Fair Value (Assets and Liabilities Measured at Fair Value on Recurring Basis) (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 7,142.0	$ 6,983.4
Equity securities	582.7	576.6
Other investments	454.8	393.4
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	7,142.0	6,983.4
Equity securities	574.0	567.7
Other investments	94.1	104.5
Total investment assets at fair value	7,810.1	7,655.6
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	226.3	246.1
Equity securities	572.7	566.4
Total investment assets at fair value	799.0	812.5
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	6,859.8	6,681.7
Other investments	90.5	100.9
Total investment assets at fair value	6,950.3	6,782.6
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	55.9	55.6
Equity securities	1.3	1.3
Other investments	3.6	3.6
Total investment assets at fair value	60.8	60.5
Fair Value, Measurements, Recurring | U.S. Treasury And Government Agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	376.5	449.1
Fair Value, Measurements, Recurring | U.S. Treasury And Government Agencies | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	178.8	193.6
Fair Value, Measurements, Recurring | U.S. Treasury And Government Agencies | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	197.7	255.5
Fair Value, Measurements, Recurring | Foreign Government [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	236.6	245.8
Fair Value, Measurements, Recurring | Foreign Government [Member] | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	47.5	52.5
Fair Value, Measurements, Recurring | Foreign Government [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	189.1	193.3
Fair Value, Measurements, Recurring | Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,159.9	1,120.3
Fair Value, Measurements, Recurring | Municipal [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,125.5	1,085.9
Fair Value, Measurements, Recurring | Municipal [Member] | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	34.4	34.4
Fair Value, Measurements, Recurring | Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	3,882.7	3,691.0
Fair Value, Measurements, Recurring | Corporate [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	3,878.3	3,687.3
Fair Value, Measurements, Recurring | Corporate [Member] | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	4.4	3.7
Fair Value, Measurements, Recurring | Residential Mortgage Backed Securities U S Agency Backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	815.1	824.5
Fair Value, Measurements, Recurring | Residential Mortgage Backed Securities U S Agency Backed | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	815.1	824.5
Fair Value, Measurements, Recurring | Residential Mortgage Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	59.1	71.6
Fair Value, Measurements, Recurring | Residential Mortgage Backed [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	59.1	71.6
Fair Value, Measurements, Recurring | Commercial Mortgage Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	532.9	501.1
Fair Value, Measurements, Recurring | Commercial Mortgage Backed [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	516.2	484.1
Fair Value, Measurements, Recurring | Commercial Mortgage Backed [Member] | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	16.7	17.0
Fair Value, Measurements, Recurring | Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	79.2	80.0
Fair Value, Measurements, Recurring | Asset-backed Securities [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	78.8	79.5
Fair Value, Measurements, Recurring | Asset-backed Securities [Member] | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 0.4	$ 0.5