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Investments - Additional Information (Detail)
 1 Months Ended 12 Months Ended
Apr. 30, 2017
USD ($)
Aug. 31, 2016
USD ($)
Dec. 31, 2017
USD ($)
security
Dec. 31, 2016
USD ($)
security
Dec. 31, 2015
USD ($)
Schedule of Investments [Line Items]          
Principal amount of bonds and certificates of deposit on deposit to meet state insurance regulatory and/or rating agency requirements     $ 222,600,000    
Securities exceeding 10% of Shareholders' Equity     0 $ 0  
Fair value of fixed-maturity securities that were non-income producing during the preceding 12 months     0    
Short-term Investments     2,869,400,000 3,572,900,000  
Securities Sold under Agreements to Repurchase     0 0  
Open reverse repurchase commitments     0 0  
Gross Unrealized Losses     $ 99,400,000 $ 139,800,000  
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security     1,166 1,369  
Trading securities     $ 0 $ 0  
Net realized gains (losses) on securities     114,100,000 137,900,000 $ 136,500,000
Gain (Loss) on Derivative Instruments, Net, Pretax     0 (20,000,000) (20,700,000)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax     300,000 1,900,000 1,800,000
Derivative Assets (Liabilities), at Fair Value, Net     0 0  
4.125% Senior Notes due 2047          
Schedule of Investments [Line Items]          
Senior notes, issuance amount $ 850,000,000   $ 850,000,000 0  
Debt Instrument, Interest Rate, Stated Percentage 4.125%   4.125%    
Unrealized Gain (Loss) on Interest Rate Cash Flow Hedges, Pretax, Accumulated Other Comprehensive Income (Loss) $ (8,000,000)        
2.45% Senior Notes Due 2027          
Schedule of Investments [Line Items]          
Senior notes, issuance amount   $ 500,000,000 $ 500,000,000 500,000,000  
Debt Instrument, Interest Rate, Stated Percentage   2.45% 2.45%    
6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067          
Schedule of Investments [Line Items]          
Senior notes, issuance amount     $ 0 1,000,000,000  
Debt Instrument, Interest Rate, Stated Percentage     6.70%    
Debt Instrument, Repurchased Face Amount     $ 31,000,000 20,000,000 18,000,000
Equity securities          
Schedule of Investments [Line Items]          
Gross Unrealized Losses     $ 9,000,000 $ 18,200,000  
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security     23 88  
Equity securities | Common equities          
Schedule of Investments [Line Items]          
Gross Unrealized Losses     $ 200,000 $ 2,100,000  
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security     19 75  
Equity securities | Nonredeemable preferred stocks          
Schedule of Investments [Line Items]          
Gross Unrealized Losses     $ 8,800,000 $ 16,100,000  
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security     4 13  
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Percentage Decline     6.00%    
Fixed maturities          
Schedule of Investments [Line Items]          
Gross Unrealized Losses     $ 90,400,000 $ 121,600,000  
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security     1,143 1,281  
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment     $ 400,000 $ 0 0
Fixed maturities | Corporate debt securities          
Schedule of Investments [Line Items]          
Gross Unrealized Losses     $ 14,400,000 $ 24,300,000  
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security     222 184  
Fixed maturities | Redeemable preferred stocks          
Schedule of Investments [Line Items]          
Gross Unrealized Losses     $ 1,500,000 $ 2,000,000  
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security     2 2  
Fixed maturities | Commercial mortgage-backed securities          
Schedule of Investments [Line Items]          
Gross Unrealized Losses     $ 13,300,000 $ 25,500,000  
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security     105 111  
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment     $ 400,000 $ 0 0
Fixed Income Securities          
Schedule of Investments [Line Items]          
Gross Unrealized Losses     99,200,000    
Foreign Government Debt          
Schedule of Investments [Line Items]          
Short-term Investments     0 0  
Interest Rate Swaps          
Schedule of Investments [Line Items]          
Cash collateral received from counterparty       4,900,000  
Categories of Investments, Marketable Securities, Trading Securities          
Schedule of Investments [Line Items]          
Net realized gains (losses) on securities     $ 0 0 0
Russell One Thousand | Equity securities | Common equities          
Schedule of Investments [Line Items]          
Percentage of common stock portfolio     96.00%    
Managed Equity Strategy | Equity securities | Common equities          
Schedule of Investments [Line Items]          
Percentage of common stock portfolio     4.00%    
Designated as Hedging Instrument [Member] | Ineffective Cash Flow Hedge [Member]          
Schedule of Investments [Line Items]          
Derivative, Notional Amount   $ 350,000,000 $ 31,000,000 370,000,000 18,000,000
Gain (Loss) on Derivative Instruments, Net, Pretax     0 (1,300,000) 200,000
Derivative Assets (Liabilities), at Fair Value, Net     0 0  
Designated as Hedging Instrument [Member] | Ineffective Cash Flow Hedge [Member] | 2.45% Senior Notes Due 2027          
Schedule of Investments [Line Items]          
Gain (Loss) on Derivative Instruments, Net, Pretax   $ (1,400,000) (1,400,000)    
Non-hedging Instruments | Interest Rate Swaps          
Schedule of Investments [Line Items]          
Derivative Assets (Liabilities), at Fair Value, Net     0 0  
Non-hedging Instruments | Future [Member]          
Schedule of Investments [Line Items]          
Derivative Assets (Liabilities), at Fair Value, Net     0 0  
Non-hedging Instruments | Fixed maturities | Interest Rate Swaps          
Schedule of Investments [Line Items]          
Derivative, Notional Amount     0 0 750,000,000
Gain (Loss) on Derivative Instruments, Net, Pretax     0 0 $ (23,400,000)
Derivative Assets (Liabilities), at Fair Value, Net     $ 0 $ 0