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Long-term Debt, Collateralized and Short-Term Borrowings - Notes Payable, Collateralized and Short-Term Borrowings (Parenthetical) (Detail)
Dec. 31, 2017
Dec. 31, 2016
May 27, 2016
8.125% Senior Notes [Member]      
Debt Instrument [Line Items]      
Stated interest rate 8.125% 8.125%  
5.375% Senior Notes [Member]      
Debt Instrument [Line Items]      
Stated interest rate 5.375% 5.375%  
8.375% Senior Notes [Member]      
Debt Instrument [Line Items]      
Stated interest rate 8.375% 8.375%  
5.125% Senior Notes [Member]      
Debt Instrument [Line Items]      
Stated interest rate 5.125% 5.125% 5.125%