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Short-Term Financing - Additional Information (Detail)
6 Months Ended
Jun. 30, 2017
USD ($)
banks
program
Dec. 31, 2016
USD ($)
Short-term Debt [Line Items]    
Short-term financing $ 330,423,000 $ 418,832,000
Commercial paper (secured)    
Short-term Debt [Line Items]    
Number of commercial paper programs | program 3  
Short-term financing $ 112,386,000 147,021,000
Commercial paper (secured) | Minimum    
Short-term Debt [Line Items]    
Debt term 27 days  
Commercial paper (secured) | Maximum    
Short-term Debt [Line Items]    
Debt term 270 days  
Commercial paper (secured) | CP Series A | Weighted Average    
Short-term Debt [Line Items]    
Debt term 25 days  
Commercial paper (secured) | CP Series II A | Weighted Average    
Short-term Debt [Line Items]    
Debt term 16 days  
Commercial paper (secured) | CP Series III A    
Short-term Debt [Line Items]    
Excess net capital required $ 120,000,000.0  
Commercial paper (secured) | CP Series III A | Weighted Average    
Short-term Debt [Line Items]    
Debt term 21 days  
Bank lines (secured)    
Short-term Debt [Line Items]    
Short-term financing $ 104,000,000 $ 0
Bank lines (secured) | Committed Credit Facility    
Short-term Debt [Line Items]    
Debt term 1 year  
Line of credity, maximum borrowing capacity $ 200,000,000  
Minimum net capital required 120,000,000  
Short-term financing 104,000,000  
Bank lines (secured) | Uncommitted Credit Facility    
Short-term Debt [Line Items]    
Line of credity, maximum borrowing capacity 185,000,000  
Short-term financing $ 0  
Number of banks | banks 2