Accounting Developments (Details) $ / shares in Units, $ in Millions		3 Months Ended
		Mar. 31, 2023 USD ($) $ / shares	Mar. 31, 2022 USD ($) $ / shares		Mar. 31, 2022 USD ($) $ / shares		Dec. 31, 2022 USD ($)		Dec. 31, 2021 USD ($)	Jan. 01, 2021 USD ($)		Dec. 31, 2020 USD ($)
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest		$ (3,162.3)	$ (4,438.5)		$ (4,438.5)		$ (3,448.3)		$ (5,164.6)	$ 4,166.0
Other Comprehensive Income (Loss), Net of Tax		286.0	726.1
Liability for Future Policy Benefits		39,351.5	46,474.7	[1]	46,474.7	[1]	38,577.1	[2]
Policyholders' Account Balances		5,709.6	5,772.3	[1]	5,772.3	[1]	5,740.2	[2]
Unearned Premiums		451.6	444.8		444.8		365.5
Deferred Income Tax		23.8	104.5		104.5		25.2
Liabilities		53,176.4	60,878.0		60,878.0		52,413.5
Accumulated Other Comprehensive Loss		(3,162.3)	(4,438.5)		(4,438.5)		(3,448.3)		(5,164.6)
Retained Earnings		13,430.4	12,166.9		12,166.9		13,141.3		11,989.4
Stockholders' Equity Attributable to Parent		9,247.4	6,887.8		6,887.8		8,735.0
Deferred Acquisition Costs		2,602.4	2,464.3		2,464.3		2,560.0					$ 2,272.6
Deferred Income Tax Assets, Net		489.8	889.8		889.8		586.0
Other Assets		1,677.2	1,704.9	[3]	1,704.9	[3]	1,666.6
Total Assets		62,423.8	67,765.8		67,765.8		61,148.5
Policy and Contract Benefits			0.0	[1]	0.0	[1]	0.0	[2]
Reserves for Future Policy and Contract Benefits			0.0	[1]	0.0	[1]	0.0	[2]
Reinsurance Recoverable (net of allowance for credit losses of $3.9; $1.7)		9,628.7	10,939.5		$ 10,939.5		9,608.0
Amortization of Deferred Acquisition Costs		115.9	103.6
Other Expenses		273.2	240.5
Deferred		27.4	(28.0)
Net Income		358.3	240.4
Increase (Decrease) in Receivables		89.9	246.2
Change in Deferred Acquisition Costs		(41.8)	(38.2)
Change in Insurance Reserves and Liabilities		(119.6)	27.5
Change in Income Taxes		76.2	54.4
Amortization of the Cost of Reinsurance		11.0	13.4
Change in the Effect of Discount Rate Assumptions on the Liability for Future Policy Benefits, Net of Reinsurance (net of tax expense (benefit) of $(152.9); $982.8)	[4]		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 $(152.9); $982.8)		(573.9)	3,658.1	[4]
Change in Foreign Currency Translation Adjustment (net of tax expense (benefit) of $(0.2); $0.9)		24.7	(32.0)
Comprehensive Income (Loss)		$ 644.3	$ 966.5
Basic Earnings Per Share | (per share)		$ 1.81	$ 1.19		$ 1.19
Diluted Earnings Per Share | (per share)		$ 1.80	$ 1.18		$ 1.18
Premium Income		$ 2,459.3	$ 2,401.4
Benefits and Change in Reserves for Future Benefits		1,882.1	1,983.0
Policy Benefits - Remeasurement Gain		(145.7)	(68.8)
Operating Segments
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Liability for Future Policy Benefits		39,351.5	46,474.7		$ 46,474.7
Policyholders' Account Balances		5,709.6	5,772.3		5,772.3		5,598.6		5,679.6
Liability for Future Policy Benefit, Reinsurance Recoverable, after Allowance		8,148.2	9,424.6		9,424.6					10,496.0	[5]
Liability for Future Policy Benefit, after Reinsurance		30,975.9	36,826.5		36,826.5					45,089.4
Deferred Acquisition Costs		2,602.4	2,464.3		2,464.3		2,560.0		2,427.4
Amortization of Deferred Acquisition Costs		115.9	103.6
Premium Income		2,459.3	2,401.4
Liability for Future Policy Benefit, before Reinsurance, after other		39,124.1	46,251.1		46,251.1							45,704.4	[6]
Previously Reported [Member]
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest												10,871.0
Other Comprehensive Income (Loss), Net of Tax			(934.2)
Liability for Future Policy Benefits			0.0	[1]	0.0	[1]	0.0	[2]
Policyholders' Account Balances			0.0	[1]	0.0	[1]	0.0	[2]
Unearned Premiums			431.9		431.9		352.7
Deferred Income Tax			202.8		202.8		9.2
Liabilities			55,848.6		55,848.6		52,237.4
Accumulated Other Comprehensive Loss			(580.1)		(580.1)		(2,756.6)		354.1
Retained Earnings			12,043.8		12,043.8		12,912.1		11,853.2
Stockholders' Equity Attributable to Parent			10,623.1		10,623.1		9,197.5
Deferred Acquisition Costs			2,229.2		2,229.2		2,252.3
Deferred Income Tax Assets, Net			0.0		0.0		449.8
Other Assets			1,834.9	[3]	1,834.9	[3]	1,786.3
Total Assets			66,471.7		66,471.7		61,434.9
Policy and Contract Benefits			1,874.2	[1]	1,874.2	[1]	1,839.8	[2]
Reserves for Future Policy and Contract Benefits			45,258.0	[1]	45,258.0	[1]	42,330.2	[2]
