Investments - Contractual Maturities (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Amortized Cost
Due in less than one year, Amortized Cost	$ 27.8
Due in one to two years, Amortized Cost	2.3
Total marketable investments	30.1	$ 160.0
Fair Value
Due in less than one year, Fair Value	27.8
Due in one to two years, Fair Value	2.3
Fair Value	$ 30.1	$ 159.8

Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

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Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Fair value of held-to-maturity debt securities maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 320
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-Paragraph 3
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Carrying amount of debt securities accounted for as held-to-maturity, net of adjustments including, but not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized, and fair value hedge accounting adjustments, maturing in the next fiscal year following the latest fiscal year.

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Fair value of investments in debt securities classified as held-to-maturity.

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This item represents the fair value of debt securities which are expected to mature after one year and through two years from the balance sheet date and which are categorized as held-to-maturity.

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