Investments - Schedule of Aging of Unrealized Losses on Investments (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	$ 981.3	$ 490.3
Unrealized Losses, Less Than One Year	36.1	17.0
Fair Value, One Year or Greater	7.9	28.2
Unrealized Losses, One Year or Greater	0.7	3.1
Fair Value, Total	989.2	518.5
Unrealized Losses, Total	36.8	20.1
Fixed maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	924.8	418.5
Unrealized Losses, Less Than One Year	33.0	7.7
Fair Value, One Year or Greater	7.9	21.9
Unrealized Losses, One Year or Greater	0.7	2.2
Fair Value, Total	932.7	440.4
Unrealized Losses, Total	33.7	9.9
Fixed maturities [Member] | U.S. Governments [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	36.8	11.9
Unrealized Losses, Less Than One Year	0.4	0
Fair Value, One Year or Greater	0	0
Unrealized Losses, One Year or Greater	0	0
Fair Value, Total	36.8	11.9
Unrealized Losses, Total	0.4	0
Fixed maturities [Member] | Non-US Governments [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	51.3	8.5
Unrealized Losses, Less Than One Year	2.4	0.1
Fair Value, One Year or Greater	0	0.5
Unrealized Losses, One Year or Greater	0	0
Fair Value, Total	51.3	9.0
Unrealized Losses, Total	2.4	0.1
Fixed maturities [Member] | Obligations of states and political subdivisions [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	216.9	28.2
Unrealized Losses, Less Than One Year	13.5	0.3
Fair Value, One Year or Greater	0	0.8
Unrealized Losses, One Year or Greater	0	0.2
Fair Value, Total	216.9	29.0
Unrealized Losses, Total	13.5	0.5
Fixed maturities [Member] | Credit-Financial [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	82.6	25.0
Unrealized Losses, Less Than One Year	2.3	0.2
Fair Value, One Year or Greater	1.3	8.6
Unrealized Losses, One Year or Greater	0	0.5
Fair Value, Total	83.9	33.6
Unrealized Losses, Total	2.3	0.7
Fixed maturities [Member] | Credit-Industrial [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	95.9	32.2
Unrealized Losses, Less Than One Year	3.6	0.4
Fair Value, One Year or Greater	2.5	3.0
Unrealized Losses, One Year or Greater	0.2	0.3
Fair Value, Total	98.4	35.2
Unrealized Losses, Total	3.8	0.7
Fixed maturities [Member] | Credit-Utility [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	34.9	8.4
Unrealized Losses, Less Than One Year	1.4	0.2
Fair Value, One Year or Greater	0	0.8
Unrealized Losses, One Year or Greater	0	0.2
Fair Value, Total	34.9	9.2
Unrealized Losses, Total	1.4	0.4
Fixed maturities [Member] | CMO/MBS-agency [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	44.6	26.4
Unrealized Losses, Less Than One Year	1.5	0.2
Fair Value, One Year or Greater	0.2	0.3
Unrealized Losses, One Year or Greater	0	0
Fair Value, Total	44.8	26.7
Unrealized Losses, Total	1.5	0.2
Fixed maturities [Member] | CMO/MBS-non agency [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year		1.7
Unrealized Losses, Less Than One Year		0.2
Fair Value, One Year or Greater		0
Unrealized Losses, One Year or Greater		0
Fair Value, Total		1.7
Unrealized Losses, Total		0.2
Fixed maturities [Member] | CMBS [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	45.5	5.7
Unrealized Losses, Less Than One Year	1.1	0
Fair Value, One Year or Greater	0	3.5
Unrealized Losses, One Year or Greater	0	0.2
Fair Value, Total	45.5	9.2
Unrealized Losses, Total	1.1	0.2
Fixed maturities [Member] | ABS-residential [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	0	0
Unrealized Losses, Less Than One Year	0	0
Fair Value, One Year or Greater	3.9	3.9
Unrealized Losses, One Year or Greater	0.5	0.8
Fair Value, Total	3.9	3.9
Unrealized Losses, Total	0.5	0.8
Fixed maturities [Member] | ABS-non residential [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	60.2	13.7
Unrealized Losses, Less Than One Year	0.1	0
Fair Value, One Year or Greater	0	0.2
Unrealized Losses, One Year or Greater	0	0
Fair Value, Total	60.2	13.9
Unrealized Losses, Total	0.1	0
Fixed maturities [Member] | Foreign denominated: Governments [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	174.4	180.5
Unrealized Losses, Less Than One Year	4.0	4.3
Fair Value, One Year or Greater	0	0.3
Unrealized Losses, One Year or Greater	0	0
Fair Value, Total	174.4	180.8
Unrealized Losses, Total	4.0	4.3
Fixed maturities [Member] | Foreign Denominated: Credit [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	81.7	76.3
Unrealized Losses, Less Than One Year	2.7	1.8
Fair Value, One Year or Greater	0	0
Unrealized Losses, One Year or Greater	0	0
Fair Value, Total	81.7	76.3
Unrealized Losses, Total	2.7	1.8
Equity securities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	54.3	60.2
Unrealized Losses, Less Than One Year	3.1	4.2
Fair Value, One Year or Greater	0	6.3
Unrealized Losses, One Year or Greater	0	0.9
Fair Value, Total	54.3	66.5
Unrealized Losses, Total	3.1	5.1
Other investments [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	0	11.6
Unrealized Losses, Less Than One Year	0	5.0
Fair Value, One Year or Greater	0	0
Unrealized Losses, One Year or Greater	0	0
Fair Value, Total	0	11.6
Unrealized Losses, Total	0	5.0
Short-term investments [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	2.2	0
Unrealized Losses, Less Than One Year	0	0.1
Fair Value, One Year or Greater	0	0
Unrealized Losses, One Year or Greater	0	0
Fair Value, Total	2.2	0
Unrealized Losses, Total	$ 0	$ 0.1