Credit-Related Losses Recognized in Earnings on Debt Securities Other-Than-Temporarily Impaired (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Cumulative Credit-Related Losses on Securities Held – Beginning of Year	$ 5.2	$ 8.8
Plus: Losses on Newly Identified Impairments	0.0	1.8
Additional Losses on Previously Identified Impairments	0.0	2.4
Less: Current and Prior Period Losses on Securities Sold During the Year	0.0	(7.8)
Cumulative Credit-Related Losses on Securities Held – End of Year	$ 5.2	$ 5.2