Retirement Plans - Reconciliation of Beginning and Ending Balances for Level 3 Assets of Plan Measured at Fair Value (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year			$ 227.4
Fair value of plan assets at end of year	217.2	254.9	227.4
Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year	96.1	91.1
Settlements	0	0
Unrealized gains	5.0	5.0
Fair value of plan assets at end of year	$ 101.1	$ 96.1