Fair Value Measurements (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	$ 452.7	$ 551.3
Total short-term investments	773.8	607.7
Corporate notes and obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	392.2	286.6
U.S. Treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	206.6	171.3
Asset backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	84.7	53.8
U.S. agency obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	37.1	27.2
Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	24.5	24.2
Certificates of deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	16.5	38.2
Foreign government obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	9.3
Municipal securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	2.9	2.4
Supranational securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments		4.0
Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	371.2	444.3
Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	6.0	2.0
Corporate notes and obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	2.9
Certificates of deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	1.0
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity investments	5.6
Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	381.1	446.3
Total short-term investments	773.8	607.7
Total cash equivalents and short-term investments	1,154.9
Equity investments		9.8
Total		1,063.8
Fair Value, Measurements, Recurring | Corporate notes and obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	392.2	286.6
Fair Value, Measurements, Recurring | U.S. Treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	206.6	171.3
Fair Value, Measurements, Recurring | Asset backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	84.7	53.8
Fair Value, Measurements, Recurring | U.S. agency obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	37.1	27.2
Fair Value, Measurements, Recurring | Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	24.5	24.2
Fair Value, Measurements, Recurring | Certificates of deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	16.5	38.2
Fair Value, Measurements, Recurring | Foreign government obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	9.3
Fair Value, Measurements, Recurring | Municipal securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	2.9	2.4
Fair Value, Measurements, Recurring | Supranational securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments		4.0
Fair Value, Measurements, Recurring | Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	371.2	444.3
Fair Value, Measurements, Recurring | Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	6.0	2.0
Fair Value, Measurements, Recurring | Corporate notes and obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	2.9
Fair Value, Measurements, Recurring | Certificates of deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	1.0
Fair Value, Measurements, Recurring | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	371.2	444.3
Total short-term investments	0.0	0.0
Total cash equivalents and short-term investments	371.2
Equity investments		9.8
Total		454.1
Fair Value, Measurements, Recurring | Level 1 | Corporate notes and obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	0.0	0.0
Fair Value, Measurements, Recurring | Level 1 | U.S. Treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	0.0	0.0
Fair Value, Measurements, Recurring | Level 1 | Asset backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	0.0	0.0
Fair Value, Measurements, Recurring | Level 1 | U.S. agency obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	0.0	0.0
Fair Value, Measurements, Recurring | Level 1 | Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	0.0	0.0
Fair Value, Measurements, Recurring | Level 1 | Certificates of deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	0.0	0.0
Fair Value, Measurements, Recurring | Level 1 | Foreign government obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	0.0
Fair Value, Measurements, Recurring | Level 1 | Municipal securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	0.0	0.0
Fair Value, Measurements, Recurring | Level 1 | Supranational securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments		0.0
Fair Value, Measurements, Recurring | Level 1 | Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	371.2	444.3
Fair Value, Measurements, Recurring | Level 1 | Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	0.0	0.0
Fair Value, Measurements, Recurring | Level 1 | Corporate notes and obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	0.0
Fair Value, Measurements, Recurring | Level 1 | Certificates of deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	0.0
Fair Value, Measurements, Recurring | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	9.9	2.0
Total short-term investments	773.8	607.7
Total cash equivalents and short-term investments	783.7
Equity investments		0.0
Total		609.7
Fair Value, Measurements, Recurring | Level 2 | Corporate notes and obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	392.2	286.6
Fair Value, Measurements, Recurring | Level 2 | U.S. Treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	206.6	171.3
Fair Value, Measurements, Recurring | Level 2 | Asset backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	84.7	53.8
Fair Value, Measurements, Recurring | Level 2 | U.S. agency obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	37.1	27.2
Fair Value, Measurements, Recurring | Level 2 | Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	24.5	24.2
Fair Value, Measurements, Recurring | Level 2 | Certificates of deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	16.5	38.2
Fair Value, Measurements, Recurring | Level 2 | Foreign government obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	9.3
Fair Value, Measurements, Recurring | Level 2 | Municipal securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	2.9	2.4
Fair Value, Measurements, Recurring | Level 2 | Supranational securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments		4.0
Fair Value, Measurements, Recurring | Level 2 | Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	0.0	0.0
Fair Value, Measurements, Recurring | Level 2 | Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	6.0	2.0
Fair Value, Measurements, Recurring | Level 2 | Corporate notes and obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	2.9
Fair Value, Measurements, Recurring | Level 2 | Certificates of deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	1.0
Fair Value, Measurements, Recurring | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	0.0	0.0
Total short-term investments	0.0	0.0
Total cash equivalents and short-term investments	0.0
Equity investments		0.0
Total		0.0
Fair Value, Measurements, Recurring | Level 3 | Corporate notes and obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	0.0	0.0
Fair Value, Measurements, Recurring | Level 3 | U.S. Treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	0.0	0.0
Fair Value, Measurements, Recurring | Level 3 | Asset backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	0.0	0.0
Fair Value, Measurements, Recurring | Level 3 | U.S. agency obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	0.0	0.0
Fair Value, Measurements, Recurring | Level 3 | Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	0.0	0.0
Fair Value, Measurements, Recurring | Level 3 | Certificates of deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	0.0	0.0
Fair Value, Measurements, Recurring | Level 3 | Foreign government obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	0.0
Fair Value, Measurements, Recurring | Level 3 | Municipal securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	0.0	0.0
Fair Value, Measurements, Recurring | Level 3 | Supranational securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments		0.0
Fair Value, Measurements, Recurring | Level 3 | Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	0.0	0.0
Fair Value, Measurements, Recurring | Level 3 | Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	0.0	$ 0.0
Fair Value, Measurements, Recurring | Level 3 | Corporate notes and obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	0.0
Fair Value, Measurements, Recurring | Level 3 | Certificates of deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	$ 0.0