Component Of Defered Income Tax Assets And Liabilities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Schedule of Deferred Income Tax Assets and Liabilities [Line Items]
Loss, LAE and unearned premium reserves, net	$ 190.6	$ 178.9
Tax credit carryforwards	126.8	112.2
Operating loss carryforwards	75.1	79.8
Employee benefit plans	56.4	45.6
Deferred Lloyd's underwriting losses	13.5	18.6
Investments, net		9.3
Other	60.7	63.0
Deferred tax assets, gross	523.1	507.4
Less: Valuation allowance	35.9	35.9	91.5	195.6
Deferred tax assets, total	523.1	471.5
Deferred policy acquisition costs	115.8	109.7
Investments, net	43.3
Software capitalization	31.4	29.9
Deferred foreign sourced income	8.0
Other	57.0	57.9
Deferred tax liabilities, total	255.5	197.5
Net deferred tax asset	$ 267.6	$ 274.0