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Cash, Cash Equivalents and Short-Term Investments - Schedule of Cash, Cash Equivalents and Short-Term Investments (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 519.3	$ 430.0	$ 352.7	$ 356.9
Total cash equivalents	519.3
Short-term investments, amortized cost	570.5
Short-term investments, unrealized gain	0.1
Short-term investments, unrealized loss	(0.6)
Short-term investments, estimated fair value	570.0
Total cash, cash equivalents, and short term investments, before unrealized gains (losses) on investments	1,089.8
Total cash, cash equivalents, and short term investments	1,089.3
Corporate notes and obligations
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	269.6
Short-term investments, unrealized gain	0.1
Short-term investments, unrealized loss	(0.5)
Short-term investments, estimated fair value	269.2
U.S. Treasury securities
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	176.0
Short-term investments, unrealized gain	0.0
Short-term investments, unrealized loss	(0.1)
Short-term investments, estimated fair value	175.9
Certificates of deposit
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	70.6
Short-term investments, unrealized gain	0.0
Short-term investments, unrealized loss	0.0
Short-term investments, estimated fair value	70.6
U.S. agency obligations
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	37.1
Short-term investments, unrealized gain	0.0
Short-term investments, unrealized loss	0.0
Short-term investments, estimated fair value	37.1
Commercial paper
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	17.2
Short-term investments, unrealized gain	0.0
Short-term investments, unrealized loss	0.0
Short-term investments, estimated fair value	17.2
Cash
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	103.0	62.9
Total cash equivalents	103.0
Money market funds
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	355.5	$ 367.1
Total cash equivalents	355.5
Commercial paper
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	27.4
Total cash equivalents	27.4
U.S. Treasury securities
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	33.4
Total cash equivalents	$ 33.4

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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