Fair Values - Schedule of Details of Book Value, Face Amount, and Fair Value of Financial Instruments (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Financial assets
Cash and cash equivalents, face amount	$ 105,662	$ 69,654
Restricted cash		32,864
Loans receivable, net, face amount	14,293,970	10,108,226
Debt securities held-to-maturity	99,000
Financial liabilities
Secured debt agreements, net, face amount	8,996,481	5,290,536
Loan participations sold, net	94,418	80,415
Loan participations sold, net, face amount	123,745	141,119
Securitized debt obligations, net	1,285,471	1,282,412
Securitized debt obligations, net,face amount	1,292,120	1,292,120
Book Value [Member]
Financial assets
Cash and cash equivalents	105,662	69,654
Restricted cash		32,864
Loans receivable, net	14,191,200	10,056,732
Debt securities held-to-maturity	96,167
Financial liabilities
Secured debt agreements, net	8,974,756	5,273,855
Loan participations sold, net	94,418	80,415
Securitized debt obligations, net	1,285,471	1,282,412
Convertible notes, net	609,911	563,911
Fair Value [Member]
Financial assets
Cash and cash equivalents	105,662	69,654
Restricted cash		32,864
Loans receivable, net	14,294,836	10,112,331
Debt securities held-to-maturity	96,600
Financial liabilities
Secured debt agreements, net	8,996,481	5,290,536
Loan participations sold, net	94,528	80,706
Securitized debt obligations, net	1,283,086	1,292,589
Convertible notes, net	605,348	610,201
Convertible Senior Note [Member]
Financial liabilities
Convertible notes, net, face amount	622,500	575,000
Senior Participation [Member]
Financial liabilities
Loan participations sold, net	94,418	80,415
Loan participations sold, net, face amount	$ 94,528	$ 80,706