Loans Receivable, Net - Principal Balance and Net Book Value of Loans Receivable Based on Internal Risk Ratings (Parenthetical) (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Jul. 31, 2018	Dec. 31, 2017	Jun. 30, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loan exposure including senior interests	$ 446.9		$ 985.4
Single asset securitization principal amount		$ 1,000.0		$ 474.6
2018 Single Asset Securitization [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Single asset securitization principal amount	$ 1,000.0