Cash Equivalents and Investments - Unrealized Loss Position on Fixed Income Securities (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Unrealized Loss
Less than 12 Months, Fair Value	$ 45.8	$ 423.2
Less than 12 Months, Unrealized Loss	0.0	(0.1)
12 Months or Greater, Fair Value	281.9	500.8
12 Months or Greater, Unrealized Loss	(0.4)	(4.1)
Total, Fair Value	327.7	924.0
Total, Unrealized Loss	(0.4)	(4.2)
Asset-backed securities
Unrealized Loss
Less than 12 Months, Fair Value	1.2	3.1
Less than 12 Months, Unrealized Loss	0.0	0.0
12 Months or Greater, Fair Value	17.4	43.0
12 Months or Greater, Unrealized Loss	0.0	(0.3)
Total, Fair Value	18.6	46.1
Total, Unrealized Loss	0.0	(0.3)
Certificates of deposit
Unrealized Loss
Less than 12 Months, Fair Value	4.9
Less than 12 Months, Unrealized Loss	0.0
12 Months or Greater, Fair Value	0.0
12 Months or Greater, Unrealized Loss	0.0
Total, Fair Value	4.9
Total, Unrealized Loss	0.0
Corporate debt securities
Unrealized Loss
Less than 12 Months, Fair Value	26.7	72.6
Less than 12 Months, Unrealized Loss	0.0	(0.1)
12 Months or Greater, Fair Value	207.6	330.7
12 Months or Greater, Unrealized Loss	(0.3)	(3.0)
Total, Fair Value	234.3	403.3
Total, Unrealized Loss	(0.3)	(3.1)
Foreign government debt securities
Unrealized Loss
Less than 12 Months, Fair Value	0.0	1.5
Less than 12 Months, Unrealized Loss	0.0	0.0
12 Months or Greater, Fair Value	10.9	18.4
12 Months or Greater, Unrealized Loss	0.0	(0.1)
Total, Fair Value	10.9	19.9
Total, Unrealized Loss	0.0	(0.1)
U.S. government agency securities
Unrealized Loss
Less than 12 Months, Fair Value	13.0	2.0
Less than 12 Months, Unrealized Loss	0.0	0.0
12 Months or Greater, Fair Value	17.1	45.2
12 Months or Greater, Unrealized Loss	(0.1)	(0.2)
Total, Fair Value	30.1	47.2
Total, Unrealized Loss	(0.1)	(0.2)
U.S. government securities
Unrealized Loss
Less than 12 Months, Fair Value	0.0	344.0
Less than 12 Months, Unrealized Loss	0.0	0.0
12 Months or Greater, Fair Value	28.9	63.5
12 Months or Greater, Unrealized Loss	0.0	(0.5)
Total, Fair Value	28.9	407.5
Total, Unrealized Loss	$ 0.0	$ (0.5)