Investments - Investments in Short and Long-Term Classified as Available-for-Sale Securities (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	$ 273.5	$ 869.3
Available-for-sale Securities, Gross Unrealized Gains	0.1	0.1
Available-for-sale Securities, Gross Unrealized Losses	(0.5)	(0.5)
Available-for-sale Securities, Fair Value	273.1	868.9
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	245.7	516.9
Available-for-sale Securities, Gross Unrealized Gains	0.1	0.1
Available-for-sale Securities, Gross Unrealized Losses	(0.4)	(0.5)
Available-for-sale Securities, Fair Value	245.4	516.5
U.S. Government and Agency Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	21.6	194.3
Available-for-sale Securities, Gross Unrealized Gains		0.0
Available-for-sale Securities, Gross Unrealized Losses	(0.1)	0.0
Available-for-sale Securities, Fair Value	21.5	194.3
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	4.2	57.8
Available-for-sale Securities, Gross Unrealized Gains		0.0
Available-for-sale Securities, Gross Unrealized Losses		0.0
Available-for-sale Securities, Fair Value	4.2	57.8
Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	2.0	100.3
Available-for-sale Securities, Gross Unrealized Gains		0.0
Available-for-sale Securities, Gross Unrealized Losses		0.0
Available-for-sale Securities, Fair Value	$ 2.0	$ 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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- Definition

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

+ References

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=51813488&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49167202&loc=d3e26610-111562

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
-Paragraph 2
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-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 320
-Section 50
-Paragraph 2
-Subparagraph (b)-(g)
-URI http://asc.fasb.org/extlink&oid=6957658&loc=d3e62557-112803

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