Collaboration and License Agreements - Schedule of Collaboration Revenue Earned From Sanofi (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Revenues	$ 1,927.8	$ 1,663.5	$ 1,608.0	$ 1,511.5	$ 1,582.4	$ 1,500.7	$ 1,470.1	$ 1,319.0	$ 6,710.8	$ 5,872.2	$ 4,860.4
Product and service, other | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues									559.0	553.7	440.2
Product and service, other | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Revenues									552.1	323.5	218.5
Product and service, other | Sanofi
Disaggregation of Revenue [Line Items]
Revenues									1,111.1	877.2	658.7
Reimbursement of Regeneron research and development expenses | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues									265.3	508.4	564.9
Reimbursement of Regeneron research and development expenses | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Revenues									311.8	240.0	138.5
Reimbursement of Regeneron commercialization-related expenses | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues									417.2	368.8	305.9
Reimbursement of Regeneron commercialization-related expenses | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Revenues									8.9	7.0	0.0
Regeneron's share of losses in connection with commercialization of antibodies | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues									(227.0)	(442.6)	(459.0)
Other | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues									103.5	119.1	28.4
Other | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Revenues									$ 231.4	$ 76.5	$ 80.0