Fair Value (Fair Value, Assets Carried on Recurring Basis) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 6,687.1	$ 6,161.5
Equity securities	575.7	464.4
Other investments	733.2	661.5
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	2.1	1.1
Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	833.5	885.8
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	6,687.1	6,161.5
Equity securities	575.7	464.4
Other investments	3.5	3.5
Total investment assets at fair value	7,266.3	6,653.9
Total investment assets at fair value, excluding held-for-sale (Chaucer)		6,629.4
Fair Value, Measurements, Recurring [Member] | Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment assets, held-for-sale		24.5
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	157.9	154.9
Equity securities	573.6	463.3
Total investment assets at fair value	731.5	623.9
Total investment assets at fair value, excluding held-for-sale (Chaucer)		618.2
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment assets, held-for-sale		5.7
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	6,503.8	5,971.6
Total investment assets at fair value	6,503.8	5,990.4
Total investment assets at fair value, excluding held-for-sale (Chaucer)		5,971.6
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment assets, held-for-sale		18.8
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	25.4	35.0
Equity securities	2.1	1.1
Other investments	3.5	3.5
Total investment assets at fair value	31.0	39.6
Total investment assets at fair value, excluding held-for-sale (Chaucer)		39.6
Fair Value, Measurements, Recurring [Member] | U.S. Treasury And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	349.8	409.9
Fair Value, Measurements, Recurring [Member] | U.S. Treasury And Government Agencies [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	157.9	154.9
Fair Value, Measurements, Recurring [Member] | U.S. Treasury And Government Agencies [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	191.9	255.0
Fair Value, Measurements, Recurring [Member] | Foreign Government [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	16.1	7.4
Fair Value, Measurements, Recurring [Member] | Foreign Government [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	16.1	7.4
Fair Value, Measurements, Recurring [Member] | Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	833.5	885.8
Fair Value, Measurements, Recurring [Member] | Municipal [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	821.4	864.7
Fair Value, Measurements, Recurring [Member] | Municipal [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	12.1	21.1
Fair Value, Measurements, Recurring [Member] | Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	3,811.2	3,410.7
Fair Value, Measurements, Recurring [Member] | Corporate [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	3,810.6	3,409.9
Fair Value, Measurements, Recurring [Member] | Corporate [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	0.6	0.8
Fair Value, Measurements, Recurring [Member] | Residential Mortgage Backed Securities U S Agency Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	919.1	713.7
Fair Value, Measurements, Recurring [Member] | Residential Mortgage Backed Securities U S Agency Backed [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	919.1	713.7
Fair Value, Measurements, Recurring [Member] | Residential Mortgage Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	2.3	2.7
Fair Value, Measurements, Recurring [Member] | Residential Mortgage Backed [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	2.3	2.7
Fair Value, Measurements, Recurring [Member] | Commercial Mortgage Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	691.9	640.3
Fair Value, Measurements, Recurring [Member] | Commercial Mortgage Backed [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	679.2	627.2
Fair Value, Measurements, Recurring [Member] | Commercial Mortgage Backed [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	12.7	13.1
Fair Value, Measurements, Recurring [Member] | Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	63.2	91.0
Fair Value, Measurements, Recurring [Member] | Asset-backed Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 63.2	$ 91.0