Balance Sheet Accounts and Supplemental Disclosures - Marketable Securities (Details) $ in Millions	Dec. 31, 2017 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 139.7
Gross Unrealized Gains	0.0
Gross Unrealized Losses	0.0
Estimated Fair Value	139.7
Cash and cash equivalents
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	119.7
Marketable securities
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	20.0
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	135.6
Gross Unrealized Gains	0.0
Gross Unrealized Losses	0.0
Estimated Fair Value	135.6
U.S. treasury notes
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4.1
Gross Unrealized Gains	0.0
Gross Unrealized Losses	0.0
Estimated Fair Value	$ 4.1

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=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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