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Significant Accounting Policies - Additional Information (Detail)
$ / shares in Units, $ in Millions
 12 Months Ended
Dec. 31, 2023
USD ($)
Integer
$ / shares
Dec. 31, 2022
USD ($)
$ / shares
Dec. 31, 2022
USD ($)
$ / shares
Dec. 31, 2021
USD ($)
$ / shares
Dec. 31, 2021
USD ($)
$ / shares
Jan. 01, 2021
USD ($)
Dec. 31, 2020
USD ($)
New Accounting Pronouncements or Change in Accounting Principle              
Number of Operating Segments | Integer 3            
Premium Income $ 10,046.0 $ 9,616.5   $ 9,475.0      
Commissions 1,170.1 1,086.4   1,038.1      
Ceded Policy Loans 3,322.5 3,312.5 $ 3,312.5        
Premium Receivable, Allowance for Credit Loss 29.5 32.5 32.5        
Premiums Receivable, Gross 612.4 557.6 557.6        
Premium Receivable, Credit Loss Expense (Reversal) (3.0) (1.7)          
Accumulated Depreciation for Property and Equipment 1,422.8 1,328.7 1,328.7        
Value of Business Acquired 63.9 62.9 62.9        
Accumulated Amortization of Value of Business Acquired 161.8 150.3 150.3        
Amortization of Value of Business Acquired 5.2 4.9   5.7      
Prepaid Reinsurance Premiums 549.4 593.5 593.5        
Deferred Gain on Reinsurance 8.8 3.1 3.1        
Reinsurance Recoverable, Allowance for Credit Loss 1.7 1.7 1.7        
Reinsurance Recoverable, Credit Loss Expense (Reversal)   (0.6)          
Premium Tax Expense 183.5 169.3   166.0      
Participation Fund Account Assets (PFA) 244.4 250.2 250.2        
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest (3,308.0) (3,448.3) (3,448.3) (5,164.6) $ (5,164.6) $ 4,166.0 $ (6,310.2)
Deferred Acquisition Costs 2,714.5 2,560.0 2,560.0     2,357.7  
Reinsurance Recoverables 9,108.4 9,608.0 9,608.0        
Deferred Income Tax Assets, Net 649.4 586.0 586.0        
Other Assets 1,661.7 1,666.6 [1] 1,666.6 [1]        
Total Assets 63,255.2 61,148.5 61,148.5        
Policy and Contract Benefits [2]   0.0 0.0        
Reserves for Future Policy and Contract Benefits [2]   0.0 0.0        
Future Policy Benefits 40,009.4 38,577.1 [2] 38,577.1 [2]        
Policyholders' Account Balances 5,667.7 5,740.2 [2] 5,740.2 [2]        
Unearned Premiums 380.2 365.5 365.5        
Deferred Income Tax 27.0 25.2 25.2        
Liabilities 53,603.8 52,413.5 52,413.5        
Accumulated Other Comprehensive Loss (3,308.0) (3,448.3) (3,448.3) (5,164.6) (5,164.6) (6,310.2)  
Retained Earnings 11,431.5 13,141.3 13,141.3 11,989.4 11,989.4 11,249.0  
Total Stockholders' Equity 9,651.4 8,735.0 8,735.0 6,033.9 6,033.9    
Policy Benefits 7,311.9 7,542.1 [3]   8,113.3 [3]      
Liability for Future Policy Benefit, Remeasurement Gain (Loss) (54.8) (547.5)   (559.9)      
Deferral of Acquisition Costs (632.2) (556.9)   (523.2)      
Amortization of Deferred Acquisition Costs 481.4 421.1   452.1      
Other Expenses 1,112.0 1,006.7   999.3      
Income Tax - Deferred (95.7) (130.7)   66.8      
Net Income $ 1,283.8 $ 1,407.2   $ 981.0      
Basic Earnings Per Share | $ / shares $ 6.53 $ 7.01   $ 4.80      
Diluted Earnings Per Share | $ / shares $ 6.50 $ 6.96   $ 4.79      
Change in Adjustment to Deferred Acquisition Costs and Reserves for Future Policy and Contract Benefits, Net of Reinsurance and Net of Tax Expense Benefit   $ 0.0   $ 0.0      
Change in the Effect of Discount Rate Assumptions on the Liability for Future Policy Benefits, Net of Reinsurance (net of tax expense (benefit) of $(256.5); $2,385.1; $622.8) $ (962.3) 8,884.6   2,361.8      
Change in Foreign Currency Translation Adjustment 69.0 (116.0)   (12.8)      
Comprehensive Income (Loss) 1,424.1 3,123.5   2,126.6      
Other Comprehensive Income (Loss), Net of Tax 140.3 1,716.3   1,145.6      
Increase (Decrease) in Receivables 602.7 590.6   676.1      
Deferral of Acquisition Costs (150.8) (135.8)   (71.1)      
Change in Insurance Reserves and Liabilities (313.2) (244.7)   37.7      
Change in Income Taxes (84.3) (31.3)   335.3      
