Cash, Cash Equivalents and Short-Term Investments - Schedule of Components (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 486.4	$ 551.3
Total cash equivalents	486.4	551.3
Short-term investments, amortized cost	594.2	606.3
Short-term investments, unrealized gain	3.0	1.5
Short-term investments, unrealized loss	(3.4)	(0.1)
Total short-term investments	593.8	607.7
Cash and cash equivalents, amortized cost	1,080.6	1,157.6
Cash and cash equivalents, estimated fair value	1,080.2	1,159.0
Corporate notes and obligations
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	289.0	285.5
Short-term investments, unrealized gain	0.6	1.2
Short-term investments, unrealized loss	(2.7)	(0.1)
Total short-term investments	286.9	286.6
U.S. Treasury securities
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	173.3	171.0
Short-term investments, unrealized gain	2.1	0.3
Short-term investments, unrealized loss	0.0	0.0
Total short-term investments	175.4	171.3
Asset-backed securities
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	59.1	53.8
Short-term investments, unrealized gain	0.1	0.0
Short-term investments, unrealized loss	(0.7)	0.0
Total short-term investments	58.5	53.8
Certificates of deposit
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	30.8	38.2
Short-term investments, unrealized gain	0.0	0.0
Short-term investments, unrealized loss	0.0	0.0
Total short-term investments	30.8	38.2
Commercial Paper
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	21.4	24.2
Short-term investments, unrealized gain	0.0	0.0
Short-term investments, unrealized loss	0.0	0.0
Total short-term investments	21.4	24.2
U.S. agency obligations
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	15.3	27.2
Short-term investments, unrealized gain	0.2	0.0
Short-term investments, unrealized loss	0.0	0.0
Total short-term investments	15.5	27.2
Supranational securities
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	2.8	4.0
Short-term investments, unrealized gain	0.0	0.0
Short-term investments, unrealized loss	0.0	0.0
Total short-term investments	2.8	4.0
Municipal securities
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	2.5	2.4
Short-term investments, unrealized gain	0.0	0.0
Short-term investments, unrealized loss	0.0	0.0
Total short-term investments	2.5	2.4
Cash
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	112.1	105.0
Total cash equivalents	112.1	105.0
Money market funds
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	373.0	444.3
Total cash equivalents	373.0	444.3
Commercial Paper
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	1.3	2.0
Total cash equivalents	$ 1.3	$ 2.0