Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Schedule of Incurred Claims Development) (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 ($)	Dec. 31, 2011 USD ($)
Commercial Multiple Peril Line [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	$ 4,078.7
Cumulative Paid Losses and ALAE, Net of Reinsurance	2,889.2
Unallocated loss adjustment expense	24.1
Net reserves	1,265.5
Commercial Multiple Peril Line [Member] | Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	393.6	$ 393.7	$ 393.0	$ 392.1	$ 391.8	$ 367.9	$ 362.5	$ 380.0
IBNR	$ 7.8
Cumulative Incurred Claim Count | claim	14,801
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 373.1	366.3	355.9	334.6	306.4	262.3	221.5	137.6
Commercial Multiple Peril Line [Member] | Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	441.0	444.4	449.1	459.4	464.8	439.6	443.9
IBNR	$ 13.1
Cumulative Incurred Claim Count | claim	15,881
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 411.7	407.7	395.2	363.4	316.0	267.8	171.7
Commercial Multiple Peril Line [Member] | Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	460.9	465.2	467.4	463.7	456.3	446.0
IBNR	$ 16.9
Cumulative Incurred Claim Count | claim	15,583
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 413.6	397.0	363.2	315.6	260.1	161.9
Commercial Multiple Peril Line [Member] | Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	449.6	447.4	448.7	449.6	447.1
IBNR	$ 25.8
Cumulative Incurred Claim Count | claim	15,750
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 369.6	342.2	290.2	237.9	140.3
Commercial Multiple Peril Line [Member] | Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	555.6	551.6	544.8	538.7
IBNR	$ 38.5
Cumulative Incurred Claim Count | claim	16,638
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 422.9	370.6	296.4	170.9
Commercial Multiple Peril Line [Member] | Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	556.1	560.9	578.2
IBNR	$ 77.8
Cumulative Incurred Claim Count | claim	17,123
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 363.7	306.2	178.8
Commercial Multiple Peril Line [Member] | Year 2019 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	593.0	583.8
IBNR	$ 138.0
Cumulative Incurred Claim Count | claim	16,012
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 317.2	169.3
Commercial Multiple Peril Line [Member] | Year 2020 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	628.9
IBNR	$ 245.5
Cumulative Incurred Claim Count | claim	13,136
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 217.4
Commercial Multiple Peril Line [Member] | Years 2013 to 2020 [Member]
Claims Development [Line Items]
Total reserves for accident years	1,189.5
Commercial Multiple Peril Line [Member] | Years 2012 and Prior [Member]
Claims Development [Line Items]
Total reserves for accident years	51.9
Workers' Compensation Line [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	1,596.1
Cumulative Paid Losses and ALAE, Net of Reinsurance	1,087.2
Unallocated loss adjustment expense	19.3
Net reserves	612.8
Workers' Compensation Line [Member] | Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	146.9	147.7	147.9	148.3	146.7	143.8	145.5	144.3	$ 144.2	$ 141.4
IBNR	$ 6.3
Cumulative Incurred Claim Count | claim	12,484
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 135.3	134.2	132.8	130.9	128.0	122.9	114.0	97.3	71.3	30.0
Workers' Compensation Line [Member] | Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	161.4	161.9	163.7	163.7	162.9	157.2	165.2	171.1	176.3
IBNR	$ 8.4
Cumulative Incurred Claim Count | claim	13,083
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 146.6	143.7	141.4	136.0	130.7	120.1	102.6	74.8	30.4
Workers' Compensation Line [Member] | Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	153.3	154.9	155.4	155.3	154.4	160.1	167.4	179.3
IBNR	$ 8.8
Cumulative Incurred Claim Count | claim	11,734
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 133.8	131.4	127.4	122.8	114.0	101.2	74.6	30.9
Workers' Compensation Line [Member] | Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	145.8	148.2	150.3	152.1	154.7	172.9	182.1
IBNR	$ 11.8
Cumulative Incurred Claim Count | claim	10,943
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 121.0	117.8	112.5	105.6	92.3	70.5	30.6
