Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses (Schedule of Incurred Claims Development) (Details) $ in Millions	Dec. 31, 2018 USD ($) claim	Dec. 31, 2017 USD ($)	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 ($)	Dec. 31, 2009 USD ($)
Commercial Multiple Peril Line [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	$ 3,357.8
Cumulative Paid Losses and ALAE, Net of Reinsurance	2,334.6
Unallocated loss adjustment expense	19.3
Net reserves	1,095.7
Commercial Multiple Peril Line [Member] | Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	476.6	$ 474.9	$ 475.6	$ 459.7	$ 456.2	$ 462.8	$ 480.0
IBNR	$ 8.8
Cumulative Incurred Claim Count | claim	16,983
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 454.9	442.8	424.2	389.3	350.8	307.5	175.8
Commercial Multiple Peril Line [Member] | Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	393.0	392.1	391.8	367.9	362.5	380.0
IBNR	$ 10.2
Cumulative Incurred Claim Count | claim	14,830
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 355.9	334.6	306.4	262.3	221.5	137.6
Commercial Multiple Peril Line [Member] | Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	449.1	459.4	464.8	439.6	443.9
IBNR	$ 22.6
Cumulative Incurred Claim Count | claim	15,922
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 395.2	363.4	316.0	267.8	171.7
Commercial Multiple Peril Line [Member] | Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	467.4	463.7	456.3	446.0
IBNR	$ 33.0
Cumulative Incurred Claim Count | claim	15,731
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 363.2	315.6	260.1	161.9
Commercial Multiple Peril Line [Member] | Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	448.7	449.6	447.1
IBNR	$ 55.5
Cumulative Incurred Claim Count | claim	15,981
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 290.2	237.9	140.3
Commercial Multiple Peril Line [Member] | Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	544.8	538.7
IBNR	$ 122.2
Cumulative Incurred Claim Count | claim	16,674
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 296.4	170.9
Commercial Multiple Peril Line [Member] | Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	578.2
IBNR	$ 228.3
Cumulative Incurred Claim Count | claim	16,716
Commercial Multiple Peril Line [Member] | Years 2010 And Prior [Member]
Claims Development [Line Items]
Total reserves for accident years	$ 53.2
Commercial Multiple Peril Line [Member] | Years 2011 To 2017 [Member]
Claims Development [Line Items]
Total reserves for accident years	1,023.2
Commercial Multiple Peril Line [Member] | Year 2018 [Member]
Claims Development [Line Items]
Cumulative Paid Losses and ALAE, Net of Reinsurance	178.8
Workers' Compensation Line [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	1,486.7
Cumulative Paid Losses and ALAE, Net of Reinsurance	1,001.9
Unallocated loss adjustment expense	17.0
Net reserves	592.9
Workers' Compensation Line [Member] | Year 2008 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	78.7	77.9	77.5	77.5	79.2	79.4	80.3	$ 82.5	$ 87.6	$ 93.8
IBNR	$ 1.9
Cumulative Incurred Claim Count | claim	8,032
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 73.1	72.4	70.8	69.8	68.6	65.4	61.6	54.6	41.2	$ 19.2
Workers' Compensation Line [Member] | Year 2009 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	116.5	115.9	114.7	114.9	116.3	115.9	115.8	116.9	115.5
IBNR	$ 3.3
Cumulative Incurred Claim Count | claim	10,174
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 107.2	106.3	104.2	101.0	96.5	89.5	76.9	57.2	$ 22.1
Workers' Compensation Line [Member] | Year 2010 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	147.9	148.3	146.7	143.8	145.5	144.3	144.2	141.4
IBNR	$ 7.1
Cumulative Incurred Claim Count | claim	12,465
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 132.8	130.9	128.0	122.9	114.0	97.3	71.3	30.0
Workers' Compensation Line [Member] | Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	163.7	163.7	162.9	157.2	165.2	171.1	176.3
IBNR	$ 10.2
Cumulative Incurred Claim Count | claim	13,042
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 141.4	136.0	130.7	120.1	102.6	74.8	30.4
Workers' Compensation Line [Member] | Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	155.4	155.3	154.4	160.1	167.4	179.3
IBNR	$ 12.3
Cumulative Incurred Claim Count | claim	11,683
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 127.4	122.8	114.0	101.2	74.6	30.9
Workers' Compensation Line [Member] | Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	150.3	152.1	154.7	172.9	182.1
IBNR	$ 15.2
Cumulative Incurred Claim Count | claim	10,880
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 112.5	105.6	92.3	70.5	30.6
Workers' Compensation Line [Member] | Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	150.3	157.5	164.2	189.6
IBNR	$ 22.5
Cumulative Incurred Claim Count | claim	11,417
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 99.4	87.2	65.7	28.0
Workers' Compensation Line [Member] | Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	164.6	180.5	189.6
IBNR	$ 24.5
Cumulative Incurred Claim Count | claim	15,790
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 99.5	78.1	33.9
