secured Financing Arrangements
Financial Liabilities
Financial Liabilities, Nonrecurring	$ 2,097,844	$ 1,526,449