Amortization of the Cost of Reinsurance 44.1 50.3   69.8      
Other Noncash Income (Expense) (9.2) 17.1   15.8      
Retained Earnings              
New Accounting Pronouncements or Change in Accounting Principle              
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest           11,249.0  
Total Stockholders' Equity 11,431.5 13,141.3 13,141.3 11,989.4 11,989.4   11,249.0
Net Income 1,283.8 1,407.2   981.0      
Accumulated Other Comprehensive Income (Loss)              
New Accounting Pronouncements or Change in Accounting Principle              
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest           (6,310.2)  
Total Stockholders' Equity (3,308.0) (3,448.3) (3,448.3) (5,164.6) (5,164.6)   (6,310.2)
Other Comprehensive Income (Loss), Net of Tax 140.3 1,716.3   1,145.6      
Operating Segments              
New Accounting Pronouncements or Change in Accounting Principle              
Premium Income 10,046.0 9,616.5   9,475.0      
Liability for Future Policy Benefit, before Reinsurance, after Other 39,759.1 38,357.7 38,357.7 51,779.6 51,779.6 55,585.4  
Liability for Future Policy Benefit, Reinsurance Recoverable, after Allowance 7,756.1 8,128.2 8,128.2 10,348.2 10,348.2 10,496.0 [4]  
Liability for Future Policy Benefit, after Reinsurance 32,003.0 30,229.5 30,229.5 41,431.4 41,431.4 45,089.4  
Deferred Acquisition Costs 2,714.5 2,560.0 2,560.0 2,427.4 2,427.4   2,357.7
Future Policy Benefits 40,009.4 38,577.1 38,577.1 51,991.6 51,991.6    
Policyholders' Account Balances 5,667.7 5,740.2 5,740.2 5,814.8 5,814.8    
Amortization of Deferred Acquisition Costs 481.4 421.1   452.1      
Unum US              
New Accounting Pronouncements or Change in Accounting Principle              
Liability for Future Policy Benefit, before Reinsurance, after Other 9,352.7 9,668.7 9,668.7 12,020.0 12,020.0 12,861.4  
Liability for Future Policy Benefit, Reinsurance Recoverable, after Allowance 208.6 251.2 251.2 308.4 308.4 357.7 [4]  
Liability for Future Policy Benefit, after Reinsurance 9,144.1 9,417.5 9,417.5 11,711.6 11,711.6 12,503.7  
Deferred Acquisition Costs 1,232.2 1,185.1 1,185.1 1,152.9 1,152.9 1,181.0 1,181.0
Future Policy Benefits [5] 9,352.7 9,668.7 9,668.7 12,020.0 12,020.0    
Amortization of Deferred Acquisition Costs 267.6 240.9   285.9      
Unum US | Group Disability              
New Accounting Pronouncements or Change in Accounting Principle              
Liability for Future Policy Benefit, before Reinsurance, after Other 5,147.6 5,533.7 5,533.7 6,726.2 6,726.2 7,295.7  
Liability for Future Policy Benefit, Reinsurance Recoverable, after Allowance 30.7 36.0 36.0 40.9 40.9 58.3  
Liability for Future Policy Benefit, after Reinsurance 5,116.9 5,497.7 5,497.7 6,685.3 6,685.3 7,237.4  
Deferred Acquisition Costs 63.6 61.0 61.0 60.9 60.9   95.3
Amortization of Deferred Acquisition Costs 57.6 53.0   84.2      
Unum US | Group Life and AD&D              
New Accounting Pronouncements or Change in Accounting Principle              
Liability for Future Policy Benefit, before Reinsurance, after Other 923.0 973.5 973.5 1,125.3 1,125.3 1,127.3  
Liability for Future Policy Benefit, Reinsurance Recoverable, after Allowance 7.2 7.5 7.5 4.4 4.4 3.0  
Liability for Future Policy Benefit, after Reinsurance 915.8 966.0 966.0 1,120.9 1,120.9 1,124.3  
Deferred Acquisition Costs 48.9 49.3 49.3 53.9 53.9   76.4
Amortization of Deferred Acquisition Costs 39.0 41.9   58.6      
Unum US | Individual Disability              
New Accounting Pronouncements or Change in Accounting Principle              
Liability for Future Policy Benefit, before Reinsurance, after Other 2,079.7 2,014.6 2,014.6 2,582.0 2,582.0 2,767.7  
Liability for Future Policy Benefit, Reinsurance Recoverable, after Allowance 156.7 193.6 193.6 231.9 231.9 260.6  
Liability for Future Policy Benefit, after Reinsurance 1,923.0 1,821.0 1,821.0 2,350.1 2,350.1 2,507.1  
Deferred Acquisition Costs 497.8 464.4 464.4 441.8 441.8   423.6