Reinsurance Recoverable (net of allowance for credit losses of $3.9; $1.7)			10,640.3		$ 10,640.3		10,218.6
Amortization of Deferred Acquisition Costs			156.1
Other Expenses			243.8
Deferred			(24.7)
Net Income			253.5
Increase (Decrease) in Receivables			239.2
Change in Deferred Acquisition Costs			14.3
Change in Insurance Reserves and Liabilities			(38.3)
Change in Income Taxes			58.3
Amortization of the Cost of Reinsurance			16.7
Change in the Effect of Discount Rate Assumptions on the Liability for Future Policy Benefits, Net of Reinsurance (net of tax expense (benefit) of $(152.9); $982.8)	[4]		1,997.7
Change in the Effect of Discount Rate Assumptions on the Liability for Future Policy Benefits, Net of Reinsurance (net of tax expense (benefit) of $(152.9); $982.8)	[4]		0.0
Change in Foreign Currency Translation Adjustment (net of tax expense (benefit) of $(0.2); $0.9)			(31.9)
Comprehensive Income (Loss)			(680.7)
Basic Earnings Per Share | $ / shares					$ 1.25
Diluted Earnings Per Share | $ / shares					$ 1.25
Premium Income			2,403.3
Benefits and Change in Reserves for Future Benefits			1,843.9
Policy Benefits - Remeasurement Gain			0.0
Revision of Prior Period, Accounting Standards Update, Adjustment
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest										(6,705.0)
Accounting Standards Update 2018-12 | Revision of Prior Period, Accounting Standards Update, Adjustment
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Other Comprehensive Income (Loss), Net of Tax			1,660.3
Liability for Future Policy Benefits			46,474.7	[1]	$ 46,474.7	[1]	38,577.1	[2]
Policyholders' Account Balances			5,772.3	[1]	5,772.3	[1]	5,740.2	[2]
Unearned Premiums			12.9		12.9		12.8
Deferred Income Tax			(98.3)		(98.3)		16.0
Liabilities			5,029.4		5,029.4		176.1
Accumulated Other Comprehensive Loss			(3,858.4)		(3,858.4)		(691.7)		(5,518.7)
Retained Earnings			123.1		123.1		229.2		136.2
Stockholders' Equity Attributable to Parent			(3,735.3)		(3,735.3)		(462.5)
Deferred Acquisition Costs			235.1		235.1		307.7
Deferred Income Tax Assets, Net			889.8		889.8		136.2
Other Assets			(130.0)	[3]	(130.0)	[3]	(119.7)
Total Assets			1,294.1		1,294.1		(286.4)
Policy and Contract Benefits			(1,874.2)	[1]	(1,874.2)	[1]	(1,839.8)	[2]
Reserves for Future Policy and Contract Benefits			(45,258.0)	[1]	(45,258.0)	[1]	(42,330.2)	[2]
Reinsurance Recoverable (net of allowance for credit losses of $3.9; $1.7)			299.2		$ 299.2		(610.6)
Amortization of Deferred Acquisition Costs			(52.5)
Other Expenses			(3.3)
Deferred			(3.3)
Net Income			(13.1)
Increase (Decrease) in Receivables			7.0
Change in Deferred Acquisition Costs			(52.5)
Change in Insurance Reserves and Liabilities			65.8
Change in Income Taxes			(3.9)
Amortization of the Cost of Reinsurance			(3.3)
Change in the Effect of Discount Rate Assumptions on the Liability for Future Policy Benefits, Net of Reinsurance (net of tax expense (benefit) of $(152.9); $982.8)	[4]		(1,997.7)
Change in the Effect of Discount Rate Assumptions on the Liability for Future Policy Benefits, Net of Reinsurance (net of tax expense (benefit) of $(152.9); $982.8)	[4]		3,658.1
Change in Foreign Currency Translation Adjustment (net of tax expense (benefit) of $(0.2); $0.9)			(0.1)
Comprehensive Income (Loss)			1,647.2
Basic Earnings Per Share | $ / shares					$ (0.06)
Diluted Earnings Per Share | $ / shares					$ (0.07)
Premium Income			(1.9)
Benefits and Change in Reserves for Future Benefits			139.1
Policy Benefits - Remeasurement Gain			(68.8)
Unum US
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Liability for Future Policy Benefits	[7]	9,671.3	11,093.3		$ 11,093.3
Liability for Future Policy Benefit, Reinsurance Recoverable, after Allowance		255.8	308.2		308.2					357.7	[5]
Liability for Future Policy Benefit, after Reinsurance		9,415.5	10,785.1		10,785.1					12,503.7
Deferred Acquisition Costs		1,198.3	1,164.3		1,164.3		1,185.1		1,152.9			1,168.7
Amortization of Deferred Acquisition Costs		65.8	56.9
Liability for Future Policy Benefit, before Reinsurance, after other		9,671.3	11,093.3		11,093.3							12,184.6	[6]
Unum US | Operating Segments
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Total Assets		16,085.8					16,356.3
Premium Income		1,609.6	1,543.0
