Benefit Plans - Reconciliation of Level 3 Assets (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Reconciliation of Level 3 Assets [Roll Forward]
Beginning balance	$ 18.3	$ 20.0
Net realized and unrealized gains (losses)	1.9	1.1
Net issuances and settlements	(1.2)	(2.8)
Net transfers into (out of) level 3	0	0
Ending balance	19.0	18.3
Other Investments
Reconciliation of Level 3 Assets [Roll Forward]
Beginning balance	18.3	19.0
Net realized and unrealized gains (losses)	1.9	1.1
Net issuances and settlements	(1.2)	(1.8)
Net transfers into (out of) level 3	0	0
Ending balance	19.0	18.3
Fixed Income Investments
Reconciliation of Level 3 Assets [Roll Forward]
Beginning balance		1.0
Net realized and unrealized gains (losses)		0
Net issuances and settlements		(1.0)
Net transfers into (out of) level 3		0
Ending balance		$ 0