Loans Receivable, Net - Principal Balance and Net Book Value of Loans Receivable Based on Internal Risk Ratings (Parenthetical) (Detail) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loan exposure including senior interests	$ 449.5	$ 446.9
2018 Single Asset Securitization [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Single asset securitization principal amount	$ 1,000.0	$ 1,000.0	$ 1,000.0