Fair Values - Schedule of Details of Carrying Amount, Face Amount, and Fair Value of Financial Instruments (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Financial assets
Cash and cash equivalents, face amount	$ 57,396	$ 69,654	$ 64,275
Restricted cash	17,082	32,864
Loans receivable, net, face amount	11,140,975	10,108,226
Financial liabilities
Secured debt agreements, net, face amount	6,018,670	5,290,536
Loan participations sold, net	117,926	80,415
Loan participations sold, net, face amount	178,602	141,119
Securitized debt obligations, net	1,282,279	1,282,412
Securitized debt obligations, net,face amount	1,292,120	1,292,120
Carrying Amount [Member]
Financial assets
Cash and cash equivalents	57,396	69,654
Restricted cash	17,082	32,864
Loans receivable, net	11,081,716	10,056,732
Financial liabilities
Secured debt agreements, net	5,996,880	5,273,855
Loan participations sold, net	117,926	80,415
Securitized debt obligations, net	1,282,279	1,282,412
Convertible notes, net	778,070	563,911
Fair Value [Member]
Financial assets
Cash and cash equivalents	57,396	69,654
Restricted cash	17,082	32,864
Loans receivable, net	11,142,089	10,112,331
Financial liabilities
Secured debt agreements, net	6,018,670	5,290,536
Loan participations sold, net	118,189	80,706
Securitized debt obligations, net	1,292,630	1,292,589
Convertible notes, net	811,348	610,201
Convertible Senior Note [Member]
Financial liabilities
Convertible notes, net, face amount	795,000	575,000
Senior Participation [Member]
Financial liabilities
Loan participations sold, net	117,926	80,415
Loan participations sold, net, face amount	$ 118,189	$ 80,706