Fair Value (Fair Value, Assets Measured on Recurring Basis) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 7,729.9	$ 7,779.7
Equity securities	554.0	576.5
Other investments	736.8	685.5
Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,037.7	1,076.0
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	7,729.9	7,779.7
Equity securities	554.0	568.1
Other investments	128.5	126.4
Total investment assets at fair value	8,412.4	8,474.2
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	256.5	277.7
Equity securities	552.9	567.0
Total investment assets at fair value	809.4	844.7
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	7,434.9	7,460.1
Other investments	124.9	122.8
Total investment assets at fair value	7,559.8	7,582.9
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	38.5	41.9
Equity securities	1.1	1.1
Other investments	3.6	3.6
Total investment assets at fair value	43.2	46.6
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]
Fair Value	580.5	511.4
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]
Fair Value	235.4	227.6
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]
Fair Value	345.1	283.8
Fair Value, Measurements, Recurring [Member] | Foreign Government [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	209.5	242.7
Fair Value, Measurements, Recurring [Member] | Foreign Government [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	21.1	50.1
Fair Value, Measurements, Recurring [Member] | Foreign Government [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	188.4	192.6
Fair Value, Measurements, Recurring [Member] | Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,037.7	1,076.0
Fair Value, Measurements, Recurring [Member] | Municipal [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,013.3	1,049.2
Fair Value, Measurements, Recurring [Member] | Municipal [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	24.4	26.8
Fair Value, Measurements, Recurring [Member] | Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	4,216.3	4,307.5
Fair Value, Measurements, Recurring [Member] | Corporate [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	4,215.5	4,306.6
Fair Value, Measurements, Recurring [Member] | Corporate [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0.8	0.9
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]
Fair Value	993.8	956.4
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]
Fair Value	993.8	956.4
Fair Value, Measurements, Recurring [Member] | Residential Mortgage Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3.2	29.6
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]
Fair Value	3.2	29.6
Fair Value, Measurements, Recurring [Member] | Commercial Mortgage Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	611.6	596.4
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]
Fair Value	598.3	582.2
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]
Fair Value	13.3	14.2
Fair Value, Measurements, Recurring [Member] | Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	77.3	59.7
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]
Fair Value	$ 77.3	$ 59.7