Fair Values - Schedule of Details of Book Value, Face Amount, and Fair Value of Financial Instruments (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Financial assets
Cash and cash equivalents, face amount	$ 107,057	$ 69,654	$ 33,521
Restricted cash		32,864
Loans receivable, net, face amount	12,303,596	10,108,226
Financial liabilities
Secured debt agreements, net, face amount	7,044,550	5,290,536
Loan participations sold, net	152,855	80,415
Loan participations sold, net, face amount	200,409	141,119
Securitized debt obligations, net	1,283,311	1,282,412
Securitized debt obligations, net,face amount	1,292,120	1,292,120
Book Value [Member]
Financial assets
Cash and cash equivalents	107,057	69,654
Restricted cash		32,864
Loans receivable, net	12,214,565	10,056,732
Financial liabilities
Secured debt agreements, net	7,022,500	5,273,855
Loan participations sold, net	152,855	80,415
Securitized debt obligations, net	1,283,311	1,282,412
Convertible notes, net	779,416	563,911
Fair Value [Member]
Financial assets
Cash and cash equivalents	107,057	69,654
Restricted cash		32,864
Loans receivable, net	12,304,909	10,112,331
Financial liabilities
Secured debt agreements, net	7,044,550	5,290,536
Loan participations sold, net	153,090	80,706
Securitized debt obligations, net	1,294,444	1,292,589
Convertible notes, net	808,331	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	152,855	80,415
Loan participations sold, net, face amount	$ 153,090	$ 80,706