Acquisitions (Summary of Total Consideration) (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
	Jan. 28, 2020	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Consideration
Upfront payment, fair value of equity compensation attributable to the post-combination service period		$ 0.0	$ 0.0	$ (25.7)	$ 0.0
Contingent consideration		374.7		374.7		$ 192.4
Assets Acquired and Liabilities Assumed
Goodwill		5,075.2		5,075.2		$ 5,037.4
Achillion
Consideration
Upfront payment to shareholders and option 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	$ 190.1		$ 190.1
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
Goodwill	37.8
Deferred tax liabilities, net	(62.9)
Other assets and liabilities, net	(6.3)
Total net assets acquired	1,061.2
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)