Credit-Related Losses Recognized in Earnings on Debt Securities Other-Than-Temporarily Impaired (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Cumulative Credit-Related Losses on Securities Held - Beginning of Period	$ 5.2	$ 12.7	$ 5.2	$ 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	0.0	0.0	0.0	0.0
Cumulative Credit-Related Losses on Securities Held - End of Period	$ 5.2	$ 12.7	$ 5.2	$ 12.7