Amortization of Deferred Acquisition Costs 54.0 49.4   45.7      
Unum US | Voluntary Benefits              
New Accounting Pronouncements or Change in Accounting Principle              
Liability for Future Policy Benefit, before Reinsurance, after Other 1,202.4 1,146.9 1,146.9 1,586.5 1,586.5 1,659.2  
Liability for Future Policy Benefit, Reinsurance Recoverable, after Allowance 14.0 14.1 14.1 31.2 31.2 35.7  
Liability for Future Policy Benefit, after Reinsurance 1,188.4 1,132.8 1,132.8 1,555.3 1,555.3 1,623.5  
Deferred Acquisition Costs 610.6 601.0 601.0 588.6 588.6   569.7
Policyholders' Account Balances 678.1 679.7 679.7 681.6 681.6    
Amortization of Deferred Acquisition Costs 106.2 87.6   80.5      
Unum US | Dental and Vision              
New Accounting Pronouncements or Change in Accounting Principle              
Liability for Future Policy Benefit, before Reinsurance, after Other           11.5  
Liability for Future Policy Benefit, Reinsurance Recoverable, after Allowance           0.1  
Liability for Future Policy Benefit, after Reinsurance           11.4  
Deferred Acquisition Costs 11.3 9.4 9.4 7.7 7.7   16.0
Amortization of Deferred Acquisition Costs 10.8 9.0   16.9      
Unum US | Operating Segments              
New Accounting Pronouncements or Change in Accounting Principle              
Premium Income 6,579.2 6,251.4   6,072.0      
Total Assets 15,561.1 16,356.3 16,356.3        
Amortization of Deferred Acquisition Costs 267.6 240.9   285.9      
Unum US | Operating Segments | Voluntary Benefits              
New Accounting Pronouncements or Change in Accounting Principle              
Premium Income 850.1 833.7   840.7      
Unum US | Operating Segments | Dental and Vision              
New Accounting Pronouncements or Change in Accounting Principle              
Premium Income 278.1 275.8   272.7      
Unum International              
New Accounting Pronouncements or Change in Accounting Principle              
Liability for Future Policy Benefit, before Reinsurance, after Other 2,293.2 2,063.2 2,063.2 2,955.8 2,955.8 3,203.4  
Liability for Future Policy Benefit, Reinsurance Recoverable, after Allowance 78.7 70.3 70.3 125.3 125.3 132.5 [4]  
Liability for Future Policy Benefit, after Reinsurance 2,214.5 1,992.9 1,992.9 2,830.5 2,830.5 3,070.9  
Deferred Acquisition Costs 46.9 37.0 37.0 36.4 36.4 32.0 32.0
Future Policy Benefits 2,293.2 2,063.2 2,063.2 2,955.8 2,955.8    
Amortization of Deferred Acquisition Costs 8.4 8.2   7.0      
Unum International | Operating Segments              
New Accounting Pronouncements or Change in Accounting Principle              
Premium Income 825.2 718.8   717.0      
Total Assets 3,372.9 3,129.4 3,129.4        
Amortization of Deferred Acquisition Costs 8.4 8.2   7.0      
Colonial Life              
New Accounting Pronouncements or Change in Accounting Principle              
Liability for Future Policy Benefit, before Reinsurance, after Other 1,997.8 1,858.4 1,858.4 2,482.9 2,482.9 2,528.6  
Liability for Future Policy Benefit, Reinsurance Recoverable, after Allowance 1.8 1.1 1.1 2.3 2.3 4.4 [4]  
Liability for Future Policy Benefit, after Reinsurance 1,996.0 1,857.3 1,857.3 2,480.6 2,480.6 2,524.2  
Deferred Acquisition Costs 1,435.4 1,337.9 1,337.9 1,238.1 1,238.1 1,144.7 1,144.7
Future Policy Benefits 1,997.8 1,858.4 1,858.4 2,482.9 2,482.9    
Policyholders' Account Balances 869.8 869.1 869.1 870.7 870.7    
Amortization of Deferred Acquisition Costs 205.4 172.0   159.2      
Colonial Life | Operating Segments              
New Accounting Pronouncements or Change in Accounting Principle              
Premium Income 1,726.1 1,702.0   1,690.2      
Total Assets 4,830.4 4,575.0 4,575.0        
Amortization of Deferred Acquisition Costs 205.4 172.0   159.2      
Closed Block              
New Accounting Pronouncements or Change in Accounting Principle              
Liability for Future Policy Benefit, before Reinsurance, after Other 26,115.4 24,767.4 24,767.4 34,320.9 34,320.9 36,992.0  
