Property and Casualty Insurance Reserves - Short-duration Insurance Contracts, 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 ($)
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Property and Casualty Insurance Reserves, Net of Reinsurance	$ 1,756.7
Specialty Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	5,668.0
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	4,559.4
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance	3.9
Property and Casualty Insurance Reserves, Net of Reinsurance	1,112.5
Specialty Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	2,600.6
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	2,548.7
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance	1.6
Property and Casualty Insurance Reserves, Net of Reinsurance	53.5
Commercial Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	609.5
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	412.2
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance	11.8
Property and Casualty Insurance Reserves, Net of Reinsurance	209.1
Commercial Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	130.2
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	124.3
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance	0.4
Property and Casualty Insurance Reserves, Net of Reinsurance	6.3
Preferred Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	841.5
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	617.5
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance	8.6
Property and Casualty Insurance Reserves, Net of Reinsurance	232.6
Preferred Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	543.6
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	539.1
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance	(0.1)
Property and Casualty Insurance Reserves, Net of Reinsurance	4.4
Homeowners Insurance
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	947.3
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	864.0
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance	5.1
Property and Casualty Insurance Reserves, Net of Reinsurance	88.4
2015 | Specialty Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	1,022.7	$ 1,026.0	$ 1,027.2	$ 1,021.6	$ 969.4
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 4.7
Cumulative Number of Reported Claims | claim	417,219
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 1,008.3	987.7	943.4	831.1	459.7
2015 | Specialty Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	457.4	457.0	456.9	456.9	462.2
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (0.1)
Cumulative Number of Reported Claims | claim	246,239
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 457.5	457.5	458.0	460.2	436.4
2015 | Commercial Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	115.6	117.7	115.6	112.4	120.5
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 1.1
Cumulative Number of Reported Claims | claim	20,427
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 109.7	102.3	89.7	71.6	36.2
2015 | Commercial Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	24.2	24.2	24.1	24.2	24.2
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.0
Cumulative Number of Reported Claims | claim	10,561
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 24.2	24.2	24.1	24.2	22.4
2015 | Preferred Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	177.2	176.8	179.1	174.5	162.1
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 1.6
Cumulative Number of Reported Claims | claim	36,766
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 169.3	161.1	145.6	114.6	61.2
2015 | Preferred Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	106.2	106.2	106.3	106.6	106.6
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.0
Cumulative Number of Reported Claims | claim	65,191
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 106.2	106.3	106.3	106.9	105.2
2015 | Homeowners Insurance
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	201.1	202.2	204.2	201.7	200.3
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.3
Cumulative Number of Reported Claims | claim	20,019
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 199.5	198.9	195.8	190.1	$ 141.2
2016 | Specialty Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	999.1	1,004.5	999.9	997.7
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 13.7
Cumulative Number of Reported Claims | claim	397,059
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 967.7	926.7	808.6	441.9
2016 | Specialty Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	465.6	465.1	465.6	475.6
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.1
Cumulative Number of Reported Claims | claim	251,935
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 465.9	466.0	468.7	443.0
2016 | Commercial Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	114.3	118.3	120.0	120.5
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 2.9
Cumulative Number of Reported Claims | claim	19,951
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 101.7	90.7	72.3	36.3
2016 | Commercial Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	23.4	23.5	23.5	24.2
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.1
Cumulative Number of Reported Claims | claim	9,792
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 23.4	23.4	23.5	22.2
2016 | Preferred Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	158.2	155.8	157.8	164.4
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 1.9
Cumulative Number of Reported Claims | claim	33,731
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 143.2	134.1	108.9	59.2
2016 | Preferred Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	105.1	105.2	105.8	109.2
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.0
Cumulative Number of Reported Claims | claim	62,385
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 105.1	105.2	106.1	104.4
2016 | Homeowners Insurance
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	243.8	245.2	259.5	261.2
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.6
Cumulative Number of Reported Claims | claim	20,796
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 239.5	235.7	242.5	$ 165.8
2017 | Specialty Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	1,120.0	1,119.1	1,128.1
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 33.9
Cumulative Number of Reported Claims | claim	447,610
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 1,039.3	903.8	467.5
2017 | Specialty Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	496.4	496.9	504.9
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (0.4)
Cumulative Number of Reported Claims | claim	270,000
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 497.4	501.5	463.6
2017 | Commercial Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	113.0	116.5	123.2
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 12.6
Cumulative Number of Reported Claims | claim	20,146
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 88.1	68.8	36.8
2017 | Commercial Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	23.6	23.5	23.6
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.1
Cumulative Number of Reported Claims | claim	9,567
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 23.6	23.6	21.7
2017 | Preferred Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	161.7	156.3	157.6
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 6.1
Cumulative Number of Reported Claims | claim	32,246
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 132.7	107.6	55.5
2017 | Preferred Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	110.4	111.0	113.9
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (0.1)
Cumulative Number of Reported Claims | claim	60,707
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 110.4	111.4	107.2
2017 | Homeowners Insurance
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	183.6	183.0	185.9
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 3.0
Cumulative Number of Reported Claims | claim	17,104
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 180.0	180.2	$ 127.4
2018 | Specialty Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	1,306.9	1,270.7
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 91.4
Cumulative Number of Reported Claims | claim	485,455
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 1,052.5	497.2
2018 | Specialty Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	581.0	574.7
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (10.7)
Cumulative Number of Reported Claims | claim	286,954
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 585.7	525.8
2018 | Commercial Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	126.1	128.4
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 19.4
Cumulative Number of Reported Claims | claim	19,404
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 75.7	32.4
2018 | Commercial Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	27.1	26.0
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (0.3)
Cumulative Number of Reported Claims | claim	9,290
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 26.9	23.0
2018 | Preferred Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	195.5	172.2
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 20.0
Cumulative Number of Reported Claims | claim	35,891
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 127.9	62.7
2018 | Preferred Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	125.8	126.4
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (0.7)
Cumulative Number of Reported Claims | claim	63,776
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 126.5	120.7
2018 | Homeowners Insurance
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	161.8	162.9
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 4.1
Cumulative Number of Reported Claims | claim	15,311
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 150.4	$ 111.1
2019 | Specialty Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	1,219.3
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 368.6
Cumulative Number of Reported Claims | claim	399,216
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 491.6
2019 | Specialty Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	600.2
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 52.0
Cumulative Number of Reported Claims | claim	252,237
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 542.2
2019 | Commercial Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	140.5
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 60.5
Cumulative Number of Reported Claims | claim	16,457
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 37.0
2019 | Commercial Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	31.9
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 4.6
Cumulative Number of Reported Claims | claim	10,936
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 26.2
2019 | Preferred Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	148.9
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 45.8
Cumulative Number of Reported Claims | claim	23,543
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 44.4
2019 | Preferred Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	96.1
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (0.7)
Cumulative Number of Reported Claims | claim	43,539
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 90.9
2019 | Homeowners Insurance
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	157.0
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 17.5
Cumulative Number of Reported Claims | claim	12,529
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 94.6