Fair Values - Schedule of Details of Carrying Amount, Face Amount, and Fair Value of Financial Instruments (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financial assets
Cash and cash equivalents, face amount	$ 69,654	$ 75,567
Restricted cash	32,864
Loans receivable, net, face amount	10,108,226	8,727,218
Financial liabilities
Secured debt agreements, net, face amount	5,290,536	5,731,626
Loan participations sold, net	80,415	348,077
Loan participations sold, net, face amount	141,119	419,560
Securitized debt obligations, net	1,282,412	0
Securitized debt obligations, net,face amount	1,292,120
Carrying Amount [Member]
Financial assets
Cash and cash equivalents	69,654	75,567
Restricted cash	32,864
Loans receivable, net	10,056,732	8,692,978
Financial liabilities
Secured debt agreements, net	5,273,855	5,716,354
Loan participations sold, net	80,415	348,077
Securitized debt obligations, net	1,282,412
Convertible notes, net	563,911	166,762
Fair Value [Member]
Financial assets
Cash and cash equivalents	69,654	75,567
Restricted cash	32,864
Loans receivable, net	10,112,331	8,733,784
Financial liabilities
Secured debt agreements, net	5,290,536	5,731,626
Loan participations sold, net	80,706	349,633
Securitized debt obligations, net	1,292,589
Convertible notes, net	610,201	191,763
Convertible Senior Note [Member]
Financial liabilities
Convertible notes, net, face amount	575,000	172,500
Senior Participation [Member]
Financial liabilities
Loan participations sold, net	80,415	348,077
Loan participations sold, net, face amount	$ 80,706	$ 349,633