Fair Value (Fair Value,Investment Assets Measured on Recurring Basis) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 6,559.4	$ 6,161.5
Equity securities	504.7	464.4
Other investments	680.4	661.5
Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	865.5	885.8
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	1.1	1.1
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	6,559.4	6,161.5
Equity securities	504.7	464.4
Other investments	3.5	3.5
Total investment assets at fair value, excluding held-for-sale	7,067.6	6,653.9
Investment assets, held-for-sale		24.5
Fair Value, Measurements, Recurring [Member] | Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment assets at fair value, excluding held-for-sale (Chaucer)		6,629.4
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	406.7	409.9
Fair Value, Measurements, Recurring [Member] | Foreign Government [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	7.4	7.4
Fair Value, Measurements, Recurring [Member] | Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	865.5	885.8
Fair Value, Measurements, Recurring [Member] | Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	3,674.6	3,410.7
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	817.2	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	2.7	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] | Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	93.4	91.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	159.3	154.9
Equity securities	503.6	463.3
Total investment assets at fair value, excluding held-for-sale	662.9	623.9
Investment assets, held-for-sale		5.7
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment assets at fair value, excluding held-for-sale (Chaucer)		618.2
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | U.S. Treasury And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	159.3	154.9
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	6,366.1	5,971.6
Total investment assets at fair value, excluding held-for-sale	6,366.1	5,990.4
Investment assets, held-for-sale		18.8
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment assets at fair value, excluding held-for-sale (Chaucer)		5,971.6
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. Treasury And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	247.4	255.0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Foreign Government [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	7.4	7.4
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	845.0	864.7
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	3,673.9	3,409.9
Fair Value, Measurements, Recurring [Member] | Level 2 [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	817.2	713.7
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Residential Mortgage Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	2.7	2.7
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Commercial Mortgage Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	679.1	627.2
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	93.4	91.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	34.0	35.0
Equity securities	1.1	1.1
Other investments	3.5	3.5
Total investment assets at fair value, excluding held-for-sale	38.6	39.6
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment assets at fair value, excluding held-for-sale (Chaucer)		39.6
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	20.5	21.1
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	0.7	0.8
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Commercial Mortgage Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 12.8	$ 13.1