Investments - Investments in Short and Long-Term Classified as Available-for-Sale Securities (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	$ 855.8	$ 915.3
Available-for-sale Securities, Gross Unrealized Gains	0.5	0.5
Available-for-sale Securities, Gross Unrealized Losses	(0.3)	(0.1)
Available-for-sale Securities, Fair Value	856.0	915.7
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	443.5	383.6
Available-for-sale Securities, Gross Unrealized Gains	0.3	0.3
Available-for-sale Securities, Gross Unrealized Losses	(0.3)	(0.1)
Available-for-sale Securities, Fair Value	443.5	383.8
U.S. government and agency debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	252.9	295.8
Available-for-sale Securities, Gross Unrealized Gains	0.2	0.1
Available-for-sale Securities, Fair Value	253.1	295.9
Foreign government debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	3.1	5.0
Available-for-sale Securities, Fair Value	3.1	5.0
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	89.0	138.7
Available-for-sale Securities, Fair Value	89.0	138.7
Certificates of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	67.3	92.2
Available-for-sale Securities, Gross Unrealized Gains		0.1
Available-for-sale Securities, Fair Value	$ 67.3	$ 92.3