Investments - Investments in Short and Long-Term Classified as Available-for-Sale Securities (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	$ 1,033.3	$ 807.3
Available-for-sale Securities, Gross Unrealized Gains	0.4	0.5
Available-for-sale Securities, Gross Unrealized Losses	(0.2)	(0.1)
Available-for-sale Securities, Fair Value	1,033.5	807.7
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	454.1	457.6
Available-for-sale Securities, Gross Unrealized Gains	0.3	0.4
Available-for-sale Securities, Gross Unrealized Losses	(0.2)	(0.1)
Available-for-sale Securities, Fair Value	454.2	457.9
U.S. government and agency debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	258.2	211.1
Available-for-sale Securities, Gross Unrealized Gains	0.1	0.1
Available-for-sale Securities, Fair Value	258.3	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	203.5	68.3
Available-for-sale Securities, Fair Value	203.5	68.3
Certificates of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	109.4	67.2
Available-for-sale Securities, Fair Value	$ 109.4	$ 67.2