Unaudited Quarterly Results - Schedule of unaudited quarterly results (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 2,422.9	$ 2,294.0	$ 1,952.0	$ 1,828.2	$ 1,863.5	$ 1,743.7	$ 1,577.8	$ 1,372.6	$ 8,497.1	$ 6,557.6	$ 5,145.6
Operating expenses	1,255.9	1,240.9	1,295.6	1,128.1	1,187.8	1,005.2	1,262.2	892.6	4,920.5	4,347.8	2,611.2
Net income	$ 1,149.2	$ 842.1	$ 897.3	$ 624.6	$ 792.0	$ 669.6	$ 193.1	$ 461.1	$ 3,513.2	$ 2,115.8	$ 2,444.4
Net income per share - basic (in dollars per share)	$ 10.90	$ 7.98	$ 8.19	$ 5.69	$ 7.25	$ 6.12	$ 1.77	$ 4.23	$ 32.65	$ 19.38	$ 22.65
Net income per share - diluted (in dollars per shares)	$ 10.24	$ 7.39	$ 7.61	$ 5.43	$ 6.93	$ 5.86	$ 1.68	$ 3.99	$ 30.52	$ 18.46	$ 21.29
Amgen v.s. Regeneron
Disaggregation of Revenue [Line Items]
Reversal of accruals for litigation-related contingencies	$ 95.0
Intellia Collaboration Agreement
Disaggregation of Revenue [Line Items]
Upfront payment made			$ 85.0
Sanofi IO, Teva and MTPC Collaboration Agreements
Disaggregation of Revenue [Line Items]
Cumulative catch-up adjustment to revenue, modification of contract	$ 99.8
Alnylam Pharmaceuticals, Inc. Collaboration Agreement
Disaggregation of Revenue [Line Items]
Upfront payment made							$ 400.0			$ 400.0