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Warehouse Receivables & Warehouse Lines of Credit - Additional Information (Detail)
9 Months Ended
Sep. 30, 2017
USD ($)
Warehouse Receivables And Warehouse Lines Of Credit [Line Items]  
Period of repayment for warehouse lines of credit 1 month
Warehouse Agreement Borrowings [Member]  
Warehouse Receivables And Warehouse Lines Of Credit [Line Items]  
Lines of credit principal outstanding $ 1,400,000,000