Loans Receivable, Net - Overall Statistics for Loans Receivable Portfolio (Parenthetical) (Detail)	Sep. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of loans subject to yield maintenance, or other prepayment restrictions	86.00%	75.00%
Percentage of loans open to repayment by borrower without penalty	14.00%	25.00%
USD LIBOR [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of loans receivable by type	83.00%	92.00%