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Fair Value of Financial Instruments - Additional Information (Detail) - Recourse debt [Member] - 5.30% Senior Notes due in 2025 ("2025 Notes") [Member]
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Interest Rate 5.30% 5.30%
Maturity Dates Aug. 31, 2025 Aug. 31, 2025