Effect of Derivative Instruments on Statements of Income (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Deferred gain (loss) on settled rollover derivatives amount	$ 0.01	$ (0.90)	$ 0.61	$ 6.90