Common Stock Repurchase Programs (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended		3 Months Ended		9 Months Ended
	Aug. 31, 2011	Feb. 28, 2011	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Repurchase of common stock, authorized amount	$ 30	$ 30
Aggregate shares purchased			0		0		2.5
Aggregate purchase price							40
Available amount for repurchase of common stock canceled and replaced					20
New authorized purchase of additional common stock			100		100
Total of repurchased common stock shares					0.4
Aggregate purchase price					6.4
Restricted shares surrendered to satisfy tax withholding obligations			0.2	0.1	0.7	0.4
Restricted value of shares surrendered to satisfy tax withholding obligations			$ 2.6	$ 2.1	$ 12.0	$ 5.4