Fair Value (Fair Value, Assets Measured on Recurring Basis) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 7,340.8	$ 7,331.3
Equity securities	600.5	584.4
Other investments	588.8	533.8
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	7,340.8	7,331.3
Equity securities	591.7	574.6
Other investments	117.5	106.3
Total investment assets at fair value	8,050.0	8,012.2
Fair Value, Measurements, Recurring | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	259.8	256.8
Equity securities	590.2	573.1
Total investment assets at fair value	850.0	829.9
Fair Value, Measurements, Recurring | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	7,031.7	7,024.3
Other investments	113.4	102.2
Total investment assets at fair value	7,145.1	7,126.5
Fair Value, Measurements, Recurring | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	49.3	50.2
Equity securities	1.5	1.5
Other investments	4.1	4.1
Total investment assets at fair value	54.9	55.8
Fair Value, Measurements, Recurring | U.S. Treasury And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	338.1	341.1
Fair Value, Measurements, Recurring | 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	207.6	209.5
Fair Value, Measurements, Recurring | 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	130.5	131.6
Fair Value, Measurements, Recurring | Foreign Government [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	248.0	240.7
Fair Value, Measurements, Recurring | Foreign Government [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	52.2	47.3
Fair Value, Measurements, Recurring | Foreign Government [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	195.8	193.4
Fair Value, Measurements, Recurring | Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,079.7	1,095.4
Fair Value, Measurements, Recurring | Municipal [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,048.9	1,064.4
Fair Value, Measurements, Recurring | Municipal [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	30.8	31.0
Fair Value, Measurements, Recurring | Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	4,094.1	4,053.8
Fair Value, Measurements, Recurring | Corporate [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	4,090.2	4,049.6
Fair Value, Measurements, Recurring | Corporate [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	3.9	4.2
Fair Value, Measurements, Recurring | Residential Mortgage Backed Securities U S Agency Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	918.6	924.4
Fair Value, Measurements, Recurring | 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	918.6	924.4
Fair Value, Measurements, Recurring | Residential Mortgage Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	44.8	49.8
Fair Value, Measurements, Recurring | Residential Mortgage Backed [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	44.8	49.8
Fair Value, Measurements, Recurring | Commercial Mortgage Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	558.9	554.3
Fair Value, Measurements, Recurring | Commercial Mortgage Backed [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	544.3	539.3
Fair Value, Measurements, Recurring | Commercial Mortgage Backed [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	14.6	15.0
Fair Value, Measurements, Recurring | Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	58.6	71.8
Fair Value, Measurements, Recurring | Asset-backed Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 58.6	$ 71.8