Credit-Related Losses Recognized in Earnings on Debt Securities Other-Than-Temporarily Impaired (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2014	Sep. 30, 2013	Jun. 30, 2013
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Cumulative Credit-Related Losses on Securities Held - Beginning of Period	$ 12.7	$ 8.8	$ 42.3	$ 8.8
Plus: Losses on Newly Identified Impairments		1.8
Additional Losses on Previously Identified Impairments		2.1
Less: Current and Prior Period Losses on Securities Sold During the Period	(1.8)	(1.8)	(33.5)
Cumulative Credit-Related Losses on Securities Held - End of Period	$ 10.9	$ 10.9	$ 8.8	$ 8.8