Workers' Compensation Line [Member] | Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	172.1	186.1
IBNR	$ 35.4
Cumulative Incurred Claim Count | claim	16,405
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 73.0	32.8
Workers' Compensation Line [Member] | Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	187.2
IBNR	$ 69.4
Cumulative Incurred Claim Count | claim	15,711
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 35.6
Workers' Compensation Line [Member] | Years 2008 To 2017 [Member]
Claims Development [Line Items]
Total reserves for accident years	484.8
Workers' Compensation Line [Member] | Years 2007 And Prior [Member]
Claims Development [Line Items]
Total reserves for accident years	91.1
Commercial Automobile Line [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	1,099.8
Cumulative Paid Losses and ALAE, Net of Reinsurance	711.1
Unallocated loss adjustment expense	4.9
Net reserves	408.0
Commercial Automobile Line [Member] | Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	231.0	225.6	227.5	205.3	179.4		177.7
IBNR	$ 1.4
Cumulative Incurred Claim Count | claim	15,274
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 214.9	200.6	176.3	137.6	89.6		35.9
Commercial Automobile Line [Member] | Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	185.1	181.7	177.3	163.3	168.5
IBNR	$ 6.4
Cumulative Incurred Claim Count | claim	13,477
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 168.2	137.1	102.7	70.8	33.1
Commercial Automobile Line [Member] | Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	167.8	166.9	168.3	163.4
IBNR	$ 10.0
Cumulative Incurred Claim Count | claim	12,838
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 129.3	96.4	63.8	32.2
Commercial Automobile Line [Member] | Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	163.0	157.7	157.0
IBNR	$ 16.4
Cumulative Incurred Claim Count | claim	11,629
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 98.3	60.7	27.8
Commercial Automobile Line [Member] | Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	170.1	159.5
IBNR	$ 43.4
Cumulative Incurred Claim Count | claim	11,343
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 71.2	26.9
Commercial Automobile Line [Member] | Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	182.8
IBNR	$ 110.7
Cumulative Incurred Claim Count | claim	10,338
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 29.2
Commercial Automobile Line [Member] | Years 2012 To 2017 [Member]
Claims Development [Line Items]
Total reserves for accident years	388.7
Commercial Automobile Line [Member] | Years 2011 And Prior [Member]
Claims Development [Line Items]
Total reserves for accident years	14.4
General Liability And Umbrella - Occurrence [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	712.0
Cumulative Paid Losses and ALAE, Net of Reinsurance	348.7
Unallocated loss adjustment expense	11.5
Net reserves	400.6
General Liability And Umbrella - Occurrence [Member] | Year 2010 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	57.5	55.4	55.6	51.0	42.2	38.2	43.8	47.6
IBNR	$ 3.3
Cumulative Incurred Claim Count | claim	1,398
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 50.6	47.8	45.8	39.8	31.1	19.4	6.7	$ 1.6
General Liability And Umbrella - Occurrence [Member] | Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	69.1	74.2	71.1	64.3	62.2	59.3	77.2
IBNR	$ 3.7
Cumulative Incurred Claim Count | claim	1,779
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 63.0	60.6	52.1	43.8	29.8	12.6	$ 2.2
General Liability And Umbrella - Occurrence [Member] | Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	81.2	84.4	88.6	71.5	67.4	84.1
IBNR	$ 8.2
Cumulative Incurred Claim Count | claim	2,031
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 63.5	56.1	43.1	26.8	11.0	2.4
General Liability And Umbrella - Occurrence [Member] | Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	98.3	101.4	98.6	82.3	100.9
IBNR	$ 10.6
Cumulative Incurred Claim Count | claim	2,133
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 70.4	52.8	31.4	14.5	3.1
General Liability And Umbrella - Occurrence [Member] | Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	97.0	99.6	99.3	104.2
IBNR	$ 21.3
Cumulative Incurred Claim Count | claim	2,550
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 48.5	30.8	15.2	3.3
General Liability And Umbrella - Occurrence [Member] | Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	101.5	100.7	95.6
IBNR	$ 33.9
Cumulative Incurred Claim Count | claim	2,015
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 31.6	15.0	3.1
General Liability And Umbrella - Occurrence [Member] | Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	107.6	97.7
IBNR	$ 54.2
Cumulative Incurred Claim Count | claim	1,844
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 17.0	4.4
General Liability And Umbrella - Occurrence [Member] | Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	99.8
IBNR	$ 74.4
Cumulative Incurred Claim Count | claim	1,917
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 4.1
General Liability And Umbrella - Occurrence [Member] | Years 2010 To 2017 [Member]
Claims Development [Line Items]
