Cash, Cash Equivalents and Short-Term Investments - Schedule of Components (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 452.7	$ 551.3
Total cash equivalents	452.7	551.3
Short-term investments, amortized cost	768.8	606.3
Short-term investments, unrealized gain	5.1	1.5
Short-term investments, unrealized loss	(0.1)	(0.1)
Short-term investments, estimated fair value	773.8	607.7
Total cash, cash equivalents, and short term investments, before unrealized gains (losses) on investments	1,221.5	1,157.6
Total cash, cash equivalents, and short term investments	1,226.5	1,159.0
Corporate notes and obligations
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	389.3	285.5
Short-term investments, unrealized gain	3.0	1.2
Short-term investments, unrealized loss	(0.1)	(0.1)
Short-term investments, estimated fair value	392.2	286.6
U.S. Treasury securities
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	205.3	171.0
Short-term investments, unrealized gain	1.3	0.3
Short-term investments, unrealized loss	0.0	0.0
Short-term investments, estimated fair value	206.6	171.3
Asset backed securities
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	84.0	53.8
Short-term investments, unrealized gain	0.7	0.0
Short-term investments, unrealized loss	0.0	0.0
Short-term investments, estimated fair value	84.7	53.8
U.S. agency obligations
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	37.0	27.2
Short-term investments, unrealized gain	0.1	0.0
Short-term investments, unrealized loss	0.0	0.0
Short-term investments, estimated fair value	37.1	27.2
Commercial paper
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	24.5	24.2
Short-term investments, unrealized gain	0.0	0.0
Short-term investments, unrealized loss	0.0	0.0
Short-term investments, estimated fair value	24.5	24.2
Certificates of deposit
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	16.5	38.2
Short-term investments, unrealized gain	0.0	0.0
Short-term investments, unrealized loss	0.0	0.0
Short-term investments, estimated fair value	16.5	38.2
Foreign government obligations
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	9.3
Short-term investments, unrealized gain	0.0
Short-term investments, unrealized loss	0.0
Short-term investments, estimated fair value	9.3
Municipal securities
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	2.9	2.4
Short-term investments, unrealized gain	0.0	0.0
Short-term investments, unrealized loss	0.0	0.0
Short-term investments, estimated fair value	2.9	2.4
Supranational securities
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost		4.0
Short-term investments, unrealized gain		0.0
Short-term investments, unrealized loss		0.0
Short-term investments, estimated fair value		4.0
Cash
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	71.6	105.0
Total cash equivalents	71.6	105.0
Money market funds
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	371.2	444.3
Total cash equivalents	371.2	444.3
Commercial paper
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	6.0	2.0
Total cash equivalents	6.0	$ 2.0
Corporate notes and obligations
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	2.9
Total cash equivalents	2.9
Certificates of deposit
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	1.0
Total cash equivalents	$ 1.0