Liability for Future Policy Benefit, Reinsurance Recoverable, after Allowance 7,466.9 7,805.5 7,805.5 9,912.1 9,912.1 10,001.4 [4]  
Liability for Future Policy Benefit, after Reinsurance 18,648.5 16,961.9 16,961.9 24,408.8 24,408.8 26,990.6  
Future Policy Benefits [5] 26,115.4 24,767.4 24,767.4 34,320.9 34,320.9    
Closed Block | Long-term Care              
New Accounting Pronouncements or Change in Accounting Principle              
Liability for Future Policy Benefit, before Reinsurance, after Other 16,597.3 14,811.6 14,811.6 21,714.7 21,714.7 22,740.9  
Liability for Future Policy Benefit, Reinsurance Recoverable, after Allowance 4.5 5.7 5.7 7.4 7.4 44.4  
Liability for Future Policy Benefit, after Reinsurance 16,592.8 14,805.9 14,805.9 21,707.3 21,707.3 22,696.5  
Closed Block | Other Insurance Product Line              
New Accounting Pronouncements or Change in Accounting Principle              
Liability for Future Policy Benefit, before Reinsurance, after Other 9,518.1 9,955.8 9,955.8 12,606.2 12,606.2 14,251.1  
Liability for Future Policy Benefit, Reinsurance Recoverable, after Allowance 7,462.4 7,799.8 7,799.8 9,904.7 9,904.7 9,957.0  
Liability for Future Policy Benefit, after Reinsurance 2,055.7 2,156.0 2,156.0 2,701.5 2,701.5 4,294.1  
Policyholders' Account Balances 4,119.8 4,191.4 4,191.4 4,262.5 4,262.5    
Closed Block | Operating Segments              
New Accounting Pronouncements or Change in Accounting Principle              
Premium Income 915.5 944.3   995.8      
Total Assets 35,272.8 33,776.6 33,776.6        
Amortization of Deferred Acquisition Costs 0.0 0.0   0.0      
Closed Block | Operating Segments | Long-term Care              
New Accounting Pronouncements or Change in Accounting Principle              
Premium Income 696.0 697.4   704.3      
Closed Block | Operating Segments | Other Insurance Product Line              
New Accounting Pronouncements or Change in Accounting Principle              
Premium Income 219.5 246.9   291.5      
Previously Reported              
New Accounting Pronouncements or Change in Accounting Principle              
Premium Income   9,623.4   9,481.0      
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest             10,871.0
Deferred Acquisition Costs   2,252.3 2,252.3       2,272.6
Reinsurance Recoverables   10,218.6 10,218.6        
Deferred Income Tax Assets, Net   449.8 449.8        
Other Assets [1]   1,786.3 1,786.3        
Total Assets   61,434.9 61,434.9        
Policy and Contract Benefits [2]   1,839.8 1,839.8        
Reserves for Future Policy and Contract Benefits [2]   42,330.2 42,330.2        
Future Policy Benefits [2]   0.0 0.0        
Policyholders' Account Balances [2]   0.0 0.0        
Unearned Premiums   352.7 352.7        
Deferred Income Tax   9.2 9.2        
Liabilities   52,237.4 52,237.4        
Accumulated Other Comprehensive Loss   (2,756.6) (2,756.6) 354.1 354.1 374.2  
Retained Earnings   12,912.1 12,912.1 11,853.2 $ 11,853.2 11,269.6  
Total Stockholders' Equity   9,197.5 $ 9,197.5        
Policy Benefits [3]   6,936.7   7,598.6      
Liability for Future Policy Benefit, Remeasurement Gain (Loss)   0.0   0.0      
Deferral of Acquisition Costs       (508.1)      
Amortization of Deferred Acquisition Costs   591.0   586.1      
Other Expenses   1,020.2   1,008.6      
Income Tax - Deferred   (156.3)   26.0      
Net Income   1,314.2   824.2      
Basic Earnings Per Share | $ / shares     $ 6.55   $ 4.04    
Diluted Earnings Per Share | $ / shares     $ 6.50   $ 4.02    
Change in Adjustment to Deferred Acquisition Costs and Reserves for Future Policy and Contract Benefits, Net of Reinsurance and Net of Tax Expense Benefit   4,056.9   1,195.9      
Change in the Effect of Discount Rate Assumptions on the Liability for Future Policy Benefits, Net of Reinsurance (net of tax expense (benefit) of $(256.5); $2,385.1; $622.8)   0.0   0.0      
Change in Foreign Currency Translation Adjustment   (115.2)   (12.6)      
