Fair Value Measurements (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	$ 334.1	$ 551.3
Total short-term investments	783.5	607.7
Supranational securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments		4.0
Corporate notes and obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	361.4	286.6
US Treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	235.8	171.3
Asset-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	83.6	53.8
Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	40.0	24.2
US Agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	27.9	27.2
Certificates of deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	26.0	38.2
Municipal securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	6.0	2.4
Foreign government obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	2.8
Corporate notes and obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	1.1
Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	241.5	444.3
Municipal securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	1.8
Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	1.2	2.0
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 short-term investments	783.5	607.7
Total cash equivalents and short-term investments	1,029.1
Equity investments		9.8
Total		1,063.8
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 | Corporate notes and obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	361.4	286.6
Fair Value, Measurements, Recurring | US Treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	235.8	171.3
Fair Value, Measurements, Recurring | Asset-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	83.6	53.8
Fair Value, Measurements, Recurring | Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	40.0	24.2
Fair Value, Measurements, Recurring | US Agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	27.9	27.2
Fair Value, Measurements, Recurring | Certificates of deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	26.0	38.2
Fair Value, Measurements, Recurring | Municipal securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	6.0	2.4
Fair Value, Measurements, Recurring | Foreign government obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	2.8
Fair Value, Measurements, Recurring | Corporate notes and obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	1.1
Fair Value, Measurements, Recurring | Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	241.5	444.3
Fair Value, Measurements, Recurring | Municipal securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	1.8
Fair Value, Measurements, Recurring | Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	1.2	2.0
Fair Value, Measurements, Recurring | Cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	245.6	446.3
Fair Value, Measurements, Recurring | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	0.0	0.0
Total cash equivalents and short-term investments	241.5
Equity investments		9.8
Total		454.1
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 | 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 | US 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 | 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 | US Agency 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 | 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 | 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 | 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 | 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 | Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	241.5	444.3
Fair Value, Measurements, Recurring | Level 1 | Municipal securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	0.0
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 | Cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	241.5	444.3
Fair Value, Measurements, Recurring | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	783.5	607.7
Total cash equivalents and short-term investments	787.6
Equity investments		0.0
Total		609.7
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 | Corporate notes and obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	361.4	286.6
Fair Value, Measurements, Recurring | Level 2 | US Treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	235.8	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	83.6	53.8
Fair Value, Measurements, Recurring | Level 2 | Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	40.0	24.2
Fair Value, Measurements, Recurring | Level 2 | US Agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	27.9	27.2
Fair Value, Measurements, Recurring | Level 2 | Certificates of deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	26.0	38.2
Fair Value, Measurements, Recurring | Level 2 | Municipal securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	6.0	2.4
Fair Value, Measurements, Recurring | Level 2 | Foreign government obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	2.8
Fair Value, Measurements, Recurring | Level 2 | Corporate notes and obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	1.1
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 | Municipal securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	1.8
Fair Value, Measurements, Recurring | Level 2 | Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	1.2	2.0
Fair Value, Measurements, Recurring | Level 2 | Cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	4.1	2.0
Fair Value, Measurements, Recurring | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
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 | Supranational securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments		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 | US 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 | 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 | US Agency 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 | 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 | 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 | 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 | 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 | 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 | Municipal securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	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 | Cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	$ 0.0	$ 0.0