Income Taxes - Deferred Taxes (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Net operating loss carry-forwards	$ 48.2	$ 27.7
Research and other credit carry-forwards	252.8	236.7
Deferred revenue	43.4	40.0
Share-based compensation	15.0	24.3
Capitalized R&D expenditure	60.5	0.0
Reserves and accruals not currently deductible	43.5	55.8
Operating lease liabilities	51.3	48.3
Other	10.6	12.0
Total deferred tax assets	525.3	444.8
Valuation allowance	(261.5)	(249.4)
Deferred tax assets, net of valuation allowance	263.8	195.4
Deferred tax liabilities:
Property and equipment basis differences	(20.1)	(39.2)
Purchased intangible assets	(45.6)	(27.8)
Unremitted foreign earnings	(25.5)	(23.7)
Net unrealized gain	(21.1)	(8.7)
Operating lease assets	(44.9)	(41.1)
Total deferred tax liabilities	(157.2)	(140.5)
Net deferred tax assets	$ 106.6	$ 54.9