Summary of Significant Accounting Policies Losses, Claims and Settlement Expenses - Consolidated - Incurred and Paid Loss Development Tables (Details) $ in Millions	Dec. 31, 2019 USD ($) claim	Dec. 31, 2018 USD ($)	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 ($)	Dec. 31, 2010 USD ($)
Workers' Compensation [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	$ 6,856.8
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	4,196.1
Sub-total	2,660.7
All outstanding liabilities before 2010, net of reinsurance	628.0
Liabilities for claims and allocated claim adjustment expenses, net of reinsurance	3,288.8
General Liability [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	1,071.5
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	599.3
Sub-total	472.1
All outstanding liabilities before 2010, net of reinsurance	138.7
Liabilities for claims and allocated claim adjustment expenses, net of reinsurance	610.9
Commercial Automobile [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	7,123.2
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	5,723.7
Sub-total	1,399.5
All outstanding liabilities before 2010, net of reinsurance	3.5
Liabilities for claims and allocated claim adjustment expenses, net of reinsurance	1,403.0
Accident Year 2010 [Member] | Workers' Compensation [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	565.7	$ 563.2	$ 563.5	$ 564.0	$ 563.1	$ 569.1	$ 545.3	$ 521.5	$ 480.0	$ 485.1
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 21.4
Cumulative Number of Reported Claims | claim	57,763
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 514.0	507.2	496.1	482.8	466.5	464.0	427.9	370.1	279.8	118.9
Accident Year 2010 [Member] | General Liability [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	67.9	67.9	68.6	68.6	69.7	70.4	64.7	66.6	67.8	68.4
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 4.2
Cumulative Number of Reported Claims | claim	5,386
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 61.3	59.6	56.4	55.5	52.1	44.3	31.1	20.8	10.8	3.9
Accident Year 2010 [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	468.6	473.5	473.7	479.9	494.9	532.4	530.9	530.3	546.9	584.3
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 0.1
Cumulative Number of Reported Claims | claim	92,191
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 468.4	467.6	466.6	465.7	460.0	449.4	423.8	372.7	305.8	$ 200.0
Accident Year 2011 [Member] | Workers' Compensation [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	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	$ 42.8
Cumulative Number of Reported Claims | claim	53,329
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 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)	91.1	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	$ 7.2
Cumulative Number of Reported Claims | claim	4,739
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 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)	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	$ 2.2
Cumulative Number of Reported Claims | claim	96,789
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 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)	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	$ 34.0
Cumulative Number of Reported Claims | claim	49,910
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 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)	105.6	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	$ 8.4
Cumulative Number of Reported Claims | claim	5,277
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 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)	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	$ 3.2
Cumulative Number of Reported Claims | claim	98,042
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 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)	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	$ 80.0
Cumulative Number of Reported Claims | claim	49,005
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 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)	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	$ 9.8
Cumulative Number of Reported Claims | claim	5,520
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 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)	634.4	647.3	659.7	669.6	668.5	665.4	661.5
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 3.1
Cumulative Number of Reported Claims | claim	96,992
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 627.3	623.9	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)	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	$ 156.0
Cumulative Number of Reported Claims | claim	54,127
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 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)	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	$ 16.6
Cumulative Number of Reported Claims | claim	5,998
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 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)	688.1	689.1	688.0	691.7	689.2	687.8
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 11.4
Cumulative Number of Reported Claims | claim	103,223
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 665.1	640.8	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)	769.1	785.5	787.3	792.6	794.3
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 220.5
Cumulative Number of Reported Claims | claim	55,149
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 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)	104.8	102.3	99.2	96.3	96.0
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 24.3
Cumulative Number of Reported Claims | claim	5,540
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 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)	721.5	722.0	729.7	710.5	712.4
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 24.4
Cumulative Number of Reported Claims | claim	104,776
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 670.5	626.8	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)	730.5	745.7	752.9	756.1
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 256.3
Cumulative Number of Reported Claims | claim	52,316
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 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)	100.3	98.8	96.7	92.4
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 32.8
Cumulative Number of Reported Claims | claim	83,172
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 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)	776.7	784.5	768.9	755.9
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 25.9
Cumulative Number of Reported Claims | claim	110,045
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 673.7	583.6	469.6	$ 290.2
Accident Year 2017 [Member] | Workers' Compensation [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	700.3	713.9	727.0
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 229.6
Cumulative Number of Reported Claims | claim	51,525
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 334.8	244.6	99.6
Accident Year 2017 [Member] | General Liability [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	129.6	121.4	111.2
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 29.3
Cumulative Number of Reported Claims | claim	460,224
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 50.1	25.9	5.7
Accident Year 2017 [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	868.1	818.8	788.7
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 36.9
Cumulative Number of Reported Claims | claim	116,144
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 656.1	511.7	$ 307.9
Accident Year 2018 [Member] | Workers' Compensation [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	691.5	698.6
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 274.9
Cumulative Number of Reported Claims | claim	51,695
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 240.6	94.8
Accident Year 2018 [Member] | General Liability [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	119.7	120.5
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 39.8
Cumulative Number of Reported Claims | claim	461,270
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 28.8	6.9
Accident Year 2018 [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	948.2	883.3
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 20.2
Cumulative Number of Reported Claims | claim	126,859
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 557.7	$ 330.1
Accident Year 2019 [Member] | Workers' Compensation [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	664.6
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 380.6
Cumulative Number of Reported Claims | claim	40,042
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 102.9
Accident Year 2019 [Member] | General Liability [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	133.5
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 86.5
Cumulative Number of Reported Claims | claim	356,996
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 6.4
Accident Year 2019 [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Undiscounted)	930.9
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 74.9
Cumulative Number of Reported Claims | claim	117,696
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 330.2