Cash Equivalents and Investments - Unrealized Loss on Available-for-Sale Securities (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Fixed Income Securities
Fair Value
Less than 12 Months	$ 559.6	$ 1,238.3
12 Months or Greater	186.3	273.3
Total	745.9	1,511.6
Unrealized Loss
Less than 12 Months	(4.9)	(3.6)
12 Months or Greater	(1.7)	(1.8)
Total	(6.6)	(5.4)
Asset-backed securities
Fair Value
Less than 12 Months	65.8	215.2
12 Months or Greater	25.3	38.4
Total	91.1	253.6
Unrealized Loss
Less than 12 Months	(0.4)	(0.4)
12 Months or Greater	(0.2)	(0.2)
Total	(0.6)	(0.6)
Corporate debt securities
Fair Value
Less than 12 Months	341.4	646.7
12 Months or Greater	83.9	108.6
Total	425.3	755.3
Unrealized Loss
Less than 12 Months	(3.7)	(2.1)
12 Months or Greater	(0.9)	(0.9)
Total	(4.6)	(3.0)
Foreign government debt securities
Fair Value
Less than 12 Months	34.5	47.3
12 Months or Greater	2.0	6.6
Total	36.5	53.9
Unrealized Loss
Less than 12 Months	(0.2)	(0.2)
12 Months or Greater	0.0	0.0
Total	(0.2)	(0.2)
U.S. government agency securities
Fair Value
Less than 12 Months	30.4	68.3
12 Months or Greater	31.6	67.9
Total	62.0	136.2
Unrealized Loss
Less than 12 Months	(0.1)	(0.2)
12 Months or Greater	(0.3)	(0.5)
Total	(0.4)	(0.7)
U.S. government securities
Fair Value
Less than 12 Months	87.5	260.8
12 Months or Greater	43.5	51.8
Total	131.0	312.6
Unrealized Loss
Less than 12 Months	(0.5)	(0.7)
12 Months or Greater	(0.3)	(0.2)
Total	$ (0.8)	$ (0.9)