Quarterly Financial Information (Unaudited) (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013
Quarterly Financial Information [Line Items]
Net tax benefit (expense)	$ (8.9)	$ 15.1	$ 40.1	$ (3.7)	$ 17.0	$ 11.2	$ 4.0