Comprehensive Income (Loss)   (1,796.5)   804.1      
Other Comprehensive Income (Loss), Net of Tax   (3,110.7)   (20.1)      
Increase (Decrease) in Receivables   566.0   678.0      
Deferral of Acquisition Costs   34.1   78.0      
Change in Insurance Reserves and Liabilities   (282.0)   74.2      
Change in Income Taxes   (58.8)   295.3      
Amortization of the Cost of Reinsurance   63.8   79.1      
Other Noncash Income (Expense)   16.1          
Previously Reported | Retained Earnings              
New Accounting Pronouncements or Change in Accounting Principle              
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest             11,269.6
Previously Reported | Accumulated Other Comprehensive Income (Loss)              
New Accounting Pronouncements or Change in Accounting Principle              
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest             374.2
Previously Reported | Operating Segments              
New Accounting Pronouncements or Change in Accounting Principle              
Liability for Future Policy Benefit, before Reinsurance, after Other [6]             45,704.4
Previously Reported | Unum US              
New Accounting Pronouncements or Change in Accounting Principle              
Liability for Future Policy Benefit, before Reinsurance, after Other [6]             12,184.6
Deferred Acquisition Costs             1,168.7
Previously Reported | Unum US | Group Disability              
New Accounting Pronouncements or Change in Accounting Principle              
Liability for Future Policy Benefit, before Reinsurance, after Other             7,409.0
Previously Reported | Unum US | Group Life and AD&D              
New Accounting Pronouncements or Change in Accounting Principle              
Liability for Future Policy Benefit, before Reinsurance, after Other             1,123.9
Previously Reported | Unum US | Individual Disability              
New Accounting Pronouncements or Change in Accounting Principle              
Liability for Future Policy Benefit, before Reinsurance, after Other             2,485.3
Previously Reported | Unum US | Voluntary Benefits              
New Accounting Pronouncements or Change in Accounting Principle              
Liability for Future Policy Benefit, before Reinsurance, after Other             1,154.9
Previously Reported | Unum US | Dental and Vision              
New Accounting Pronouncements or Change in Accounting Principle              
Liability for Future Policy Benefit, before Reinsurance, after Other             11.5
Previously Reported | Unum International              
New Accounting Pronouncements or Change in Accounting Principle              
Liability for Future Policy Benefit, before Reinsurance, after Other [6]             2,969.8
Deferred Acquisition Costs             32.0
Previously Reported | Colonial Life              
New Accounting Pronouncements or Change in Accounting Principle              
Liability for Future Policy Benefit, before Reinsurance, after Other [6]             1,977.2
Deferred Acquisition Costs             1,071.9
Previously Reported | Closed Block              
New Accounting Pronouncements or Change in Accounting Principle              
Liability for Future Policy Benefit, before Reinsurance, after Other [6]             28,572.8
Previously Reported | Closed Block | Long-term Care              
New Accounting Pronouncements or Change in Accounting Principle              
Liability for Future Policy Benefit, before Reinsurance, after Other             16,283.4
Previously Reported | Closed Block | Other Insurance Product Line              
New Accounting Pronouncements or Change in Accounting Principle              
Liability for Future Policy Benefit, before Reinsurance, after Other             $ 12,289.4
Accounting Standards Update 2018-12 | Revision of Prior Period, Accounting Standards Update, Adjustment              
New Accounting Pronouncements or Change in Accounting Principle              
