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Income Taxes Reconciliation of Provision (Benefit) for income Taxes Reported in Consolidated Statements Of Income with Tax at Statutory Rate (Detail) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Apr. 01, 2015
Effective Income Tax Rate Reconciliation, Amount [Abstract]        
Income before income taxes $ 2,138,900,000 $ 1,470,700,000 $ 1,911,600,000  
Tax at statutory federal rate 748,600,000 514,800,000 669,100,000  
Net deferred tax liability revaluation1 (99,500,000) 0.00 0.00  
Tax credits (52,400,000) (62,200,000) (1,900,000)  
Stock-based compensation2 (25,100,000) 0 0  
Dividends received deduction (20,700,000) (22,600,000) (19,800,000)  
Exempt interest income (16,900,000) (15,700,000) (17,800,000)  
Tax-deductible dividends (9,700,000) (6,100,000) (7,900,000)  
State income taxes, net of federal taxes 6,000,000 6,600,000 2,300,000  
Other items, net 400,000 (1,800,000) 600,000  
Non-taxable gain4 0 0 (13,800,000)  
Total income tax provision $ 540,800,000 $ 413,500,000 $ 611,100,000  
Effective Income Tax Rate Reconciliation, Percent [Abstract]        
Tax at statutory federal rate 35.00% 35.00% 35.00%  
Net deferred tax liability revaluation1 (5.00%) 0.00% 0.00%  
Tax credits (2.00%) (4.00%) (0.00%)  
Stock-based compensation2 (1.00%) 0.00% 0.00%  
Dividends received deduction (1.00%) (2.00%) (1.00%)  
Exempt interest income (1.00%) (1.00%) (1.00%)  
Tax-deductible dividends (0.00%) (0.00%) (0.00%)  
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amount $ 10,100,000 $ 500,000 $ 300,000  
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Percent 0.00% 0.00% 0.00%  
State income taxes, net of federal taxes 0.00% 0.00% 0.00%  
Other items, net 0.00% 0.00% 0.00%  
Non-taxable gain4 0.00% 0.00% (1.00%)  
Total income tax provision 25.00% 28.00% 32.00%  
ARX Holding Corp.        
Business Acquisition [Line Items]        
Ownership interest prior to acquisition       5.00%