Unum US | Group Disability
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Liability for Future Policy Benefit, Reinsurance Recoverable, after Allowance		42.5	58.0		58.0					58.3
Liability for Future Policy Benefit, after Reinsurance		5,456.8	6,256.5		6,256.5					7,237.4
Deferred Acquisition Costs		62.5	62.5		62.5		61.0		60.9
Amortization of Deferred Acquisition Costs		13.1	11.1
Liability for Future Policy Benefit, before Reinsurance, after other		5,499.3	6,314.5		6,314.5							7,409.0
Unum US | Group Life and AD&D
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Liability for Future Policy Benefit, Reinsurance Recoverable, after Allowance		5.6	4.7		4.7					3.0
Liability for Future Policy Benefit, after Reinsurance		928.9	1,026.8		1,026.8					1,124.3
Deferred Acquisition Costs		49.3	53.6		53.6		49.3		53.9
Amortization of Deferred Acquisition Costs		9.8	9.4
Liability for Future Policy Benefit, before Reinsurance, after other		934.5	1,031.5		1,031.5							1,123.9
Unum US | Individual Disability
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Liability for Future Policy Benefit, Reinsurance Recoverable, after Allowance		195.2	216.8		216.8					260.6
Liability for Future Policy Benefit, after Reinsurance		1,864.9	2,124.7		2,124.7					2,507.1
Deferred Acquisition Costs		472.4	447.0		447.0		464.4		441.8
Amortization of Deferred Acquisition Costs		13.3	12.3
Liability for Future Policy Benefit, before Reinsurance, after other		2,060.1	2,341.5		2,341.5							2,485.3
Unum US | Voluntary Benefits
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Policyholders' Account Balances		677.2	680.6		680.6		586.8		598.7
Liability for Future Policy Benefit, Reinsurance Recoverable, after Allowance		12.5	28.7		28.7					35.7
Liability for Future Policy Benefit, after Reinsurance		1,164.9	1,377.1		1,377.1					1,623.5
Deferred Acquisition Costs		604.2	593.3		593.3		601.0		588.6
Amortization of Deferred Acquisition Costs		27.1	21.9
Liability for Future Policy Benefit, before Reinsurance, after other		1,177.4	1,405.8		1,405.8							1,154.9
Unum US | Voluntary Benefits | Operating Segments
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Premium Income		214.5	218.6
Unum US | Dental and Vision
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Liability for Future Policy Benefit, Reinsurance Recoverable, after Allowance										0.1
Liability for Future Policy Benefit, after Reinsurance										11.4
Deferred Acquisition Costs		9.9	7.9		7.9		9.4		7.7
Amortization of Deferred Acquisition Costs		2.5	2.2
Liability for Future Policy Benefit, before Reinsurance, after other												11.5
Unum US | Dental and Vision | Operating Segments
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Premium Income		69.3	70.6
Colonial Life
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Liability for Future Policy Benefits		1,942.1	2,236.7		2,236.7
Policyholders' Account Balances		867.5	869.5		869.5		852.4		849.2
Liability for Future Policy Benefit, Reinsurance Recoverable, after Allowance		0.8	1.9		1.9					4.4	[5]
Liability for Future Policy Benefit, after Reinsurance		1,941.3	2,234.8		2,234.8					2,524.2
Deferred Acquisition Costs		1,364.8	1,263.5		1,263.5		1,337.9		1,238.1			1,071.9
Amortization of Deferred Acquisition Costs		48.3	44.6
Liability for Future Policy Benefit, before Reinsurance, after other		1,942.1	2,236.7		2,236.7							1,977.2	[6]
Colonial Life | Operating Segments
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Total Assets		4,678.8					4,575.0
Premium Income		429.5	430.7
Unum International
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Liability for Future Policy Benefits		2,148.6	2,592.1		2,592.1
Liability for Future Policy Benefit, Reinsurance Recoverable, after Allowance		74.7	101.0		101.0					132.5	[5]
Liability for Future Policy Benefit, after Reinsurance		2,073.9	2,491.1		2,491.1					3,070.9
Deferred Acquisition Costs		39.3	36.5		36.5		37.0		36.4			32.0
Amortization of Deferred Acquisition Costs		1.8	2.1
Liability for Future Policy Benefit, before Reinsurance, after other		2,148.6	2,592.1		2,592.1							2,969.8	[6]
Unum International | Operating Segments
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Total Assets		3,237.7					3,129.4
