Property and Casualty Insurance Reserves - Short-duration Insurance Contracts, Claims Development (Details) $ in Millions	Dec. 31, 2019 USD ($) claim	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Loss and Allocated LAE Reserves, Net of Reinsurance	$ 1,725.1
Specialty Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	5,389.9
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	4,269.4
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance	20.4
Loss and Allocated LAE Reserves, Net of Reinsurance	1,140.9
Specialty Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	2,400.3
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	2,356.9
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance	(1.2)
Loss and Allocated LAE Reserves, Net of Reinsurance	42.2
Commercial Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	591.6
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	401.8
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance	1.4
Loss and Allocated LAE Reserves, Net of Reinsurance	191.2
Commercial Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	118.9
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	115.9
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance	0.0
Loss and Allocated LAE Reserves, Net of Reinsurance	3.0
Preferred Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	838.2
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	634.8
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance	9.5
Loss and Allocated LAE Reserves, Net of Reinsurance	212.9
Preferred Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	549.4
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	544.2
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance	(0.1)
Loss and Allocated LAE Reserves, Net of Reinsurance	5.1
Homeowners Insurance
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	957.7
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	888.6
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance	3.2
Loss and Allocated LAE Reserves, Net of Reinsurance	72.3
2015 | Specialty Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	969.6	$ 972.3	$ 965.1	$ 855.1	$ 965.1
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 8.1
Cumulative Number of Reported Claims | claim	401,025
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 954.0	934.7	898.9	794.9	450.5
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,	406.6	406.5	406.7	407.1	403.8
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.9
Cumulative Number of Reported Claims | claim	231,007
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 406.1	406.4	406.7	408.8	382.4
2015 | Commercial Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	110.7	112.3	112.9	106.9	102.5
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.6
Cumulative Number of Reported Claims | claim	19,292
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 107.6	100.3	88.6	70.2	34.8
2015 | Commercial Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	21.7	21.7	21.7	21.6	21.5
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.0
Cumulative Number of Reported Claims | claim	9,643
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 21.7	21.7	21.7	21.6	20.2
2015 | Preferred Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	177.1	177.6	176.5	171.8	168.3
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.9
Cumulative Number of Reported Claims | claim	38,961
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 169.3	163.1	147.5	122.4	73.1
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,	100.6	100.6	100.6	100.7	101.2
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.0
Cumulative Number of Reported Claims | claim	68,409
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 100.6	100.6	100.7	101.0	100.1
2015 | Homeowners Insurance
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	164.4	164.3	163.2	164.9	178.9
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.5
Cumulative Number of Reported Claims | claim	19,731
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 162.7	161.3	158.0	154.4	$ 116.9
2016 | Specialty Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	1,026.0	1,027.2	1,021.6	969.4
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 22.5
Cumulative Number of Reported Claims | claim	417,016
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 987.7	943.4	831.1	459.7
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,	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	246,183
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 457.5	458.0	460.2	436.4
2016 | Commercial Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	117.7	115.6	112.4	120.5
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 3.5
Cumulative Number of Reported Claims | claim	20,417
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 102.3	89.7	71.6	36.2
2016 | 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	24.1	24.2	24.2
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.0
Cumulative Number of Reported Claims | claim	10,560
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 24.2	24.1	24.2	22.4
2016 | Preferred Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	176.8	179.1	174.5	162.1
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 2.1
Cumulative Number of Reported Claims | claim	36,721
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 161.1	145.6	114.6	61.2
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,	106.2	106.3	106.6	106.6
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (0.1)
Cumulative Number of Reported Claims | claim	65,348
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 106.3	106.3	106.9	105.2
2016 | Homeowners Insurance
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	202.2	204.2	201.7	200.3
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 1.4
Cumulative Number of Reported Claims | claim	20,380
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 198.9	195.8	190.1	$ 141.2
2017 | Specialty Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	1,004.5	999.9	997.7
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 40.6
Cumulative Number of Reported Claims | claim	396,578
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 926.7	808.6	441.9
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,	465.1	465.6	475.6
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.0
Cumulative Number of Reported Claims | claim	251,872
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 466.0	468.7	443.0
2017 | Commercial Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	118.3	120.0	120.5
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 9.1
Cumulative Number of Reported Claims | claim	19,925
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 90.7	72.3	36.3
2017 | Commercial Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	23.5	23.5	24.2
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.2
Cumulative Number of Reported Claims | claim	9,788
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 23.4	23.5	22.2
2017 | Preferred Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	155.8	157.8	164.4
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 4.5
Cumulative Number of Reported Claims | claim	33,663
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 134.1	108.9	59.2
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,	105.2	105.8	109.2
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (0.1)
Cumulative Number of Reported Claims | claim	62,553
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 105.2	106.1	104.4
2017 | Homeowners Insurance
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	245.2	259.5	261.2
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.9
Cumulative Number of Reported Claims | claim	20,982
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 235.7	242.5	$ 165.8
2018 | Specialty Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	1,119.1	1,128.1
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 93.0
Cumulative Number of Reported Claims | claim	445,229
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 903.8	467.5
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,	496.9	504.9
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (1.6)
Cumulative Number of Reported Claims | claim	269,782
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 501.5	463.6
2018 | Commercial Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	116.5	123.2
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 19.8
Cumulative Number of Reported Claims | claim	19,969
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 68.8	36.8
2018 | Commercial Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	23.5	23.6
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.3
Cumulative Number of Reported Claims | claim	9,558
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 23.6	21.7
2018 | Preferred Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	156.3	157.6
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 15.6
Cumulative Number of Reported Claims | claim	32,020
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 107.6	55.5
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,	111.0	113.9
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (0.5)
Cumulative Number of Reported Claims | claim	60,749
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 111.4	107.2
2018 | Homeowners Insurance
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	183.0	185.9
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 1.2
Cumulative Number of Reported Claims | claim	17,122
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 180.2	$ 127.4
2019 | Specialty Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	1,270.7
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 383.0
Cumulative Number of Reported Claims | claim	447,558
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 497.2
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,	574.7
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 47.5
Cumulative Number of Reported Claims | claim	276,797
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 525.8
2019 | Commercial Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	128.4
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 59.2
Cumulative Number of Reported Claims | claim	17,772
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 32.4
2019 | Commercial Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	26.0
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 3.5
Cumulative Number of Reported Claims | claim	8,803
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 23.0
2019 | Preferred Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	172.2
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 51.3
Cumulative Number of Reported Claims | claim	32,848
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 62.7
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,	126.4
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (0.9)
Cumulative Number of Reported Claims | claim	60,548
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 120.7
2019 | Homeowners Insurance
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	162.9
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 19.2
Cumulative Number of Reported Claims | claim	13,931
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 111.1