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Investments - Additional Information (Detail) - USD ($)
1 Months Ended 3 Months Ended 6 Months Ended
Apr. 03, 2017
Apr. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Mar. 31, 2018
Dec. 31, 2017
Schedule of Investments [Line Items]                
Securities Sold under Agreements to Repurchase     $ 0 $ 0 $ 0 $ 0   $ 0
Open reverse repurchase commitments     0 0 0 0   0
Gross unrealized losses     304,100,000   304,100,000      
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment         $ 0 $ 0    
Investment Portfolio Duration         2 years 7 months 10 days 2 years 3 months 20 days    
Trading securities     0 0 $ 0 $ 0   0
Debt and Equity Securities, Gain (Loss)     0 0 0 0    
Derivative Assets (Liabilities), at Fair Value, Net     $ 0 $ 0 $ 0 $ 0   $ 0
4.20% Senior Notes due 2048                
Schedule of Investments [Line Items]                
Debt, face amount             $ 600,000,000  
Debt, stated interest rate     4.20%   4.20%   4.20%  
4.125% Senior Notes due 2047                
Schedule of Investments [Line Items]                
Debt, face amount $ 850,000,000 $ 850,000,000            
Debt, stated interest rate 4.125% 4.125% 4.125%   4.125%      
Unrealized gain (loss) on interest rate cash flow hedges $ (8,000,000) $ (8,000,000)            
Series B Preferred Stock [Member]                
Schedule of Investments [Line Items]                
Preferred Stock, Value, Issued             $ 500,000,000