Premium Income		188.6	187.8
Closed Block
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Liability for Future Policy Benefits		25,362.1	30,329.0		30,329.0
Liability for Future Policy Benefit, Reinsurance Recoverable, after Allowance		7,816.8	9,013.4		9,013.4					10,001.4	[5]
Liability for Future Policy Benefit, after Reinsurance		17,545.3	21,315.6		21,315.6					26,990.6
Liability for Future Policy Benefit, before Reinsurance, after other		25,362.1	30,329.0		30,329.0							28,572.8	[6]
Closed Block | Operating Segments
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Total Assets		34,472.5					33,776.6
Premium Income		231.6	239.9
Closed Block | Long-term Care
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Liability for Future Policy Benefit, Reinsurance Recoverable, after Allowance		5.6	6.7		6.7					44.4
Liability for Future Policy Benefit, after Reinsurance		15,393.1	18,826.9		18,826.9					22,696.5
Liability for Future Policy Benefit, before Reinsurance, after other		15,398.7	18,833.6		18,833.6							16,283.4
Closed Block | Long-term Care | Operating Segments
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Premium Income		175.1	174.8
Closed Block | Other Insurance Product Line
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Policyholders' Account Balances		4,164.9	4,222.2		4,222.2		4,159.4		4,231.7
Liability for Future Policy Benefit, Reinsurance Recoverable, after Allowance		7,811.2	9,006.7		9,006.7					9,957.0
Liability for Future Policy Benefit, after Reinsurance		2,152.2	2,488.7		2,488.7					4,294.1
Liability for Future Policy Benefit, before Reinsurance, after other		9,963.4	11,495.4		11,495.4							12,289.4
Closed Block | Other Insurance Product Line | Operating Segments
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Premium Income		56.5	65.1
Retained Earnings
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest										11,249.0
Stockholders' Equity Attributable to Parent		13,430.4	12,166.9		12,166.9
Net Income		358.3	240.4
Retained Earnings | Previously Reported [Member]
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest												11,269.6
Stockholders' Equity Attributable to Parent							13,141.3		11,853.2
Retained Earnings | Revision of Prior Period, Accounting Standards Update, Adjustment
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest										(20.6)
Retained Earnings | Accounting Standards Update 2018-12 | Revision of Prior Period, Accounting Standards Update, Adjustment
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Stockholders' Equity Attributable to Parent							0.0		136.2
Additional Paid-in Capital
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest										(6,310.2)
Stockholders' Equity Attributable to Parent		2,431.8	2,395.8		2,395.8		2,441.0		2,408.1
Additional Paid-in Capital | Previously Reported [Member]
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest												$ 374.2
Additional Paid-in Capital | Revision of Prior Period, Accounting Standards Update, Adjustment
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest										(6,684.4)
Accumulated Other Comprehensive Loss
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Other Comprehensive Income (Loss), Net of Tax		286.0	726.1
Stockholders' Equity Attributable to Parent		$ (3,162.3)	$ (4,438.5)		$ (4,438.5)		(3,448.3)		(5,164.6)
Accumulated Other Comprehensive Loss | Previously Reported [Member]
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Stockholders' Equity Attributable to Parent							(3,448.3)		354.1
Accumulated Other Comprehensive Loss | Accounting Standards Update 2018-12 | Revision of Prior Period, Accounting Standards Update, Adjustment
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Stockholders' Equity Attributable to Parent							0.0		(5,518.7)
Cumulative Effect, Period of Adoption, Adjustment | Accounting Standards Update 2018-12
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Removal of Shadow Adjustments, DAC										85.1
Cumulative Effect, Period of Adoption, Adjustment | Accounting Standards Update 2018-12 | Operating Segments
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Removal of Shadow Adjustments, LFPB	[8]									(6,225.6)
Adjustments for Loss Contracts	[9]									34.9
