Benefit Plans - Reconciliation of Level 3 Assets (Detail) - Other Investments - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of Level 3 Assets [Roll Forward]
Beginning balance	$ 6.7	$ 19.0
Net realized and unrealized gains (losses)	(0.3)	0.7
Net issuances and settlements	(2.3)	(11.8)
Net transfers into (out of) level 3	0.0	(1.2)
Ending balance	$ 4.1	$ 6.7