Loans Held for Investment - Summary of Carrying Values and Results of Internal Risk Rating Review Performed (Details) $ in Thousands	Mar. 31, 2019 USD ($) Rating	Dec. 31, 2018 USD ($) Rating
Accounts Notes And Loans Receivable [Line Items]
Carrying Value	$ 4,712,134	$ 4,293,787
Weighted Average Risk Rating | Rating	2.8	2.8
Rating 1
Accounts Notes And Loans Receivable [Line Items]
Carrying Value		$ 29,923
Rating 2
Accounts Notes And Loans Receivable [Line Items]
Carrying Value	$ 1,161,728	959,314
Rating 3
Accounts Notes And Loans Receivable [Line Items]
Carrying Value	3,416,582	3,099,401
Rating 4
Accounts Notes And Loans Receivable [Line Items]
Carrying Value	$ 133,824	$ 205,149