Investments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Schedule of Investments [Line Items]
Net unrealized gains on impaired securities	$ 23.9				$ 25.1
Overseas deposits	153.6		153.6		135.1
Other-than-temporary impairments	2.2	0.5	3.8	1.7
Net other-than-temporary impairment losses on securities recognized in earnings	1.6	0.8	3.5	2.2
Amount transferred from unrealized losses in accumulated other comprehensive income	0.6	0.3	0.3	0.5
Restricted assets, fixed maturities	404		404
Restricted assets, cash and cash equivalents	$ 125		$ 125