Workers' Compensation Line [Member] | Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	142.7	146.0	150.3	157.5	164.2	189.6
IBNR	$ 16.0
Cumulative Incurred Claim Count | claim	11,485
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 109.6	105.8	99.4	87.2	65.7	28.0
Workers' Compensation Line [Member] | Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	152.8	158.2	164.6	180.5	189.6
IBNR	$ 16.1
Cumulative Incurred Claim Count | claim	15,894
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 117.1	111.1	99.5	78.1	33.9
Workers' Compensation Line [Member] | Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	156.6	160.8	172.1	186.1
IBNR	$ 20.0
Cumulative Incurred Claim Count | claim	16,722
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 106.6	94.1	73.0	32.8
Workers' Compensation Line [Member] | Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	172.1	182.1	187.2
IBNR	$ 24.2
Cumulative Incurred Claim Count | claim	17,454
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 103.4	80.7	35.6
Workers' Compensation Line [Member] | Year 2019 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	182.0	187.9
IBNR	$ 30.1
Cumulative Incurred Claim Count | claim	17,163
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 83.1	33.3
Workers' Compensation Line [Member] | Year 2020 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	182.5
IBNR	$ 76.9
Cumulative Incurred Claim Count | claim	12,549
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 30.7
Workers' Compensation Line [Member] | Years 2011 to 2020 [Member]
Claims Development [Line Items]
Total reserves for accident years	508.9
Workers' Compensation Line [Member] | Years 2010 and Prior [Member]
Claims Development [Line Items]
Total reserves for accident years	84.6
Commercial Automobile Line [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	1,261.3
Cumulative Paid Losses and ALAE, Net of Reinsurance	807.3
Unallocated loss adjustment expense	5.8
Net reserves	473.5
Commercial Automobile Line [Member] | Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	183.5	185.0	185.1	181.7	177.3	163.3	168.5
IBNR	$ 1.7
Cumulative Incurred Claim Count | claim	13,477
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 178.2	176.3	168.2	137.1	102.7	70.8	33.1
Commercial Automobile Line [Member] | Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	170.3	167.6	167.8	166.9	168.3	163.4
IBNR	$ 1.2
Cumulative Incurred Claim Count | claim	12,829
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 152.5	144.5	129.3	96.4	63.8	32.2
Commercial Automobile Line [Member] | Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	172.5	174.3	163.0	157.7	157.0
IBNR	$ 4.3
Cumulative Incurred Claim Count | claim	11,660
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 147.2	134.0	98.3	60.7	27.8
Commercial Automobile Line [Member] | Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	188.5	182.1	170.1	159.5
IBNR	$ 7.6
Cumulative Incurred Claim Count | claim	11,504
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 137.7	105.9	71.2	26.9
Commercial Automobile Line [Member] | Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	182.9	176.1	182.8
IBNR	$ 25.5
Cumulative Incurred Claim Count | claim	11,181
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 92.5	59.7	29.2
Commercial Automobile Line [Member] | Year 2019 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	192.0	180.5
IBNR	$ 57.0
Cumulative Incurred Claim Count | claim	10,210
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 75.8	29.5
Commercial Automobile Line [Member] | Year 2020 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	171.6
IBNR	$ 115.9
Cumulative Incurred Claim Count | claim	5,772
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 23.4
Commercial Automobile Line [Member] | Years 2014 to 2020 [Member]
Claims Development [Line Items]
Total reserves for accident years	454.0
Commercial Automobile Line [Member] | Years 2013 and Prior [Member]
Claims Development [Line Items]
Total reserves for accident years	13.7
General Liability and Umbrella - Occurrence [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	928.2
Cumulative Paid Losses and ALAE, Net of Reinsurance	524.0