Premium Income   (6.9)   (6.0)      
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest           (6,705.0)  
Deferred Acquisition Costs   307.7 $ 307.7        
Removal of Shadow Adjustments, DAC           85.1  
Reinsurance Recoverables   (610.6) (610.6)        
Deferred Income Tax Assets, Net   136.2 136.2        
Other Assets [1]   (119.7) (119.7)        
Total Assets   (286.4) (286.4)        
Policy and Contract Benefits [2]   (1,839.8) (1,839.8)        
Reserves for Future Policy and Contract Benefits [2]   (42,330.2) (42,330.2)        
Future Policy Benefits [2]   38,577.1 38,577.1        
Policyholders' Account Balances [2]   5,740.2 5,740.2        
Unearned Premiums   12.8 12.8        
Deferred Income Tax   16.0 16.0        
Liabilities   176.1 176.1        
Accumulated Other Comprehensive Loss   (691.7) (691.7) (5,518.7) $ (5,518.7) (6,684.4)  
Retained Earnings   229.2 229.2 136.2 $ 136.2 (20.6)  
Total Stockholders' Equity   (462.5) $ (462.5)        
Policy Benefits [3]   605.4   514.7      
Liability for Future Policy Benefit, Remeasurement Gain (Loss)   (547.5)   (559.9)      
Deferral of Acquisition Costs       (15.1)      
Amortization of Deferred Acquisition Costs   (169.9)   (134.0)      
Other Expenses   (13.5)   (9.3)      
Income Tax - Deferred   25.6   40.8      
Net Income   93.0   156.8      
Basic Earnings Per Share | $ / shares     $ 0.46   $ 0.76    
Diluted Earnings Per Share | $ / shares     $ 0.46   $ 0.77    
Change in Adjustment to Deferred Acquisition Costs and Reserves for Future Policy and Contract Benefits, Net of Reinsurance and Net of Tax Expense Benefit   (4,056.9)   (1,195.9)      
Change in the Effect of Discount Rate Assumptions on the Liability for Future Policy Benefits, Net of Reinsurance (net of tax expense (benefit) of $(256.5); $2,385.1; $622.8)   8,884.6   2,361.8      
Change in Foreign Currency Translation Adjustment   (0.8)   (0.2)      
Comprehensive Income (Loss)   4,920.0   1,322.5      
Other Comprehensive Income (Loss), Net of Tax   4,827.0   1,165.7      
Increase (Decrease) in Receivables   24.6   (1.9)      
Deferral of Acquisition Costs   (169.9)   (149.1)      
Change in Insurance Reserves and Liabilities   37.3   (36.5)      
Change in Income Taxes   27.5   40.0      
Amortization of the Cost of Reinsurance   (13.5)   $ (9.3)      
Other Noncash Income (Expense)   $ 1.0          
Accounting Standards Update 2018-12 | Revision of Prior Period, Accounting Standards Update, Adjustment | Retained Earnings              
New Accounting Pronouncements or Change in Accounting Principle              
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest           (20.6)  
Accounting Standards Update 2018-12 | Revision of Prior Period, Accounting Standards Update, Adjustment | Accumulated Other Comprehensive Income (Loss)              
New Accounting Pronouncements or Change in Accounting Principle              
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest           (6,684.4)  
Accounting Standards Update 2018-12 | Revision of Prior Period, Accounting Standards Update, Adjustment | Operating Segments              
New Accounting Pronouncements or Change in Accounting Principle              
Removal of Shadow Adjustments, LFPB [7]           (6,225.6)  
Adjustments for Loss Contracts [8]           34.9  
Effect of Change in Discount Rate Assumptions           16,084.6  
Other increase (decrease) [9]           (12.9)  
Accounting Standards Update 2018-12 | Revision of Prior Period, Accounting Standards Update, Adjustment | Unum US              
New Accounting Pronouncements or Change in Accounting Principle              
Removal of Shadow Adjustments, LFPB [7]           (1,569.7)  
Adjustments for Loss Contracts [8]           0.0  
Effect of Change in Discount Rate Assumptions           2,251.8  
Other increase (decrease) [9]           (5.3)  
Removal of Shadow Adjustments, DAC [10]           12.3  
