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Investments Derivative Instruments (Detail) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Aug. 31, 2016
Derivative [Line Items]        
Derivative Assets (Liabilities), at Fair Value, Net $ 0 $ 0    
Gain (Loss) on Derivative Instruments, Net, Pretax 0 (20,000,000) $ (20,700,000)  
Designated as Hedging Instrument [Member] | Ineffective Cash Flow Hedge [Member]        
Derivative [Line Items]        
Derivative, Notional Amount 31,000,000 370,000,000 18,000,000 $ 350,000,000
Derivative Assets (Liabilities), at Fair Value, Net 0 0    
Gain (Loss) on Derivative Instruments, Net, Pretax 0 (1,300,000) 200,000  
Non-hedging Instruments | Interest Rate Swaps        
Derivative [Line Items]        
Derivative Assets (Liabilities), at Fair Value, Net 0 0    
Non-hedging Instruments | Interest Rate Swaps | Fixed maturities        
Derivative [Line Items]        
Derivative, Notional Amount 0 0 750,000,000  
Derivative Assets (Liabilities), at Fair Value, Net 0 0    
Gain (Loss) on Derivative Instruments, Net, Pretax 0 0 (23,400,000)  
Non-hedging Instruments | Future [Member]        
Derivative [Line Items]        
Derivative Assets (Liabilities), at Fair Value, Net 0 0    
Closed Positions [Member] | Non-hedging Instruments | Interest Rate Swaps        
Derivative [Line Items]        
Derivative, Notional Amount 0 750,000,000 0  
Derivative Assets (Liabilities), at Fair Value, Net 0 0    
Gain (Loss) on Derivative Instruments, Net, Pretax 0 (19,000,000) 0  
Closed Positions [Member] | Non-hedging Instruments | Future [Member]        
Derivative [Line Items]        
Derivative, Notional Amount 0 135,000,000 691,000,000  
Derivative Assets (Liabilities), at Fair Value, Net 0 0    
Gain (Loss) on Derivative Instruments, Net, Pretax $ 0 $ 300,000 $ 2,500,000