Cash Equivalent and Investments (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2015	Dec. 31, 2014
Schedule Of Available For Sale Securities LineItems
Fair Value	$ 36.2	$ 48.0
Money Market [Member]
Schedule Of Available For Sale Securities LineItems
Cost	$ 137.9	$ 149.7
Gross Unrealized Gain
Fair Value	$ 137.9	$ 149.7
Certificates Of Deposit [Member]
Schedule Of Available For Sale Securities LineItems
Cost	$ 1,088.9	$ 842.5
Gross Unrealized Gain
Fair Value	$ 1,088.9	$ 842.5
Fixed Maturity and Mutual Funds [Member]
Schedule Of Available For Sale Securities LineItems
Cost	33.4	47.1
Gross Unrealized Gain	2.8	0.9
Fair Value	36.2	48.0
Cash and Cash Equivalents [Member] \| Money Market [Member]
Schedule Of Available For Sale Securities LineItems
Fair Value	137.9	149.7
Cash and Cash Equivalents [Member] \| Certificates Of Deposit [Member]
Schedule Of Available For Sale Securities LineItems
Fair Value	$ 596.0	$ 380.1
Cash and Cash Equivalents [Member] \| Fixed Maturity and Mutual Funds [Member]
Schedule Of Available For Sale Securities LineItems
Fair Value
Short Term Investments [Member] \| Money Market [Member]
Schedule Of Available For Sale Securities LineItems
Cost
Fair Value
Short Term Investments [Member] \| Certificates Of Deposit [Member]
Schedule Of Available For Sale Securities LineItems
Fair Value	$ 470.2	$ 458.1
Short Term Investments [Member] \| Fixed Maturity and Mutual Funds [Member]
Schedule Of Available For Sale Securities LineItems
Fair Value
Other Assets [Member] \| Money Market [Member]
Schedule Of Available For Sale Securities LineItems
Fair Value
Other Assets [Member] \| Certificates Of Deposit [Member]
Schedule Of Available For Sale Securities LineItems
Fair Value	$ 22.7	$ 4.3
Other Assets [Member] \| Fixed Maturity and Mutual Funds [Member]
Schedule Of Available For Sale Securities LineItems
Fair Value	$ 36.2	$ 48.0

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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=28360136&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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-Section 50

-Paragraph 5

-Subparagraph (aa)

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

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Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities.

+ References

Reference 1: 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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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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