Investments - Investments in Short and Long-Term Classified as Available-for-Sale Securities (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	$ 896.6	$ 807.3
Available-for-sale Securities, Gross Unrealized Gains	0.3	0.5
Available-for-sale Securities, Gross Unrealized Losses	(0.1)	(0.1)
Available-for-sale Securities, Fair Value	896.8	807.7
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	323.6	457.6
Available-for-sale Securities, Gross Unrealized Gains	0.3	0.4
Available-for-sale Securities, Gross Unrealized Losses	(0.1)	(0.1)
Available-for-sale Securities, Fair Value	323.8	457.9
U.S. government and agency debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	185.1	211.1
Available-for-sale Securities, Gross Unrealized Gains		0.1
Available-for-sale Securities, Fair Value	185.1	211.2
Foreign government debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	8.1	3.1
Available-for-sale Securities, Fair Value	8.1	3.1
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	314.6	68.3
Available-for-sale Securities, Fair Value	314.6	68.3
Certificates of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	65.2	67.2
Available-for-sale Securities, Fair Value	$ 65.2	$ 67.2