Fair Value (Assets and Liabilities Measured at Fair Value on Recurring Basis) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 7,400.2	$ 6,983.4
Equity securities	612.5	576.6
Other investments	483.4	393.4
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	7,400.2	6,983.4
Equity securities	603.8	567.7
Other investments	99.5	104.5
Total investment assets at fair value	8,103.5	7,655.6
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	222.5	246.1
Equity securities	602.4	566.4
Total investment assets at fair value	824.9	812.5
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	7,124.9	6,681.7
Other investments	95.9	100.9
Total investment assets at fair value	7,220.8	6,782.6
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	52.8	55.6
Equity securities	1.4	1.3
Other investments	3.6	3.6
Total investment assets at fair value	57.8	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	320.4	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	174.2	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	146.2	255.5
Fair Value, Measurements, Recurring | Foreign Government [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	242.7	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	48.3	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	194.4	193.3
Fair Value, Measurements, Recurring | Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,136.4	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	1,104.1	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	32.3	34.4
Fair Value, Measurements, Recurring | Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	4,108.2	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	4,103.9	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	4.3	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	889.8	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	889.8	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	54.5	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	54.5	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	569.8	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	553.6	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	16.2	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	78.4	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	$ 78.4	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		$ 0.5