Fair Value (Fair Value, Assets Measured on Recurring Basis) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 7,890.5	$ 7,331.3
Equity securities	566.5	584.4
Other investments	616.6	533.8
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	7,890.5	7,331.3
Equity securities	558.1	574.6
Other investments	118.5	106.3
Total investment assets at fair value	8,567.1	8,012.2
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	243.1	256.8
Equity securities	557.0	573.1
Total investment assets at fair value	800.1	829.9
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	7,604.0	7,024.3
Other investments	114.4	102.2
Total investment assets at fair value	7,718.4	7,126.5
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	43.4	50.2
Equity securities	1.1	1.5
Other investments	4.1	4.1
Total investment assets at fair value	48.6	55.8
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	434.1	341.1
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]
Fixed maturities	198.4	209.5
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]
Fixed maturities	235.7	131.6
Fair Value, Measurements, Recurring [Member] | Foreign Government [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	245.0	240.7
Fair Value, Measurements, Recurring [Member] | Foreign Government [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	44.7	47.3
Fair Value, Measurements, Recurring [Member] | Foreign Government [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	200.3	193.4
Fair Value, Measurements, Recurring [Member] | Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,093.2	1,095.4
Fair Value, Measurements, Recurring [Member] | Municipal [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,065.1	1,064.4
Fair Value, Measurements, Recurring [Member] | Municipal [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	28.1	31.0
Fair Value, Measurements, Recurring [Member] | Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	4,426.5	4,053.8
Fair Value, Measurements, Recurring [Member] | Corporate [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	4,425.6	4,049.6
Fair Value, Measurements, Recurring [Member] | Corporate [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	0.9	4.2
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	1,000.2	924.4
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]
Fixed maturities	1,000.2	924.4
Fair Value, Measurements, Recurring [Member] | Residential Mortgage Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	32.9	49.8
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]
Fixed maturities	32.9	49.8
Fair Value, Measurements, Recurring [Member] | Commercial Mortgage Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	598.2	554.3
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]
Fixed maturities	583.8	539.3
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]
Fixed maturities	14.4	15.0
Fair Value, Measurements, Recurring [Member] | Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	60.4	71.8
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]
Fixed maturities	$ 60.4	$ 71.8