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	Sep. 30, 2018	Dec. 31, 2017
Carrying Value | Loans Held for Investment
Financial Assets
Financial Assets, Nonrecurring	$ 4,157,653	$ 3,175,672
Carrying Value | Collateralized Loan Obligation
Financial Liabilities
CLO (TRTX 2018-FL1)	735,415
Carrying Value | Secured Financing Arrangements
Financial Liabilities
CLO (TRTX 2018-FL1)	2,187,950	2,114,990
Estimate of Fair Value Measurement | Level III | Loans Held for Investment
Financial Assets
Financial Assets, Nonrecurring	4,187,202	3,202,150
Estimate of Fair Value Measurement | Level III | Collateralized Loan Obligation
Financial Liabilities
CLO (TRTX 2018-FL1)	735,415
Estimate of Fair Value Measurement | Level III | Secured Financing Arrangements
Financial Liabilities
CLO (TRTX 2018-FL1)	$ 2,187,950	$ 2,114,990