Summary of Significant Accounting Policies Losses, Claims and Settlement Expenses - Consolidated - Incurred and Paid Loss Development Tables (Details) $ in Millions	Dec. 31, 2020 USD ($) claim	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)
Workers' Compensation [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	$ 6,732.1
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	4,121.7
Sub-total	2,610.4
All outstanding liabilities before 2011, net of reinsurance	630.7
Liabilities for claims and allocated claim adjustment expenses, net of reinsurance	3,241.1
General Liability [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	1,120.8
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	638.5
Sub-total	482.2
All outstanding liabilities before 2011, net of reinsurance	159.2
Liabilities for claims and allocated claim adjustment expenses, net of reinsurance	641.5
Commercial Automobile [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	7,649.1
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	6,060.7
Sub-total	1,588.4
All outstanding liabilities before 2011, net of reinsurance	3.0
Liabilities for claims and allocated claim adjustment expenses, net of reinsurance	1,591.5
Accident Year 2011 [Member] | Workers' Compensation [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	627.6	$ 626.9	$ 634.9	$ 649.3	$ 641.7	$ 643.0	$ 622.5	$ 595.3	$ 567.3	$ 558.6
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 39.7
Cumulative Number of Reported Claims* | claim	53,348
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 557.7	550.0	539.7	526.4	503.4	469.8	424.0	361.4	266.7	112.6
Accident Year 2011 [Member] | General Liability [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	91.9	91.1	94.4	94.6	96.0	95.8	80.0	72.9	71.5	72.5
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 7.8
Cumulative Number of Reported Claims* | claim	4,747
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 79.8	79.7	80.7	75.3	68.9	58.7	43.6	26.0	12.1	2.5
Accident Year 2011 [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	533.2	534.3	537.2	539.3	549.0	549.9	582.3	584.4	599.9	591.6
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 0.9
Cumulative Number of Reported Claims* | claim	96,794
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 529.4	528.8	526.6	525.0	523.0	511.6	483.2	436.2	352.7	$ 223.1
Accident Year 2012 [Member] | Workers' Compensation [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	651.1	654.7	660.5	671.1	676.4	678.1	670.6	647.2	629.3
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 64.8
Cumulative Number of Reported Claims* | claim	49,922
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 539.5	531.5	518.4	496.6	469.5	426.7	361.8	265.8	113.1
Accident Year 2012 [Member] | General Liability [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	105.0	105.6	108.2	109.6	107.3	100.9	89.2	91.2	95.0
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 8.2
Cumulative Number of Reported Claims* | claim	5,283
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 93.2	90.8	86.4	75.8	67.4	50.8	36.0	18.8	5.5
Accident Year 2012 [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	551.4	552.1	558.0	558.6	573.1	575.0	603.9	619.6	622.5
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 2.7
Cumulative Number of Reported Claims* | claim	98,048
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 545.5	545.3	543.9	539.1	525.9	498.6	442.9	351.4	$ 229.0
Accident Year 2013 [Member] | Workers' Compensation [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	691.0	689.7	717.2	726.3	722.7	716.9	705.3	700.9
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 76.8
Cumulative Number of Reported Claims* | claim	49,017
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 558.3	547.0	526.8	501.9	449.8	381.2	274.3	107.6
Accident Year 2013 [Member] | General Liability [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	102.2	101.4	106.0	106.7	107.8	96.5	96.7	95.7
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 9.4
Cumulative Number of Reported Claims* | claim	5,533
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 88.1	86.9	85.1	76.1	58.5	34.4	13.6	4.0
Accident Year 2013 [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	632.5	633.4	646.4	659.7	669.6	668.5	665.4	661.5
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 7.0
Cumulative Number of Reported Claims* | claim	96,993
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 619.5	626.3	622.9	611.5	578.1	511.0	398.1	$ 248.3
Accident Year 2014 [Member] | Workers' Compensation [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	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 Claims	$ 105.7
Cumulative Number of Reported Claims* | claim	54,145
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 544.9	524.8	499.5	466.0	397.1	293.7	116.9
Accident Year 2014 [Member] | General Liability [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	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 Claims	$ 11.3
Cumulative Number of Reported Claims* | claim	6,012
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 88.9	82.8	73.5	52.8	32.0	15.8	5.8
Accident Year 2014 [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	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 Claims	$ 13.6
Cumulative Number of Reported Claims* | claim	103,227
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 658.4	664.9	640.3	605.4	536.9	430.5	$ 267.4
Accident Year 2015 [Member] | Workers' Compensation [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	742.4	769.1	785.5	787.3	792.6	794.3
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 193.6
Cumulative Number of Reported Claims* | claim	55,180
