Investments - Schedule of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Fair Value of Investments (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 3,760.4	$ 3,895.7
Gross Unrealized Gains	281.3	325.1
Gross Unrealized Losses	56.3	20.1
Fair Value	3,985.4	4,200.7
Fixed maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,752.9	2,993.1
Gross Unrealized Gains	88.3	170.8
Gross Unrealized Losses	46.6	9.9
Fair Value	2,794.6	3,154.0
Fixed maturities [Member] | U.S. Governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	302.1	401.1
Gross Unrealized Gains	3.8	9.3
Gross Unrealized Losses	0.6	0
Fair Value	305.3	410.4
Fixed maturities [Member] | Non-U.S. Governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	58.0	53.1
Gross Unrealized Gains	0.4	3.6
Gross Unrealized Losses	2.9	0.1
Fair Value	55.5	56.6
Fixed maturities [Member] | Obligations of states and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	641.1	551.7
Gross Unrealized Gains	26.6	44.9
Gross Unrealized Losses	12.0	0.5
Fair Value	655.7	596.1
Fixed maturities [Member] | Credit-Financial [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	368.1	382.4
Gross Unrealized Gains	14.5	25.7
Gross Unrealized Losses	3.7	0.7
Fair Value	378.9	407.4
Fixed maturities [Member] | Credit-Industrial [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	389.1	428.4
Gross Unrealized Gains	16.4	31.1
Gross Unrealized Losses	5.4	0.7
Fair Value	400.1	458.8
Fixed maturities [Member] | Credit-Utility [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	174.7	189.9
Gross Unrealized Gains	6.3	12.6
Gross Unrealized Losses	1.9	0.4
Fair Value	179.1	202.1
Fixed maturities [Member] | CMO/MBS-agency [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	289.3	375.1
Gross Unrealized Gains	11.1	20.2
Gross Unrealized Losses	2.7	0.2
Fair Value	297.7	395.1
Fixed maturities [Member] | CMO/MBS-non agency [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	9.5	12.5
Gross Unrealized Gains	0.9	1.0
Gross Unrealized Losses	0	0.2
Fair Value	10.4	13.3
Fixed maturities [Member] | CMBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	107.6	109.5
Gross Unrealized Gains	3.6	5.7
Gross Unrealized Losses	1.5	0.2
Fair Value	109.7	115.0
Fixed maturities [Member] | ABS-residential [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8.6	10.1
Gross Unrealized Gains	0.2	0.2
Gross Unrealized Losses	0.5	0.8
Fair Value	8.3	9.5
Fixed maturities [Member] | ABS-non residential [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	80.7	81.3
Gross Unrealized Gains	0.6	1.2
Gross Unrealized Losses	0.3	0
Fair Value	81.0	82.5
Fixed maturities [Member] | Foreign denominated: Governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	202.0	262.1
Gross Unrealized Gains	1.8	9.2
Gross Unrealized Losses	9.0	4.3
Fair Value	194.8	267.0
Fixed maturities [Member] | Foreign Denominated: Credit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	122.1	135.9
Gross Unrealized Gains	2.1	6.1
Gross Unrealized Losses	6.1	1.8
Fair Value	118.1	140.2
Equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	414.1	383.5
Gross Unrealized Gains	190.3	153.0
Gross Unrealized Losses	4.7	5.1
Fair Value	599.7	531.4
Other investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	313.7	284.8
Gross Unrealized Gains	2.7	1.2
Gross Unrealized Losses	4.8	5.0
Fair Value	311.6	281.0
Short-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	279.7	234.3
Gross Unrealized Gains	0	0.1
Gross Unrealized Losses	0.2	0.1
Fair Value	$ 279.5	$ 234.3