Fixed Maturities and Equity Securities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	$ 7.0	$ 9.4
12 months or less, Fair Value	558.1	648.6
Greater than 12 months, Gross Unrealized Losses	14.5	15.4
Greater than 12 months, Fair Value	121.2	161.6
Total, Gross Unrealized Losses	21.5	24.8
Total, Fair Value	679.3	810.2
Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	5.6	4.6
12 months or less, Fair Value	542.7	574.2
Greater than 12 months, Gross Unrealized Losses	13.7	15.4
Greater than 12 months, Fair Value	113.3	161.6
Total, Gross Unrealized Losses	19.3	20.0
Total, Fair Value	656.0	735.8
Fixed Maturities | Investment grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	4.6	3.4
12 months or less, Fair Value	511.9	546.0
Greater than 12 months, Gross Unrealized Losses	9.8	9.5
Greater than 12 months, Fair Value	72.2	106.5
Total, Gross Unrealized Losses	14.4	12.9
Total, Fair Value	584.1	652.5
Fixed Maturities | Investment grade | U.S. Treasury And Government Agencies
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.5	0.2
12 months or less, Fair Value	98.1	89.5
Greater than 12 months, Gross Unrealized Losses	0.1	0.2
Greater than 12 months, Fair Value	6.5	8.5
Total, Gross Unrealized Losses	0.6	0.4
Total, Fair Value	104.6	98.0
Fixed Maturities | Investment grade | Foreign government
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses		0.2
12 months or less, Fair Value	23.9	81.2
Greater than 12 months, Gross Unrealized Losses	0.1
Greater than 12 months, Fair Value	0.4	0.4
Total, Gross Unrealized Losses	0.1	0.2
Total, Fair Value	24.3	81.6
Fixed Maturities | Investment grade | Municipal
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.7	0.5
12 months or less, Fair Value	65.5	61.9
Greater than 12 months, Gross Unrealized Losses	0.6	0.6
Greater than 12 months, Fair Value	16.3	24.0
Total, Gross Unrealized Losses	1.3	1.1
Total, Fair Value	81.8	85.9
Fixed Maturities | Investment grade | Corporate
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	2.1	1.8
12 months or less, Fair Value	181.3	224.8
Greater than 12 months, Gross Unrealized Losses	6.9	6.6
Greater than 12 months, Fair Value	34.4	59.0
Total, Gross Unrealized Losses	9.0	8.4
Total, Fair Value	215.7	283.8
Fixed Maturities | Investment grade | Residential mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.8	0.5
12 months or less, Fair Value	66.7	47.3
Greater than 12 months, Gross Unrealized Losses	2.0	2.0
Greater than 12 months, Fair Value	9.5	9.4
Total, Gross Unrealized Losses	2.8	2.5
Total, Fair Value	76.2	56.7
Fixed Maturities | Investment grade | Commercial mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.5	0.2
12 months or less, Fair Value	52.4	29.9
Greater than 12 months, Gross Unrealized Losses	0.1	0.1
Greater than 12 months, Fair Value	4.9	4.9
Total, Gross Unrealized Losses	0.6	0.3
Total, Fair Value	57.3	34.8
Fixed Maturities | Investment grade | Asset-backed
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Fair Value	24.0	11.4
Greater than 12 months, Fair Value	0.2	0.3
Total, Fair Value	24.2	11.7
Fixed Maturities | Below Investment Grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	1.0	1.2
12 months or less, Fair Value	30.8	28.2
Greater than 12 months, Gross Unrealized Losses	3.9	5.9
Greater than 12 months, Fair Value	41.1	55.1
Total, Gross Unrealized Losses	4.9	7.1
Total, Fair Value	71.9	83.3
Fixed Maturities | Below Investment Grade | Municipal
Schedule of Available-for-sale Securities [Line Items]
Greater than 12 months, Gross Unrealized Losses	0.1
Greater than 12 months, Fair Value	3.4	2.0
Total, Gross Unrealized Losses	0.1
Total, Fair Value	3.4	2.0
Fixed Maturities | Below Investment Grade | Corporate
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.9	1.1
12 months or less, Fair Value	30.0	26.6
Greater than 12 months, Gross Unrealized Losses	3.4	5.3
Greater than 12 months, Fair Value	35.5	50.6
Total, Gross Unrealized Losses	4.3	6.4
Total, Fair Value	65.5	77.2
Fixed Maturities | Below Investment Grade | Residential mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.1	0.1
12 months or less, Fair Value	0.8	1.6
Greater than 12 months, Gross Unrealized Losses	0.4	0.6
Greater than 12 months, Fair Value	2.2	2.5
Total, Gross Unrealized Losses	0.5	0.7
Total, Fair Value	3.0	4.1
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	1.4	4.8
12 months or less, Fair Value	15.4	74.4
Greater than 12 months, Gross Unrealized Losses	0.8
Greater than 12 months, Fair Value	7.9
Total, Gross Unrealized Losses	2.2	4.8
Total, Fair Value	$ 23.3	$ 74.4