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 484.7	466.7	435.1	379.3	274.9	109.0
Accident Year 2015 [Member] | General Liability [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	105.8	104.8	102.3	99.2	96.3	96.0
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 22.0
Cumulative Number of Reported Claims* | claim	5,577
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 70.7	64.5	47.4	29.5	16.0	6.3
Accident Year 2015 [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	703.4	720.7	721.4	729.7	710.5	712.4
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 8.7
Cumulative Number of Reported Claims* | claim	104,791
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 680.6	669.7	626.2	541.8	438.9	$ 265.1
Accident Year 2016 [Member] | Workers' Compensation [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	712.6	730.5	745.7	752.9	756.1
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 235.3
Cumulative Number of Reported Claims* | claim	52,419
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 408.4	383.5	334.4	253.5	102.5
Accident Year 2016 [Member] | General Liability [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	101.0	100.3	98.8	96.7	92.4
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 27.3
Cumulative Number of Reported Claims* | claim	83,224
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 58.0	47.7	34.8	18.5	7.1
Accident Year 2016 [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	779.3	780.8	786.0	768.9	755.9
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 31.3
Cumulative Number of Reported Claims* | claim	110,110
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 710.8	677.8	585.1	469.6	$ 290.2
Short-Duration Insurance Contract, Accident Year 2020 | Workers' Compensation [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	560.9
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 310.1
Cumulative Number of Reported Claims* | claim	33,301
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 84.3
Short-Duration Insurance Contract, Accident Year 2020 | General Liability [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	112.4
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 83.3
Cumulative Number of Reported Claims* | claim	4,093
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 4.2
Short-Duration Insurance Contract, Accident Year 2020 | Commercial Automobile [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	941.1
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 142.1
Cumulative Number of Reported Claims* | claim	97,067
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 290.1
Short-Duration Insurance Contract, Accident Year 2019 | Workers' Compensation [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	657.4	664.6
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 243.7
Cumulative Number of Reported Claims* | claim	51,102
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 239.8	102.9
Short-Duration Insurance Contract, Accident Year 2019 | General Liability [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	131.9	133.5
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 61.5
Cumulative Number of Reported Claims* | claim	375,183
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 29.5	6.4
Short-Duration Insurance Contract, Accident Year 2019 | Commercial Automobile [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	959.7	931.1
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 66.6
Cumulative Number of Reported Claims* | claim	136,389
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 549.0	330.4
Short-duration Insurance Contracts, Accident Year 2018 | Workers' Compensation [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	681.0	691.5	698.6
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 232.9
Cumulative Number of Reported Claims* | claim	52,145
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 320.5	240.6	94.8
Short-duration Insurance Contracts, Accident Year 2018 | General Liability [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	125.1	119.7	120.5
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 28.8
Cumulative Number of Reported Claims* | claim	461,673
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 48.9	28.8	6.9
Short-duration Insurance Contracts, Accident Year 2018 | Commercial Automobile [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	989.9	947.9	883.2
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 46.3
Cumulative Number of Reported Claims* | claim	127,772
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 730.4	557.5	330.0
Short-duration Insurance Contracts, Accident Year 2017 | Workers' Compensation [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	683.4	700.3	713.9	727.0
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 195.1
Cumulative Number of Reported Claims* | claim	51,674
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 383.1	334.8	244.6	99.6
Short-duration Insurance Contracts, Accident Year 2017 | General Liability [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	132.8	129.6	121.4	111.2
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 22.0
Cumulative Number of Reported Claims* | claim	460,409
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 76.9	50.1	25.9	5.7
Short-duration Insurance Contracts, Accident Year 2017 | Commercial Automobile [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	874.4	869.2	819.1	788.7
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 39.3
Cumulative Number of Reported Claims* | claim	116,378
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 746.5	$ 657.1	$ 512.0	$ 307.9