Fair Value of Plan Assets and Funded Status of Plans (Detail) (USD $) In Millions, unless otherwise specified	0 Months Ended	1 Months Ended	12 Months Ended
	Jan. 04, 2010	Jan. 04, 2010	Dec. 31, 2012 United States Qualified Pension Plans	Dec. 31, 2011 United States Qualified Pension Plans	Dec. 31, 2012 United States Non Qualified Pension Plans	Dec. 31, 2011 United States Non Qualified Pension Plans	Dec. 31, 2012 Chaucer Pension Scheme	Dec. 31, 2011 Chaucer Pension Scheme
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation			$ 607.0	$ 570.8	$ 41.2	$ 39.2	$ 116.6	$ 104.4
Projected benefit obligation, beginning of period			570.8	548.0	39.2	39.1	104.4	99.7
Employee contributions							0.4	0.3
Service cost - benefits earned during the period							1.6	0.7
Interest cost			28.1	29.6	1.9	2.0	5.2	2.7
Actuarial losses			49.0	24.5	3.3	1.2	1.8	6.4
Benefits paid			(40.9)	(31.3)	(3.2)	(3.1)	(2.4)	(1.9)
Foreign currency translation							5.6	(3.5)
Projected benefit obligation, end of year			607.0	570.8	41.2	39.2	116.6	104.4
Fair value of plan assets, beginning of period			574.7	547.8			74.3	82.1
Actual return on plan assets			57.8	58.2			9.9	(4.6)
Company contribution	100.0	100.0			3.2	3.1	4.5	1.0
Employee contributions							0.4	0.3
Benefits paid			(40.9)	(31.3)	(3.2)	(3.1)	(2.4)	(1.9)
Foreign currency translation							4.2	(2.6)
Fair value of plan assets, end of year			591.6	574.7			90.9	74.3
Funded status of the plans			$ (15.4)	$ 3.9	$ (41.2)	$ (39.2)	$ (25.7)	$ (30.1)