Loans Receivable, Net - Overall Statistics for Loans Receivable Portfolio (Parenthetical) (Detail) - USD ($) $ in Billions	Sep. 30, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of loans subject to yield maintenance, or other prepayment restrictions	59.00%	75.00%
Percentage of loans open to repayment by borrower without penalty	41.00%	25.00%
USD LIBOR [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of loans receivable by type	99.00%	98.00%
Percentage of loans, fixed rate	1.00%	2.00%
Additional loan receivable of floating rate	$ 3.5	$ 1.2