Total reserves for accident years	363.3
General Liability And Umbrella - Occurrence [Member] | Years 2009 And Prior [Member]
Claims Development [Line Items]
Total reserves for accident years	25.8
General Liability - Claims Made [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	563.7
Cumulative Paid Losses and ALAE, Net of Reinsurance	342.8
Unallocated loss adjustment expense	4.8
Net reserves	229.7
General Liability - Claims Made [Member] | Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	68.4	69.2	71.4	60.6	55.4	54.9
IBNR	$ 0.9
Cumulative Incurred Claim Count | claim	645
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 65.7	64.5	61.1	53.7	36.2	$ 9.3
General Liability - Claims Made [Member] | Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	82.5	86.2	83.9	69.9	61.7
IBNR	$ 2.9
Cumulative Incurred Claim Count | claim	907
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 75.4	71.5	59.1	38.8	11.0
General Liability - Claims Made [Member] | Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	90.2	98.4	98.8	93.2
IBNR	$ 4.9
Cumulative Incurred Claim Count | claim	1,035
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 76.0	64.2	43.6	10.0
General Liability - Claims Made [Member] | Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	97.1	101.7	103.6
IBNR	$ 18.4
Cumulative Incurred Claim Count | claim	1,082
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 66.1	42.9	11.1
General Liability - Claims Made [Member] | Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	104.6	103.3
IBNR	$ 35.5
Cumulative Incurred Claim Count | claim	1,310
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 42.3	12.3
General Liability - Claims Made [Member] | Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	120.9
IBNR	$ 59.5
Cumulative Incurred Claim Count | claim	2,586
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 17.3
General Liability - Claims Made [Member] | Years 2012 To 2017 [Member]
Claims Development [Line Items]
Total reserves for accident years	220.9
General Liability - Claims Made [Member] | Years 2011 And Prior [Member]
Claims Development [Line Items]
Total reserves for accident years	4.0
Personal Automobile Lines [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	1,756.7
Cumulative Paid Losses and ALAE, Net of Reinsurance	1,239.2
Unallocated loss adjustment expense	13.8
Net reserves	596.7
Personal Automobile Lines [Member] | Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	312.5	310.2	308.8	304.1	323.1
IBNR	$ 4.7
Cumulative Incurred Claim Count | claim	43,356
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 298.4	279.4	241.1	188.6	$ 107.2
Personal Automobile Lines [Member] | Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	332.4	328.6	334.1	327.4
IBNR	$ 5.6
Cumulative Incurred Claim Count | claim	42,358
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 301.9	261.5	205.4	112.9
Personal Automobile Lines [Member] | Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	355.3	344.5	337.9
IBNR	$ 11.5
Cumulative Incurred Claim Count | claim	42,214
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 288.0	213.1	112.8
Personal Automobile Lines [Member] | Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	362.3	363.6
IBNR	$ 35.0
Cumulative Incurred Claim Count | claim	42,200
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 229.2	115.0
Personal Automobile Lines [Member] | Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	394.2
IBNR	$ 151.7
Cumulative Incurred Claim Count | claim	39,164
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 121.7
Personal Automobile Lines [Member] | Years 2013 To 2017 [Member]
Claims Development [Line Items]
Total reserves for accident years	517.5
Personal Automobile Lines [Member] | Years 2012 And Prior [Member]
Claims Development [Line Items]
Total reserves for accident years	65.4
Homeowners [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	1,115.0
Cumulative Paid Losses and ALAE, Net of Reinsurance	971.0
Unallocated loss adjustment expense	3.4
Net reserves	152.2
Homeowners [Member] | Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	274.2	272.0		267.1
IBNR	$ 1.7
Cumulative Incurred Claim Count | claim	30,436
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 267.5	260.6	253.8	$ 199.9
Homeowners [Member] | Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	231.4	226.5	222.1
IBNR	$ 3.2
Cumulative Incurred Claim Count | claim	25,456
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 218.2	207.9	$ 154.3
Homeowners [Member] | Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	296.5	291.4
IBNR	$ 3.4
Cumulative Incurred Claim Count | claim	33,394
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 271.6	$ 204.5
Homeowners [Member] | Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	312.9
IBNR	$ 46.0
Cumulative Incurred Claim Count | claim	30,487
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 213.7
Homeowners [Member] | Years 2015 And 2017 [Member]
Claims Development [Line Items]
Total reserves for accident years	144.0
Homeowners [Member] | Years 2014 And Prior [Member]
Claims Development [Line Items]
Total reserves for accident years	$ 4.8