Supplementary Insurance Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	$ 489.5		$ 458.6		$ 309.7
Future policy benefits, losses, claims and loss expenses	6,192.9		5,757.3		3,275.8
Unearned premiums	2,474.8		2,292.1		1,520.3
Other policy claims and benefits payable	4.1		3.0		1.9
Premium revenue	4,239.1		3,598.6		2,841.0
Net investment income	276.6		258.2		247.2
Benefits, claims, losses and settlement expenses	2,974.4		2,550.8		1,856.3
Amortization of deferred policy acquisition costs	938.1		778.9		600.8
Other operating expenses	641.4		557.0		487.5
Premiums written	4,368.4		3,593.4		3,048.0
Property And Casualty Insurance
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	346.6		325.3		309.7
Future policy benefits, losses, claims and loss expenses	3,784.9		3,424.5		3,275.8
Unearned premiums	1,711.1		1,588.9		1,520.3
Other policy claims and benefits payable	4.1		3.0		1.9
Premium revenue	3,272.3		3,092.3		2,841.0
Net investment income	236.1		241.3		247.2
Benefits, claims, losses and settlement expenses	2,467.2		2,237.6		1,856.3
Amortization of deferred policy acquisition costs	704.2		658.8		600.8
Other operating expenses	428.4		439.3		447.8
Premiums written	3,377.9		3,164.6		3,048.0
Chaucer
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	142.9	[1]	133.3	[1]
Future policy benefits, losses, claims and loss expenses	2,408.0	[1]	2,332.8	[1]
Unearned premiums	763.7	[1]	703.2	[1]
Premium revenue	966.8	[1]	506.3	[1]
Net investment income	40.2	[1]	16.9	[1]
Benefits, claims, losses and settlement expenses	507.2	[1]	313.2	[1]
Amortization of deferred policy acquisition costs	233.9	[1]	120.1	[1]
Other operating expenses	157.6	[1]	67.9	[1]
Premiums written	990.5	[1]	428.8	[1]
Interest On Debt
Supplementary Insurance Information, by Segment [Line Items]
Other operating expenses	61.9		55.0		44.3
Eliminations
Supplementary Insurance Information, by Segment [Line Items]
Net investment income	0.3
Other operating expenses	$ (6.5)		$ (5.2)		$ (4.6)

[1] 2011 includes results of Chaucer Holdings plc ("Chaucer") since the July 1, 2011 acquisition date. 2012 includes results of Chaucer for the entire year.