Fixed Maturities and Equity Securities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	$ 9.4	$ 45.8
12 months or less, Fair Value	648.6	1,389.6
Greater than 12 months, Gross Unrealized Losses	15.4	18.0
Greater than 12 months, Fair Value	161.6	123.6
Total, Gross Unrealized Losses	24.8	63.8
Total, Fair Value	810.2	1,513.2
Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	4.6	37.0
12 months or less, Fair Value	574.2	1,302.4
Greater than 12 months, Gross Unrealized Losses	15.4	18.0
Greater than 12 months, Fair Value	161.6	123.6
Total, Gross Unrealized Losses	20.0	55.0
Total, Fair Value	735.8	1,426.0
Fixed Maturities | Investment grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	3.4	27.1
12 months or less, Fair Value	546.0	1,175.5
Greater than 12 months, Gross Unrealized Losses	9.5	14.3
Greater than 12 months, Fair Value	106.5	108.9
Total, Gross Unrealized Losses	12.9	41.4
Total, Fair Value	652.5	1,284.4
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	0.2	0.2
12 months or less, Fair Value	89.5	57.7
Greater than 12 months, Gross Unrealized Losses	0.2
Greater than 12 months, Fair Value	8.5
Total, Gross Unrealized Losses	0.4	0.2
Total, Fair Value	98.0	57.7
Fixed Maturities | Investment grade | Foreign Governments
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.2	0.5
12 months or less, Fair Value	81.2	148.8
Greater than 12 months, Fair Value	0.4
Total, Gross Unrealized Losses	0.2	0.5
Total, Fair Value	81.6	148.8
Fixed Maturities | Investment grade | Municipal
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.5	0.5
12 months or less, Fair Value	61.9	28.0
Greater than 12 months, Gross Unrealized Losses	0.6	3.4
Greater than 12 months, Fair Value	24.0	58.8
Total, Gross Unrealized Losses	1.1	3.9
Total, Fair Value	85.9	86.8
Fixed Maturities | Investment grade | Corporate
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	1.8	19.9
12 months or less, Fair Value	224.8	699.6
Greater than 12 months, Gross Unrealized Losses	6.6	8.2
Greater than 12 months, Fair Value	59.0	35.6
Total, Gross Unrealized Losses	8.4	28.1
Total, Fair Value	283.8	735.2
Fixed Maturities | Investment grade | Residential Mortgage-Backed
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.5	5.1
12 months or less, Fair Value	47.3	115.8
Greater than 12 months, Gross Unrealized Losses	2.0	2.4
Greater than 12 months, Fair Value	9.4	9.9
Total, Gross Unrealized Losses	2.5	7.5
Total, Fair Value	56.7	125.7
Fixed Maturities | Investment grade | Commercial Mortgage-Backed
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.2	0.7
12 months or less, Fair Value	29.9	58.0
Greater than 12 months, Gross Unrealized Losses	0.1	0.3
Greater than 12 months, Fair Value	4.9	4.6
Total, Gross Unrealized Losses	0.3	1.0
Total, Fair Value	34.8	62.6
Fixed Maturities | Investment grade | Asset-Backed
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses		0.2
12 months or less, Fair Value	11.4	67.6
Greater than 12 months, Fair Value	0.3
Total, Gross Unrealized Losses		0.2
Total, Fair Value	11.7	67.6
Fixed Maturities | Below Investment Grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	1.2	9.9
12 months or less, Fair Value	28.2	126.9
Greater than 12 months, Gross Unrealized Losses	5.9	3.7
Greater than 12 months, Fair Value	55.1	14.7
Total, Gross Unrealized Losses	7.1	13.6
Total, Fair Value	83.3	141.6
Fixed Maturities | Below Investment Grade | Municipal
Schedule of Available-for-sale Securities [Line Items]
Greater than 12 months, Fair Value	2.0
Total, Fair Value	2.0
Fixed Maturities | Below Investment Grade | Corporate
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	1.1	8.5
12 months or less, Fair Value	26.6	118.0
Greater than 12 months, Gross Unrealized Losses	5.3	3.7
Greater than 12 months, Fair Value	50.6	14.7
Total, Gross Unrealized Losses	6.4	12.2
Total, Fair Value	77.2	132.7
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	0.9
12 months or less, Fair Value	1.6	8.0
Greater than 12 months, Gross Unrealized Losses	0.6
Greater than 12 months, Fair Value	2.5
Total, Gross Unrealized Losses	0.7	0.9
Total, Fair Value	4.1	8.0
Fixed Maturities | Below Investment Grade | Asset-Backed
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses		0.5
12 months or less, Fair Value		0.9
Total, Gross Unrealized Losses		0.5
Total, Fair Value		0.9
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	4.8	8.8
12 months or less, Fair Value	74.4	87.2
Total, Gross Unrealized Losses	4.8	8.8
Total, Fair Value	$ 74.4	$ 87.2