Basis of Presentation and Significant Accounting Policies - Narrative (Details) $ / shares in Units, loan in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020 USD ($) loan $ / shares	Sep. 30, 2020 $ / shares	Jun. 30, 2020 $ / shares	Mar. 31, 2020 $ / shares	Dec. 31, 2019 USD ($) $ / shares	Sep. 30, 2019 $ / shares	Jun. 30, 2019 $ / shares	Mar. 31, 2019 $ / shares	Dec. 31, 2020 USD ($) loan segment $ / shares	Dec. 31, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of reportable segments | segment									1
Threshold period when delinquent loans are placed on nonaccrual status									90 days
Number of commercial real estate loans included in third-party historical loan loss database | loan	100								100
Basic (in usd per share) | $ / shares	$ 0.42	$ (0.45)	$ (0.03)	$ (0.68)	$ 0.32	$ 0.32	$ 0.34	$ 0.35	$ (0.73)	$ 1.32	$ 1.45
Accounts receivable, concurrent, threshold period past due	30 days								30 days
REIT taxable income distribution requirement	90.00%								90.00%
Percent per annum of equity used to calculate management fees									1.50%
Cumulative effect of adoption of ASU 2016-13 | $	$ (933,846)				$ (1,019,136)				$ (933,846)	$ (1,019,136)	$ (827,531)	$ (828,621)
Cumulative effect of adoption of new accounting principle
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Basic (in usd per share) | $ / shares										$ 0.34
Cumulative effect of adoption of ASU 2016-13 | $					$ 18,472					$ 18,472	$ 0