Unallocated loss adjustment expense	13.7
Net reserves	436.0
General Liability and Umbrella - Occurrence [Member] | Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	59.3	58.8	57.5	55.4	55.6	51.0	42.2	38.2	43.8	47.6
IBNR	$ 2.4
Cumulative Incurred Claim Count | claim	1,393
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 54.8	52.9	50.6	47.8	45.8	39.8	31.1	19.4	6.7	$ 1.6
General Liability and Umbrella - Occurrence [Member] | Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	72.8	71.2	69.1	74.2	71.1	64.3	62.2	59.3	77.2
IBNR	$ 2.5
Cumulative Incurred Claim Count | claim	1,774
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 65.0	64.5	63.0	60.6	52.1	43.8	29.8	12.6	$ 2.2
General Liability and Umbrella - Occurrence [Member] | Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	79.1	80.4	81.2	84.4	88.6	71.5	67.4	84.1
IBNR	$ 4.1
Cumulative Incurred Claim Count | claim	2,001
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 70.1	67.6	63.5	56.1	43.1	26.8	11.0	$ 2.4
General Liability and Umbrella - Occurrence [Member] | Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	98.4	99.8	98.3	101.4	98.6	82.3	100.9
IBNR	$ 6.4
Cumulative Incurred Claim Count | claim	2,108
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 87.3	83.6	70.4	52.8	31.4	14.5	3.1
General Liability and Umbrella - Occurrence [Member] | Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	99.7	98.3	97.0	99.6	99.3	104.2
IBNR	$ 9.9
Cumulative Incurred Claim Count | claim	2,448
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 77.3	65.7	48.5	30.8	15.2	3.3
General Liability and Umbrella - Occurrence [Member] | Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	98.7	100.8	101.5	100.7	95.6
IBNR	$ 15.7
Cumulative Incurred Claim Count | claim	1,913
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 63.4	52.5	31.6	15.0	3.1
General Liability and Umbrella - Occurrence [Member] | Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	110.7	109.2	107.6	97.7
IBNR	$ 24.9
Cumulative Incurred Claim Count | claim	1,806
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 49.8	34.5	17.0	4.4
General Liability and Umbrella - Occurrence [Member] | Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	104.0	106.4	99.8
IBNR	$ 35.2
Cumulative Incurred Claim Count | claim	1,823
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 35.7	15.6	4.1
General Liability and Umbrella - Occurrence [Member] | Year 2019 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	105.7	104.2
IBNR	$ 55.8
Cumulative Incurred Claim Count | claim	1,605
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 16.7	7.5
General Liability and Umbrella - Occurrence [Member] | Year 2020 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	99.8
IBNR	$ 81.0
Cumulative Incurred Claim Count | claim	1,213
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 3.9
General Liability and Umbrella - Occurrence [Member] | Years 2011 to 2020 [Member]
Claims Development [Line Items]
Total reserves for accident years	404.2
General Liability and Umbrella - Occurrence [Member] | Years 2010 and Prior [Member]
Claims Development [Line Items]
Total reserves for accident years	18.1
General Liability - Claims Made [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	767.0
Cumulative Paid Losses and ALAE, Net of Reinsurance	482.8
Unallocated loss adjustment expense	5.1
Net reserves	293.1
General Liability - Claims Made [Member] | Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	82.4	82.9	82.5	86.2	83.9	69.9	61.7
IBNR	$ 0.9
Cumulative Incurred Claim Count | claim	906
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 79.9	79.1	75.4	71.5	59.1	38.8	$ 11.0
General Liability - Claims Made [Member] | Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	91.0	90.3	90.2	98.4	98.8	93.2
IBNR	$ 1.9
Cumulative Incurred Claim Count | claim	1,002
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 83.7	81.8	76.0	64.2	43.6	$ 10.0
General Liability - Claims Made [Member] | Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	95.2	89.7	97.1	101.7	103.6
IBNR	$ 4.7
Cumulative Incurred Claim Count | claim	1,011
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 80.4	75.5	66.1	42.9	11.1
General Liability - Claims Made [Member] | Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	96.0	98.1	104.6	103.3
