Cash, Cash Equivalents, and Marketable Debt Securities - Schedule of Company's Cash and Available-For-Sale (AFS) Debt Securities by Major Asset Category (Details) - USD ($) $ in Millions		6 Months Ended
		Jun. 30, 2020	Dec. 31, 2019
Financial Instruments [Line Items]
Adjusted Cost Basis		$ 975.5	$ 825.5
Unrealized Gain		1.0
Fair Value		976.5	825.5	[1]
Cash and cash equivalents		853.3	679.5
Short-term marketable debt securities		86.9	87.9
Long-term marketable debt securities		36.3	58.1
Level 1
Financial Instruments [Line Items]
Adjusted Cost Basis	[2]	56.8	27.2
Unrealized Gain	[2]	0.3
Fair Value	[2]	57.1	27.2	[1]
Cash and cash equivalents	[2]	29.0	3.2
Short-term marketable debt securities	[2]	17.3	11.5
Long-term marketable debt securities	[2]	10.8	12.5
Level 2
Financial Instruments [Line Items]
Adjusted Cost Basis	[3]	94.4	122.7
Unrealized Gain	[3]	0.7
Fair Value	[3]	95.1	122.7	[1]
Cash and cash equivalents	[3]		0.7
Short-term marketable debt securities	[3]	69.6	76.4
Long-term marketable debt securities	[3]	25.5	45.6
Cash and Cash Equivalents
Financial Instruments [Line Items]
Adjusted Cost Basis		824.3	675.6
Fair Value		824.3	675.6	[1]
Cash and cash equivalents		824.3	675.6
Money Market Fund | Level 1
Financial Instruments [Line Items]
Adjusted Cost Basis	[2]	29.0	0.1
Fair Value	[2]	29.0	0.1	[1]
Cash and cash equivalents	[2]	29.0	0.1
U.S. Treasury Securities | Level 1
Financial Instruments [Line Items]
Adjusted Cost Basis	[2]	27.8	27.1
Unrealized Gain	[2]	0.3
Fair Value	[2]	28.1	27.1	[1]
Cash and cash equivalents	[2]		3.1
Short-term marketable debt securities	[2]	17.3	11.5
Long-term marketable debt securities	[2]	10.8	12.5
Certificates of Deposit | Level 2
Financial Instruments [Line Items]
Adjusted Cost Basis	[3]	0.7	3.9
Fair Value	[3]	0.7	3.9	[1]
Short-term marketable debt securities	[3]	0.7	3.9
Commercial Paper | Level 2
Financial Instruments [Line Items]
Adjusted Cost Basis	[3]		5.6
Fair Value	[1],[3]		5.6
Cash and cash equivalents	[3]		0.7
Short-term marketable debt securities	[3]		4.9
U.S. Government Agency Securities | Level 2
Financial Instruments [Line Items]
Adjusted Cost Basis	[3]	5.6	7.0
Fair Value	[3]	5.6	7.0	[1]
Short-term marketable debt securities	[3]	3.0	3.0
Long-term marketable debt securities	[3]	2.6	4.0
Corporate Debt Securities | Level 2
Financial Instruments [Line Items]
Adjusted Cost Basis	[3]	88.1	106.2
Unrealized Gain	[3]	0.7
Fair Value	[3]	88.8	106.2	[1]
Short-term marketable debt securities	[3]	65.9	64.6
Long-term marketable debt securities	[3]	$ 22.9	$ 41.6

[1] Unrealized gains and losses were insignificant as of December 31, 2019. Therefore, the fair value approximates the adjusted cost basis for each major asset category.

[2] Valuations based on quoted prices for identical assets or liabilities in active markets.

[3] Valuations based on observable inputs other than quoted prices included in Level 1, such as quoted prices for similar assets and liabilities in active markets, quoted prices for identical or similar assets and liabilities in markets that are not active, or other inputs that are observable or can be corroborated by observable market data.