Effect of Change in Discount Rate Assumptions										16,084.6
Other increase (decrease)	[10]									(12.9)
Cumulative Effect, Period of Adoption, Adjustment | Unum US | Accounting Standards Update 2018-12
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Removal of Shadow Adjustments, LFPB	[8]									(1,569.7)
Adjustments for Loss Contracts	[9]									0.0
Effect of Change in Discount Rate Assumptions										2,251.8
Other increase (decrease)	[10]									(5.3)
Removal of Shadow Adjustments, DAC	[11]									12.3
Cumulative Effect, Period of Adoption, Adjustment | Unum US | Accounting Standards Update 2018-12 | Group Disability
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Removal of Shadow Adjustments, LFPB										(1,025.2)
Effect of Change in Discount Rate Assumptions										911.9
Other increase (decrease)										0.0
Cumulative Effect, Period of Adoption, Adjustment | Unum US | Accounting Standards Update 2018-12 | Group Life and AD&D
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Removal of Shadow Adjustments, LFPB										(88.5)
Effect of Change in Discount Rate Assumptions										91.9
Other increase (decrease)										0.0
Cumulative Effect, Period of Adoption, Adjustment | Unum US | Accounting Standards Update 2018-12 | Individual Disability
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Removal of Shadow Adjustments, LFPB										(446.1)
Effect of Change in Discount Rate Assumptions										728.5
Other increase (decrease)										0.0
Cumulative Effect, Period of Adoption, Adjustment | Unum US | Accounting Standards Update 2018-12 | Voluntary Benefits
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Removal of Shadow Adjustments, LFPB										(9.9)
Effect of Change in Discount Rate Assumptions										519.5
Other increase (decrease)										(5.3)
Cumulative Effect, Period of Adoption, Adjustment | Unum US | Accounting Standards Update 2018-12 | Dental and Vision
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Removal of Shadow Adjustments, LFPB										0.0
Effect of Change in Discount Rate Assumptions										0.0
Other increase (decrease)										0.0
Cumulative Effect, Period of Adoption, Adjustment | Colonial Life | Accounting Standards Update 2018-12
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Removal of Shadow Adjustments, LFPB	[8]									(44.5)
Adjustments for Loss Contracts	[9]									34.9
Effect of Change in Discount Rate Assumptions										568.6
Other increase (decrease)	[10]									(7.6)
Removal of Shadow Adjustments, DAC										72.8
Cumulative Effect, Period of Adoption, Adjustment | Unum International | Accounting Standards Update 2018-12
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Removal of Shadow Adjustments, LFPB	[8]									(545.8)
Adjustments for Loss Contracts	[9]									0.0
Effect of Change in Discount Rate Assumptions										779.4
Other increase (decrease)	[10]									0.0
Removal of Shadow Adjustments, DAC										0.0
Cumulative Effect, Period of Adoption, Adjustment | Closed Block | Accounting Standards Update 2018-12
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Removal of Shadow Adjustments, LFPB	[8]									(4,065.6)
Adjustments for Loss Contracts	[9]									0.0
Effect of Change in Discount Rate Assumptions										12,484.8
Other increase (decrease)	[10]									0.0
Cumulative Effect, Period of Adoption, Adjustment | Closed Block | Accounting Standards Update 2018-12 | Long-term Care
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Removal of Shadow Adjustments, LFPB										(3,465.4)
Effect of Change in Discount Rate Assumptions										9,922.9
Cumulative Effect, Period of Adoption, Adjustment | Closed Block | Accounting Standards Update 2018-12 | Other Insurance Product Line
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Removal of Shadow Adjustments, LFPB										(600.2)
Effect of Change in Discount Rate Assumptions										2,561.9
Cumulative Effect, Period of Adoption, Adjusted Balance
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Deferred Acquisition Costs										2,357.7
Cumulative Effect, Period of Adoption, Adjusted Balance | Operating Segments
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Liability for Future Policy Benefit, before Reinsurance, after other										55,585.4
