Acquisitions and Discontinued Operations - Additional Information (Detail) In Millions, except Per Share data, unless otherwise specified	1 Months Ended	12 Months Ended				12 Months Ended
	Jun. 30, 2011 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2012 GBP (£)	Dec. 31, 2012 Citizens Management, Inc. USD ($)	Dec. 31, 2012 Discontinued Accident And Health Insurance Business USD ($)	Dec. 31, 2011 Discontinued Accident And Health Insurance Business USD ($)
Significant Acquisitions and Disposals [Line Items]
Per share amount received by shareholders, pence					£ 0.533
Business acquisition, purchase price, settlement loss			$ 11.3
Gain (loss) on foreign exchange			6.4
Proceeds from issuance of senior unsecured notes	300
Current exchange rate between GBP and US dollars			1.60	1.55
Assets of discontinued operations		130.5	121.2				72.9	70.4
Liabilities of discontinued operations		126.3	129.3				52.5	52.1
Gain on discontinued operations, after taxes		0.7	5.2	1.8		10.8
Contribution to charitable foundation that offset gain from disposal of discontinued operation		$ 1.7