Losses, Claims and Settlement Expenses - Consolidated - Incurred and Paid Loss Development Tables (Details) $ in Millions	Dec. 31, 2023 USD ($) claim	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Workers' Compensation [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	$ 5,802.1
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	3,651.7
Subtotal	2,150.3
All outstanding liabilities before 2014, net of reinsurance	754.9
Liabilities for loss and allocated loss adjustment expenses, net of reinsurance	2,905.3
General Liability [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	1,204.4
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	665.9
Subtotal	538.4
All outstanding liabilities before 2014, net of reinsurance	167.0
Liabilities for loss and allocated loss adjustment expenses, net of reinsurance	705.5
Commercial Automobile [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	8,917.7
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	7,123.8
Subtotal	1,793.9
All outstanding liabilities before 2014, net of reinsurance	14.5
Liabilities for loss and allocated loss adjustment expenses, net of reinsurance	1,808.4
Short-duration Insurance Contracts, Accident Year 2014 | Workers' Compensation [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	673.2	$ 685.8	$ 705.4	$ 724.4	$ 763.3	$ 777.0	$ 784.9	$ 786.4	$ 792.8	$ 780.9
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Losses	$ 58.8
Cumulative Number of Reported Losses* | claim	54,174
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 567.9	561.0	554.6	544.9	524.8	499.5	466.0	397.1	293.7	116.9
Short-duration Insurance Contracts, Accident Year 2014 | General Liability [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	117.9	115.6	112.8	112.3	117.1	117.0	111.0	109.4	110.4	107.0
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Losses	$ 10.7
Cumulative Number of Reported Losses* | claim	6,047
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 103.0	98.9	94.9	88.9	82.8	73.5	52.8	32.0	15.8	5.8
Short-duration Insurance Contracts, Accident Year 2014 | Commercial Automobile [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	679.2	682.1	683.0	683.8	687.8	688.6	688.0	691.7	689.2	687.8
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Losses	$ 5.9
Cumulative Number of Reported Losses* | claim	103,242
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 672.2	669.9	664.3	658.4	664.9	640.3	605.4	536.9	430.5	$ 267.4
Short-duration Insurance Contracts, Accident Year 2015 | Workers' Compensation [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	622.3	659.8	695.8	742.4	769.1	785.5	787.3	792.6	794.3
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Losses	$ 69.8
Cumulative Number of Reported Losses* | claim	55,234
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 512.7	507.3	499.8	484.7	466.7	435.1	379.3	274.9	109.0
Short-duration Insurance Contracts, Accident Year 2015 | General Liability [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	99.0	99.2	99.8	105.8	104.8	102.3	99.2	96.3	96.0
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Losses	$ 12.1
Cumulative Number of Reported Losses* | claim	5,616
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 82.5	79.9	75.0	70.7	64.5	47.4	29.5	16.0	6.3
Short-duration Insurance Contracts, Accident Year 2015 | Commercial Automobile [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	699.5	699.9	700.6	703.4	720.7	721.4	729.7	710.5	712.4
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Losses	$ 2.2
Cumulative Number of Reported Losses* | claim	104,829
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 695.4	693.0	687.3	680.6	669.7	626.2	541.8	438.9	$ 265.1
Short-duration Insurance Contracts, Accident Year 2016 | Workers' Compensation [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	584.9	624.2	692.8	712.6	730.5	745.7	752.9	756.1
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Losses	$ 95.5
Cumulative Number of Reported Losses* | claim	52,495
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 442.5	435.8	425.2	408.4	383.5	334.4	253.5	102.5
Short-duration Insurance Contracts, Accident Year 2016 | General Liability [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	95.9	98.8	104.4	101.0	100.3	98.8	96.7	92.4
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Losses	$ 13.2
Cumulative Number of Reported Losses* | claim	83,293
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 75.1	71.9	66.9	58.0	47.7	34.8	18.5	7.1
Short-duration Insurance Contracts, Accident Year 2016 | Commercial Automobile [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	752.9	754.9	762.1	779.3	780.8	786.0	768.9	755.9
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Losses	$ 4.3
Cumulative Number of Reported Losses* | claim	110,766
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 742.7	737.3	725.5	710.8	677.8	585.1	469.6	$ 290.2
Short-duration Insurance Contracts, Accident Year 2017 | Workers' Compensation [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	609.3	654.2	676.3	683.4	700.3	713.9	727.0
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Losses	$ 102.6
Cumulative Number of Reported Losses* | claim	51,787
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 453.5	444.1	414.3	383.1	334.8	244.6	99.6
Short-duration Insurance Contracts, Accident Year 2017 | General Liability [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	143.9	138.0	135.2	132.8	129.6	121.4	111.2
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Losses	$ 18.2
Cumulative Number of Reported Losses* | claim	460,523
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 113.2	105.3	95.5	76.9	50.1	25.9	5.7
Short-duration Insurance Contracts, Accident Year 2017 | Commercial Automobile [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	843.0	847.1	867.9	874.4	869.2	819.1	788.7
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Losses	$ 6.7
Cumulative Number of Reported Losses* | claim	117,639
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 826.9	814.1	791.2	746.5	657.1	512.0	$ 307.9
Short-duration Insurance Contracts, Accident Year 2018 | Workers' Compensation [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	605.4	644.8	665.9	681.0	691.5	698.6
