Acquisitions (Narrative) (Details) - USD ($)			3 Months Ended		6 Months Ended
	Jul. 02, 2020	Jan. 28, 2020	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Schedule Of Asset Acquisition, By Acquisition [Line Items]
Contingent consideration			$ 374,700,000		$ 374,700,000		$ 192,400,000
Change in fair value of contingent consideration			15,800,000	$ 6,100,000	21,600,000	$ (22,600,000)
Upfront payment, fair value of equity compensation attributable to the post-combination service period			0	0	$ (25,700,000)	0
Fair Value Inputs, Weighted Average Cost of Capital					11.50%
Achillion
Schedule Of Asset Acquisition, By Acquisition [Line Items]
Total acquisition value, per share (in dollars per share)		$ 6.30
Upfront payment to shareholders and option holders		$ 926,200,000
Goodwill adjustments			3,100,000
Contingent consideration		160,700,000	190,100,000		$ 190,100,000
Contingent consideration, post-combination service period compensation		5,700,000
Change in fair value of contingent consideration			27,600,000		29,300,000
Upfront payment, fair value of equity compensation attributable to the post-combination service period		(20,000,000.0)			(25,700,000)
Net deferred tax liability		(62,900,000)
Deferred tax assets		$ 142,400,000
Operating loss since acquisition			12,900,000		$ 26,800,000
Acquisition costs not expensed				$ (100,000)		$ 53,200,000
Portola Pharmaceuticals, Inc. | Subsequent Event
Schedule Of Asset Acquisition, By Acquisition [Line Items]
Total acquisition value, per share (in dollars per share)	$ 18.00
Upfront payment to shareholders and option holders	$ 1,380,100,000
Repayments of debt	196,900,000
Assumed royalty payment debt	$ 290,000,000.0
Danicopan | Achillion
Schedule Of Asset Acquisition, By Acquisition [Line Items]
Contingent value rights per share (in dollars per share)		$ 1.00
ACH-5528, Phase 3 | Achillion
Schedule Of Asset Acquisition, By Acquisition [Line Items]
Contingent value rights per share (in dollars per share)		$ 1.00
Minimum | Achillion
Schedule Of Asset Acquisition, By Acquisition [Line Items]
Estimated milestone payments		$ 0
Maximum | Achillion
Schedule Of Asset Acquisition, By Acquisition [Line Items]
Estimated milestone payments		$ 306,300,000
Acquisition-related contingent consideration | Minimum | Level 3
Schedule Of Asset Acquisition, By Acquisition [Line Items]
Fair Value Inputs, Cost of Debt		2.10%			0.40%
Fair Value Inputs, Weighted Average Cost of Capital					9.00%
Acquisition-related contingent consideration | Maximum | Level 3
Schedule Of Asset Acquisition, By Acquisition [Line Items]
Fair Value Inputs, Cost of Debt		2.30%			0.70%
In-process research & development assets (IPR&D)
Schedule Of Asset Acquisition, By Acquisition [Line Items]
Impairment			$ 11,000,000.0
In-process research & development assets (IPR&D) | Achillion
Schedule Of Asset Acquisition, By Acquisition [Line Items]
In-process research & development assets (IPR&D)		$ 918,000,000.0
Net deferred tax liability		$ (205,300,000)