Fair Value (Assets and Liabilities Measured at Fair Value on Recurring Basis) (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 7,084.9	$ 7,378.1
Equity securities	550.7	580.8
Other investments	334.9	291.4
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	101.3	136.4
Total investment assets at fair value	7,727.3	8,086.0
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment assets at fair value	765.8	847.1
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	97.5	132.6
Total investment assets at fair value	6,902.8	7,177.2
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	3.8	3.8
Total investment assets at fair value	58.7	61.7
Fair Value, Measurements, Recurring | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	7,084.9	7,378.1
Fair Value, Measurements, Recurring | Fixed Maturities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	226.0	276.8
Fair Value, Measurements, Recurring | Fixed Maturities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	6,805.3	7,044.6
Fair Value, Measurements, Recurring | Fixed Maturities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	53.6	56.7
Fair Value, Measurements, Recurring | Fixed Maturities | U.S. Treasury And Government Agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	463.2	520.4
Fair Value, Measurements, Recurring | Fixed Maturities | U.S. Treasury And Government Agencies | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	169.2	233.5
Fair Value, Measurements, Recurring | Fixed Maturities | U.S. Treasury And Government Agencies | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	294.0	286.9
Fair Value, Measurements, Recurring | Fixed Maturities | Foreign Government Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	268.6	354.0
Fair Value, Measurements, Recurring | Fixed Maturities | Foreign Government Debt Securities [Member] | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	56.8	43.3
Fair Value, Measurements, Recurring | Fixed Maturities | Foreign Government Debt Securities [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	211.8	310.7
Fair Value, Measurements, Recurring | Fixed Maturities | Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,143.8	1,138.0
Fair Value, Measurements, Recurring | Fixed Maturities | Municipal [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,113.8	1,112.3
Fair Value, Measurements, Recurring | Fixed Maturities | Municipal [Member] | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	30.0	25.7
Fair Value, Measurements, Recurring | Fixed Maturities | Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	3,746.7	3,880.8
Fair Value, Measurements, Recurring | Fixed Maturities | Corporate Debt Securities [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	3,742.0	3,871.2
Fair Value, Measurements, Recurring | Fixed Maturities | Corporate Debt Securities [Member] | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	4.7	9.6
Fair Value, Measurements, Recurring | Fixed Maturities | Residential Mortgage Backed Securities U S Agency Backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	751.0	689.2
Fair Value, Measurements, Recurring | Fixed Maturities | 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	751.0	689.2
Fair Value, Measurements, Recurring | Fixed Maturities | Residential Mortgage Backed Securities, Non-Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	78.7	99.9
Fair Value, Measurements, Recurring | Fixed Maturities | Residential Mortgage Backed Securities, Non-Agency [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	78.7	99.9
Fair Value, Measurements, Recurring | Fixed Maturities | Commercial Mortgage Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	535.0	527.8
Fair Value, Measurements, Recurring | Fixed Maturities | Commercial Mortgage Backed Securities [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	517.3	506.4
Fair Value, Measurements, Recurring | Fixed Maturities | Commercial Mortgage Backed Securities [Member] | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	17.7	21.4
Fair Value, Measurements, Recurring | Fixed Maturities | Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	97.9	168.0
Fair Value, Measurements, Recurring | Fixed Maturities | Asset-backed Securities [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	96.7	168.0
Fair Value, Measurements, Recurring | Fixed Maturities | Asset-backed Securities [Member] | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1.2
Fair Value, Measurements, Recurring | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	541.1	571.5
Fair Value, Measurements, Recurring | Equity Securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	539.8	570.3
Fair Value, Measurements, Recurring | Equity Securities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	$ 1.3	$ 1.2