Fair Values - Schedule of Details of Book Value, Face Amount, and Fair Value of Financial Instruments (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financial assets
Cash and cash equivalents	$ 79,437	$ 105,662
Loans receivable, net	14,603,831	14,293,970
Debt securities held-to-maturity	98,579	99,000
Financial liabilities
Secured debt agreements, net, face amount	9,235,365	8,996,481
Loan participations sold, net	107,237	94,418
Loan participations sold, net, face amount	140,504	123,745
Securitized debt obligations, net	1,286,417	1,285,471
Securitized debt obligations, net,face amount	1,292,120	1,292,120
Book Value [Member]
Financial assets
Cash and cash equivalents	79,437	105,662
Loans receivable, net	14,508,735	14,191,200
Debt securities held-to-maturity	95,877	96,167
Financial liabilities
Secured debt agreements, net	9,208,010	8,974,756
Loan participations sold, net	107,237	94,418
Securitized debt obligations, net	1,286,417	1,285,471
Convertible notes, net	610,684	609,911
Fair Value [Member]
Financial assets
Cash and cash equivalents	79,437	105,662
Loans receivable, net	14,605,743	14,294,836
Debt securities held-to-maturity	97,200	96,600
Financial liabilities
Secured debt agreements, net	9,235,365	8,996,481
Loan participations sold, net	107,330	94,528
Securitized debt obligations, net	1,290,833	1,283,086
Convertible notes, net	627,050	605,348
Convertible Senior Note [Member]
Financial liabilities
Convertible notes, net, face amount	622,500	622,500
Senior Participation [Member]
Financial liabilities
Loan participations sold, net	107,237	94,418
Loan participations sold, net, face amount	$ 107,330	$ 94,528