Investments - Investments in Short and Long-Term Classified as Available-for-Sale Securities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	$ 700.2	$ 869.3
Available-for-sale Securities, Gross Unrealized Gains	0.4	0.1
Available-for-sale Securities, Gross Unrealized Losses	(0.2)	(0.5)
Available-for-sale Securities, Fair Value	700.4	868.9
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	494.2	516.9
Available-for-sale Securities, Gross Unrealized Gains	0.4	0.1
Available-for-sale Securities, Gross Unrealized Losses	(0.2)	(0.5)
Available-for-sale Securities, Fair Value	494.4	516.5
U.S. Government and Agency Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	133.7	194.3
Available-for-sale Securities, Fair Value	133.7	194.3
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	34.4	57.8
Available-for-sale Securities, Fair Value	34.4	57.8
Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	37.9	100.3
Available-for-sale Securities, Fair Value	$ 37.9	$ 100.3

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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-SubTopic 10

-Section 50

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Amount of debt securities categorized neither as held-to-maturity nor trading.

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