Fair Value (Assets and Liabilities Measured at Fair Value on Recurring Basis) (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 7,058.5	$ 6,970.6
Equity securities	530.5	430.2
Other investments	238.5	192.5
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	138.5	153.2
Total investment assets at fair value	7,718.2	7,544.7
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	723.3	595.1
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	134.9	149.6
Total investment assets at fair value	6,927.6	6,845.4
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	3.6	3.6
Total investment assets at fair value	67.3	104.2
Fair Value, Measurements, Recurring | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	7,058.5	6,970.6
Fair Value, Measurements, Recurring | Fixed Maturities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	203.3	212.8
Fair Value, Measurements, Recurring | Fixed Maturities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	6,792.7	6,695.8
Fair Value, Measurements, Recurring | Fixed Maturities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	62.5	62.0
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	403.2	406.6
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	160.5	167.2
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	242.7	239.4
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	321.0	305.0
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	42.8	45.6
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	278.2	259.4
Fair Value, Measurements, Recurring | Fixed Maturities | Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,133.5	1,126.3
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	1,106.7	1,100.7
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	26.8	25.6
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	3,908.7	3,824.2
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	3,895.5	3,811.2
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	13.2	13.0
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	586.2	573.2
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	586.2	573.2
Fair Value, Measurements, Recurring | Fixed Maturities | Residential Mortgage Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	127.4	155.6
Fair Value, Measurements, Recurring | Fixed Maturities | Residential Mortgage Backed Securities [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	127.4	155.1
Fair Value, Measurements, Recurring | Fixed Maturities | Residential Mortgage Backed Securities [Member] | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities		0.5
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	406.1	411.6
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	383.6	388.7
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	22.5	22.9
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	172.4	168.1
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	172.4	168.1
Fair Value, Measurements, Recurring | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	521.2	420.9
Fair Value, Measurements, Recurring | Equity Securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	520.0	382.3
Fair Value, Measurements, Recurring | Equity Securities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	$ 1.2	$ 38.6