Fair Values - Schedule of Details of Book Value, Face Amount, and Fair Value of Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Financial assets
Cash and cash equivalents, face amount	$ 86,987	$ 69,654	$ 61,221
Restricted cash		32,864
Loans receivable, net, face amount	12,260,103	10,108,226
Debt securities held-to-maturity	96,050
Debt securities held-to-maturity	99,000
Financial liabilities
Secured debt agreements, net, face amount	6,862,256	5,290,536
Loan participations sold, net	81,044	80,415
Loan participations sold, net, face amount	106,259	141,119
Securitized debt obligations, net	1,284,362	1,282,412
Securitized debt obligations, net,face amount	1,292,120	1,292,120
Book Value [Member]
Financial assets
Cash and cash equivalents	86,987	69,654
Restricted cash		32,864
Loans receivable, net	12,176,516	10,056,732
Debt securities held-to-maturity	96,050
Financial liabilities
Secured debt agreements, net	6,841,759	5,273,855
Loan participations sold, net	81,044	80,415
Securitized debt obligations, net	1,284,362	1,282,412
Convertible notes, net	780,951	563,911
Fair Value [Member]
Financial assets
Cash and cash equivalents	86,987	69,654
Restricted cash		32,864
Loans receivable, net	12,261,013	10,112,331
Debt securities held-to-maturity	97,607
Financial liabilities
Secured debt agreements, net	6,862,256	5,290,536
Loan participations sold, net	81,170	80,706
Securitized debt obligations, net	1,297,032	1,292,589
Convertible notes, net	832,407	610,201
Convertible Senior Note [Member]
Financial liabilities
Convertible notes, net, face amount	794,828	575,000
Senior Participation [Member]
Financial liabilities
Loan participations sold, net	81,044	80,415
Loan participations sold, net, face amount	$ 81,170	$ 80,706