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Losses	$ 141.3
Cumulative Number of Reported Losses* | claim	52,390
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 416.0	396.8	367.2	320.5	240.6	94.8
Short-duration Insurance Contracts, Accident Year 2018 | General Liability [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	152.0	141.9	135.5	125.1	119.7	120.5
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Losses	$ 32.0
Cumulative Number of Reported Losses* | claim	462,021
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 102.0	91.0	71.0	48.9	28.8	6.9
Short-duration Insurance Contracts, Accident Year 2018 | Commercial Automobile [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	971.3	976.1	992.1	989.9	947.9	883.2
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Losses	$ 20.1
Cumulative Number of Reported Losses* | claim	129,290
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 924.1	900.0	836.7	730.4	557.5	$ 330.0
Short-duration Insurance Contracts, Accident Year 2019 | Workers' Compensation [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	658.8	667.5	653.2	657.4	664.6
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Losses	$ 154.0
Cumulative Number of Reported Losses* | claim	51,887
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 412.2	382.3	329.6	239.8	102.9
Short-duration Insurance Contracts, Accident Year 2019 | General Liability [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	146.9	146.0	138.7	131.9	133.5
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Losses	$ 39.5
Cumulative Number of Reported Losses* | claim	375,990
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 87.8	72.1	53.4	29.5	6.4
Short-duration Insurance Contracts, Accident Year 2019 | Commercial Automobile [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	942.0	947.4	954.8	959.7	931.1
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Losses	$ 25.0
Cumulative Number of Reported Losses* | claim	138,735
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 875.3	787.2	681.6	549.0	$ 330.4
Short-duration Insurance Contracts, Accident Year 2020 | Workers' Compensation [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	574.7	571.7	569.4	560.9
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Losses	$ 144.7
Cumulative Number of Reported Losses* | claim	45,859
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 329.7	284.3	211.6	84.3
Short-duration Insurance Contracts, Accident Year 2020 | General Liability [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	116.1	114.9	111.7	112.4
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Losses	$ 50.0
Cumulative Number of Reported Losses* | claim	6,112
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 45.0	28.5	12.4	4.2
Short-duration Insurance Contracts, Accident Year 2020 | Commercial Automobile [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	845.5	854.0	913.7	941.1
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Losses	$ 70.6
Cumulative Number of Reported Losses* | claim	117,927
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 692.7	602.2	464.3	$ 290.1
Short-Duration Insurance Contract, Accident Year 2021 | Workers' Compensation [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	493.8	502.4	500.3
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Losses	$ 153.0
Cumulative Number of Reported Losses* | claim	46,975
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 252.9	187.8	80.1
Short-Duration Insurance Contract, Accident Year 2021 | General Liability [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	99.0	92.7	94.2
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Losses	$ 44.3
Cumulative Number of Reported Losses* | claim	6,022
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 30.9	14.7	5.6
Short-Duration Insurance Contract, Accident Year 2021 | Commercial Automobile [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	935.3	954.6	989.4
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Losses	$ 83.9
Cumulative Number of Reported Losses* | claim	125,608
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 662.9	508.6	$ 302.7
Short-Duration Insurance Contract, Accident Year 2022 | Workers' Compensation [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	487.4	488.1
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Losses	$ 184.7
Cumulative Number of Reported Losses* | claim	46,691
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 188.2	74.2
Short-Duration Insurance Contract, Accident Year 2022 | General Liability [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	100.7	97.8
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Losses	$ 49.4
Cumulative Number of Reported Losses* | claim	5,758
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 22.2	6.4
Short-Duration Insurance Contract, Accident Year 2022 | Commercial Automobile [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	1,044.5	1,074.2
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Losses	$ 123.6
Cumulative Number of Reported Losses* | claim	125,309
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 606.8	$ 354.8
Short-duration Insurance Contracts, Accident Year 2023 | Workers' Compensation [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	491.7
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Losses	$ 276.1
Cumulative Number of Reported Losses* | claim	35,679
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 75.5
Short-duration Insurance Contracts, Accident Year 2023 | General Liability [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	132.6
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Losses	$ 98.5
Cumulative Number of Reported Losses* | claim	12,208
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 3.9
Short-duration Insurance Contracts, Accident Year 2023 | Commercial Automobile [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Undiscounted)	1,204.1
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Losses	$ 100.9
Cumulative Number of Reported Losses* | claim	107,783
Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 424.4