Cumulative Effect, Period of Adoption, Adjusted Balance | Unum US
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Deferred Acquisition Costs										1,181.0
Liability for Future Policy Benefit, before Reinsurance, after other										12,861.4
Cumulative Effect, Period of Adoption, Adjusted Balance | Unum US | Group Disability
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Liability for Future Policy Benefit, before Reinsurance, after other										7,295.7
Cumulative Effect, Period of Adoption, Adjusted Balance | Unum US | Group Life and AD&D
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Liability for Future Policy Benefit, before Reinsurance, after other										1,127.3
Cumulative Effect, Period of Adoption, Adjusted Balance | Unum US | Individual Disability
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Liability for Future Policy Benefit, before Reinsurance, after other										2,767.7
Cumulative Effect, Period of Adoption, Adjusted Balance | Unum US | Voluntary Benefits
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Liability for Future Policy Benefit, before Reinsurance, after other										1,659.2
Cumulative Effect, Period of Adoption, Adjusted Balance | Unum US | Dental and Vision
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Liability for Future Policy Benefit, before Reinsurance, after other										11.5
Cumulative Effect, Period of Adoption, Adjusted Balance | Colonial Life
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Deferred Acquisition Costs										1,144.7
Liability for Future Policy Benefit, before Reinsurance, after other										2,528.6
Cumulative Effect, Period of Adoption, Adjusted Balance | Unum International
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Deferred Acquisition Costs										32.0
Liability for Future Policy Benefit, before Reinsurance, after other										3,203.4
Cumulative Effect, Period of Adoption, Adjusted Balance | Closed Block
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Liability for Future Policy Benefit, before Reinsurance, after other										36,992.0
Cumulative Effect, Period of Adoption, Adjusted Balance | Closed Block | Long-term Care
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Liability for Future Policy Benefit, before Reinsurance, after other										22,740.9
Cumulative Effect, Period of Adoption, Adjusted Balance | Closed Block | Other Insurance Product Line
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Liability for Future Policy Benefit, before Reinsurance, after other										$ 14,251.1
Cumulative Effect, Period of Adoption, Adjusted Balance | Retained Earnings | Accounting Standards Update 2018-12
Cumulative Effect on Opening Balance in Period of Adoption [Line Items]
Stockholders' Equity Attributable to Parent							$ 13,141.3		$ 11,989.4

[1] 2We previously reported policy and contract benefits and reserves for future policy and contract benefits liabilities in our consolidated balance sheet as of March 31, 2022 of $1,874.2 million and $45,258.0 million, respectively, resulting in total policyholder liabilities of $47,132.2 million. These balances were reclassified into new line items, future policy benefits and policyholders' account balances, which would have reported balances of $41,368.2 million and $5,764.0 million, respectively, as of March 31, 2022, resulting in total policyholder liabilities of $47,132.2 million under the historical accounting method.

[2] 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, 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] 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.

[4] 1 Previously disclosed as Benefits and Change in Reserves for Future Benefits

[5] 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.

[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] 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.

[8] 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.

[9] 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.

[10] 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.

[11] 1The $12.3 million removal of shadow adjustments is related to the Unum US voluntary benefits product line.