Accounting Standards Update 2018-12 | Revision of Prior Period, Accounting Standards Update, Adjustment | Unum US | Group Disability              
New Accounting Pronouncements or Change in Accounting Principle              
Removal of Shadow Adjustments, LFPB           (1,025.2)  
Effect of Change in Discount Rate Assumptions           911.9  
Other increase (decrease)           0.0  
Accounting Standards Update 2018-12 | Revision of Prior Period, Accounting Standards Update, Adjustment | Unum US | Group Life and AD&D              
New Accounting Pronouncements or Change in Accounting Principle              
Removal of Shadow Adjustments, LFPB           (88.5)  
Effect of Change in Discount Rate Assumptions           91.9  
Other increase (decrease)           0.0  
Accounting Standards Update 2018-12 | Revision of Prior Period, Accounting Standards Update, Adjustment | Unum US | Individual Disability              
New Accounting Pronouncements or Change in Accounting Principle              
Removal of Shadow Adjustments, LFPB           (446.1)  
Effect of Change in Discount Rate Assumptions           728.5  
Other increase (decrease)           0.0  
Accounting Standards Update 2018-12 | Revision of Prior Period, Accounting Standards Update, Adjustment | Unum US | Voluntary Benefits              
New Accounting Pronouncements or Change in Accounting Principle              
Removal of Shadow Adjustments, LFPB           (9.9)  
Effect of Change in Discount Rate Assumptions           519.5  
Other increase (decrease)           (5.3)  
Accounting Standards Update 2018-12 | Revision of Prior Period, Accounting Standards Update, Adjustment | Unum US | Dental and Vision              
New Accounting Pronouncements or Change in Accounting Principle              
Removal of Shadow Adjustments, LFPB           0.0  
Effect of Change in Discount Rate Assumptions           0.0  
Other increase (decrease)           0.0  
Accounting Standards Update 2018-12 | Revision of Prior Period, Accounting Standards Update, Adjustment | Unum International              
New Accounting Pronouncements or Change in Accounting Principle              
Removal of Shadow Adjustments, LFPB [7]           (545.8)  
Adjustments for Loss Contracts [8]           0.0  
Effect of Change in Discount Rate Assumptions           779.4  
Other increase (decrease) [9]           0.0  
Removal of Shadow Adjustments, DAC           0.0  
Accounting Standards Update 2018-12 | Revision of Prior Period, Accounting Standards Update, Adjustment | Colonial Life              
New Accounting Pronouncements or Change in Accounting Principle              
Removal of Shadow Adjustments, LFPB [7]           (44.5)  
Adjustments for Loss Contracts [8]           34.9  
Effect of Change in Discount Rate Assumptions           568.6  
Other increase (decrease) [9]           (7.6)  
Removal of Shadow Adjustments, DAC           72.8  
Accounting Standards Update 2018-12 | Revision of Prior Period, Accounting Standards Update, Adjustment | Closed Block              
New Accounting Pronouncements or Change in Accounting Principle              
Removal of Shadow Adjustments, LFPB [7]           (4,065.6)  
Adjustments for Loss Contracts [8]           0.0  
Effect of Change in Discount Rate Assumptions           12,484.8  
Other increase (decrease) [9]           0.0  
Accounting Standards Update 2018-12 | Revision of Prior Period, Accounting Standards Update, Adjustment | Closed Block | Long-term Care              
New Accounting Pronouncements or Change in Accounting Principle              
Removal of Shadow Adjustments, LFPB           (3,465.4)  
Effect of Change in Discount Rate Assumptions           9,922.9  
Accounting Standards Update 2018-12 | Revision of Prior Period, Accounting Standards Update, Adjustment | Closed Block | Other Insurance Product Line              
New Accounting Pronouncements or Change in Accounting Principle              
Removal of Shadow Adjustments, LFPB           (600.2)  
