Acquisitions (Summary of Total Consideration) (Details) - USD ($) $ in Millions			3 Months Ended		9 Months Ended
	Jul. 02, 2020	Jan. 28, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Consideration
Upfront payment, fair value of equity compensation attributable to the post-combination service period			$ (11.1)	$ 0.0	$ (36.8)	$ 0.0
Contingent consideration			398.1		398.1		$ 192.4
Assets Acquired and Liabilities Assumed
Goodwill			5,100.7		5,100.7		$ 5,037.4
Portola Pharmaceuticals
Consideration
Fair value of equity shares held by Alexion at closing	$ 47.8
Achillion
Consideration
Upfront payment to shareholders and equity holders		$ 926.2
Upfront payment, fair value of equity compensation attributable to the post-combination service period		(20.0)			(25.7)
Upfront cash paid, net		906.2
Contingent consideration		160.7	209.4		209.4
Contingent consideration, fair value of equity compensation attributable to the post-combination service period		(5.7)
Total consideration		1,061.2
Assets Acquired and Liabilities Assumed
Cash and cash equivalents		68.5
Marketable securities		106.1
Deferred tax assets, net		142.4
Goodwill		37.8
Deferred tax liabilities, net		(62.9)
Other assets and liabilities, net		(6.3)
Total net assets acquired		1,061.2
Portola Pharmaceuticals, Inc.
Consideration
Upfront payment to shareholders and equity holders	1,380.8
Upfront payment, fair value of equity compensation attributable to the post-combination service period	(11.1)
Upfront cash paid, net	1,369.7
Fair value of replacement equity awards attributable to the pre-combination period	7.2
Total consideration to acquire outstanding equity, net	1,424.7
Repayments of debt	196.9
Total consideration	1,621.6
Assets Acquired and Liabilities Assumed
Cash and cash equivalents	288.5
Marketable securities	17.8
Inventories, including noncurrent portion of $169.1 and validation batches of $60.9	362.5
Noncurrent inventory	169.1
Validation batches	60.9
Intangible assets	1,051.0
Deferred tax assets, net	116.0
Other assets	41.9
Goodwill	25.5
Accounts payable and accrued expenses	(75.6)
Long-term debt, including current portion of $7.7	(182.0)
Current debt	7.7
Other liabilities	(24.0)
Total net assets acquired	$ 1,621.6
In-process research & development assets (IPR&D) | Achillion
Assets Acquired and Liabilities Assumed
In-process research & development assets (IPR&D)		918.0
Deferred tax liabilities, net		$ (205.3)
In-process research & development assets (IPR&D) | Portola Pharmaceuticals, Inc.
Assets Acquired and Liabilities Assumed
In-process research & development assets (IPR&D)			$ 15.0		$ 15.0