Investments - Investments in Short and Long-Term Classified as Available-for-Sale Securities (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	$ 466.4	$ 869.3
Available-for-sale Securities, Gross Unrealized Gains	0.3	0.1
Available-for-sale Securities, Gross Unrealized Losses	(0.2)	(0.5)
Available-for-sale Securities, Fair Value	466.5	868.9
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	372.1	516.9
Available-for-sale Securities, Gross Unrealized Gains	0.3	0.1
Available-for-sale Securities, Gross Unrealized Losses	(0.2)	(0.5)
Available-for-sale Securities, Fair Value	372.2	516.5
U.S. Government and Agency Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	66.9	194.3
Available-for-sale Securities, Fair Value	66.9	194.3
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	11.5	57.8
Available-for-sale Securities, Fair Value	11.5	57.8
Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	15.9	100.3
Available-for-sale Securities, Fair Value	$ 15.9	$ 100.3

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

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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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- Definition

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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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