Effect of Change in Discount Rate Assumptions           $ 2,561.9  
Change in Accounting Method Accounted for as Change in Estimate              
New Accounting Pronouncements or Change in Accounting Principle              
Premium Income (13.4)            
Commissions (1.0)            
Net Income $ (9.8)            
Basic Earnings Per Share | $ / shares $ 0.05            
Diluted Earnings Per Share | $ / shares $ 0.05            
[1]
1The change in other assets is driven by the cost of reinsurance associated with the second phase of our Closed Block individual disability reinsurance transaction which was completed in the first quarter of 2021. In accordance with the provisions of the ASU related to non-contemporaneous reinsurance, we were then required to establish the ceded reserves using an upper-medium grade fixed-income instrument as of the reinsurance transaction date in March 2021 which resulted in higher ceded reserves compared to that which was reported historically.
[2]
2We previously reported policy and contract benefits and reserves for future policy and contract benefits liabilities in our consolidated balance sheet as of December 31, 2022 of $1,839.8 million and $42,330.2 million, respectively, resulting in total policyholder liabilities of $44,170.0 million. These balances were reclassified into new line items, future policy benefits and policyholders' account balances, which would have reported balances of $38,443.2 million and $5,726.8 million, respectively, as of December 31, 2022, resulting in total policyholder liabilities of $44,170.0 million under the historical accounting method.
[3]
1 Previously disclosed as Benefits and Change in Reserves for Future Benefits
[4]
5These amounts represent the portion of reinsurance recoverable related to the liability for future policy benefits. These amounts include the adjustments for the removal of shadow adjustments and for the effect of change in discount rate assumptions for the liability for future policy benefits ceded to third party reinsurers.
[5]
1Unum US excludes dental & vision and medical stop-loss product lines and Closed Block excludes our participating fund account, which represents policies issued by one of our subsidiaries prior to its 1986 conversion from a mutual stock life insurance company. The liabilities associated with these products are included within Other products.
[6]
1We previously reported policy and contract benefits and reserves for future policy and contract benefits liabilities in our consolidated balance sheet as of December 31, 2020 of $1,855.4 million and $49,653.0 million, respectively, resulting in total policyholder liabilities of $51,508.4 million. These balances were reclassified into new line items, future policy benefits and policyholders' account balances, which would have reported balances of $45,704.4 million and $5,804.0 million as of December 31, 2020, respectively, resulting in total policyholder liabilities of $51,508.4 million under the historical accounting method. These balances were reclassified to more closely align with the new disclosure requirements of ASU 2018-12.
[7]
2Shadow adjustments represent the adjustments related to unrealized investment gains and losses previously included in the reserves balance prior to the adoption of ASU 2018-12.
[8]
3Adjustment for loss contracts represents the adjustment for those cohorts whose net premium ratio exceeded 100 percent as of the transition date. For those cohorts, the net premiums were set equal to the gross premiums and the difference was recorded as a transition adjustment to the liability for future policy benefits.
[9]
4This amount reflects the effect of the reclassification of certain unearned premium. This amount was reclassified to more closely align with the presentation of liabilities under ASU 2018-12 and had no impact to stockholders' equity.
[10]
1The $12.3 million removal of shadow adjustments is related to the Unum US voluntary benefits product line.