IBNR	$ 8.8
Cumulative Incurred Claim Count | claim	1,122
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 76.5	67.2	42.3	12.3
General Liability - Claims Made [Member] | Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	132.5	127.8	120.9
IBNR	$ 14.3
Cumulative Incurred Claim Count | claim	1,386
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 87.5	56.1	17.3
General Liability - Claims Made [Member] | Year 2019 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	127.1	126.8
IBNR	$ 31.7
Cumulative Incurred Claim Count | claim	1,554
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 56.7	17.8
General Liability - Claims Made [Member] | Year 2020 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	142.8
IBNR	$ 76.3
Cumulative Incurred Claim Count | claim	2,514
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 18.1
General Liability - Claims Made [Member] | Years 2014 to 2020 [Member]
Claims Development [Line Items]
Total reserves for accident years	284.2
General Liability - Claims Made [Member] | Years 2013 and Prior [Member]
Claims Development [Line Items]
Total reserves for accident years	3.8
Personal Automobile Lines [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	1,970.0
Cumulative Paid Losses and ALAE, Net of Reinsurance	1,355.6
Unallocated loss adjustment expense	15.3
Net reserves	681.4
Personal Automobile Lines [Member] | Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	367.7	365.8	355.3	344.5	337.9
IBNR	$ 4.2
Cumulative Incurred Claim Count | claim	42,375
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 346.3	329.4	288.0	213.1	$ 112.8
Personal Automobile Lines [Member] | Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	392.2	383.4	362.3	363.6
IBNR	$ 9.7
Cumulative Incurred Claim Count | claim	42,993
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 343.8	302.7	229.2	115.0
Personal Automobile Lines [Member] | Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	400.3	395.3	394.2
IBNR	$ 23.9
Cumulative Incurred Claim Count | claim	42,841
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 307.4	237.2	121.7
Personal Automobile Lines [Member] | Year 2019 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	431.8	431.2
IBNR	$ 67.0
Cumulative Incurred Claim Count | claim	41,703
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 262.2	131.0
Personal Automobile Lines [Member] | Year 2020 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	378.0
IBNR	$ 199.2
Cumulative Incurred Claim Count | claim	24,339
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 95.9
Personal Automobile Lines [Member] | Years 2016 to 2020 [Member]
Claims Development [Line Items]
Total reserves for accident years	614.4
Personal Automobile Lines [Member] | Years 2015 and Prior [Member]
Claims Development [Line Items]
Total reserves for accident years	51.7
Homeowners [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	1,376.6
Cumulative Paid Losses and ALAE, Net of Reinsurance	1,183.0
Unallocated loss adjustment expense	3.4
Net reserves	207.5
Homeowners [Member] | Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	295.2	297.3	296.5	291.4
IBNR	$ 0.6
Cumulative Incurred Claim Count | claim	33,474
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 289.0	283.5	271.6	$ 204.5
Homeowners [Member] | Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	317.3	316.2	312.9
IBNR	$ 6.1
Cumulative Incurred Claim Count | claim	32,725
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 300.8	290.1	$ 213.7
Homeowners [Member] | Year 2019 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	345.3	344.5
IBNR	$ 14.3
Cumulative Incurred Claim Count | claim	33,582
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 311.7	$ 234.7
Homeowners [Member] | Year 2020 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	418.8
IBNR	$ 77.2
Cumulative Incurred Claim Count | claim	31,392
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 281.5
Homeowners [Member] | Years 2017 to 2020 [Member]
Claims Development [Line Items]
Total reserves for accident years	193.6
Homeowners [Member] | Years 2016 and Prior [Member]
Claims Development [Line Items]
Total reserves for accident years	$ 10.5