Fixed Maturities and Equity Securities (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	$ 15.4	$ 45.8
12 months or less, Fair Value	620.0	1,389.6
Greater than 12 months, Gross Unrealized Losses	24.2	18.0
Greater than 12 months, Fair Value	200.1	123.6
Total, Gross Unrealized Losses	39.6	63.8
Total, Fair Value	820.1	1,513.2
Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	10.5	37.0
12 months or less, Fair Value	543.1	1,302.4
Greater than 12 months, Gross Unrealized Losses	23.1	18.0
Greater than 12 months, Fair Value	192.2	123.6
Total, Gross Unrealized Losses	33.6	55.0
Total, Fair Value	735.3	1,426.0
Fixed Maturities | Investment grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	5.1	27.1
12 months or less, Fair Value	497.8	1,175.5
Greater than 12 months, Gross Unrealized Losses	16.9	14.3
Greater than 12 months, Fair Value	131.0	108.9
Total, Gross Unrealized Losses	22.0	41.4
Total, Fair Value	628.8	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
12 months or less, Fair Value	0.7	57.7
Greater than 12 months, Gross Unrealized Losses	0.2
Greater than 12 months, Fair Value	8.8
Total, Gross Unrealized Losses	0.2	0.2
Total, Fair Value	9.5	57.7
Fixed Maturities | Investment grade | Foreign Governments
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.1	0.5
12 months or less, Fair Value	25.0	148.8
Total, Gross Unrealized Losses	0.1	0.5
Total, Fair Value	25.0	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	33.5	28.0
Greater than 12 months, Gross Unrealized Losses	2.2	3.4
Greater than 12 months, Fair Value	33.8	58.8
Total, Gross Unrealized Losses	2.7	3.9
Total, Fair Value	67.3	86.8
Fixed Maturities | Investment grade | Corporate
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	3.2	19.9
12 months or less, Fair Value	323.0	699.6
Greater than 12 months, Gross Unrealized Losses	8.3	8.2
Greater than 12 months, Fair Value	62.3	35.6
Total, Gross Unrealized Losses	11.5	28.1
Total, Fair Value	385.3	735.2
Fixed Maturities | Investment grade | Residential Mortgage-Backed
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	1.2	5.1
12 months or less, Fair Value	58.1	115.8
Greater than 12 months, Gross Unrealized Losses	5.9	2.4
Greater than 12 months, Fair Value	21.3	9.9
Total, Gross Unrealized Losses	7.1	7.5
Total, Fair Value	79.4	125.7
Fixed Maturities | Investment grade | Commercial Mortgage-Backed
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses		0.7
12 months or less, Fair Value	27.3	58.0
Greater than 12 months, Gross Unrealized Losses	0.3	0.3
Greater than 12 months, Fair Value	4.8	4.6
Total, Gross Unrealized Losses	0.3	1.0
Total, Fair Value	32.1	62.6
Fixed Maturities | Investment grade | Asset-Backed
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.1	0.2
12 months or less, Fair Value	30.2	67.6
Total, Gross Unrealized Losses	0.1	0.2
Total, Fair Value	30.2	67.6
Fixed Maturities | Below Investment Grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	5.4	9.9
12 months or less, Fair Value	45.3	126.9
Greater than 12 months, Gross Unrealized Losses	6.2	3.7
Greater than 12 months, Fair Value	61.2	14.7
Total, Gross Unrealized Losses	11.6	13.6
Total, Fair Value	106.5	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	4.0	8.5
12 months or less, Fair Value	35.2	118.0
Greater than 12 months, Gross Unrealized Losses	6.2	3.7
Greater than 12 months, Fair Value	59.2	14.7
Total, Gross Unrealized Losses	10.2	12.2
Total, Fair Value	94.4	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	0.7	0.9
12 months or less, Fair Value	9.2	8.0
Total, Gross Unrealized Losses	0.7	0.9
Total, Fair Value	9.2	8.0
Fixed Maturities | Below Investment Grade | Asset-Backed
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.7	0.5
12 months or less, Fair Value	0.9	0.9
Total, Gross Unrealized Losses	0.7	0.5
Total, Fair Value	0.9	0.9
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	4.9	8.8
12 months or less, Fair Value	76.9	87.2
Greater than 12 months, Gross Unrealized Losses	1.1
Greater than 12 months, Fair Value	7.9
Total, Gross Unrealized Losses	6.0	8.8
Total, Fair Value	$ 84.8	$ 87.2