Stockholders' Equity - Narrative (Details) - USD ($)		3 Months Ended
	Dec. 31, 2020	Mar. 31, 2021	Mar. 31, 2020	Jan. 31, 2021
Sanofi | Libtayo development
Class of Stock [Line Items]
Number of shares repurchased (in shares)		43,627
Cost of treasury stock received		$ 21,400,000
Sanofi | Dupilumab/REGN3500 Eligible Investments
Class of Stock [Line Items]
Number of shares repurchased (in shares)		85,287
Cost of treasury stock received		$ 41,800,000
2019 Share Repurchase Program
Class of Stock [Line Items]
Share repurchase program, authorized amount	$ 1,000,000,000.0
Number of shares repurchased (in shares)			719,167
Cost of treasury stock received	$ 1,000,000,000.0		$ 272,800,000
2021 Share Repurchase Program
Class of Stock [Line Items]
Share repurchase program, authorized amount				$ 1,500,000,000
Number of shares repurchased (in shares)		690,265
Cost of treasury stock received		$ 323,500,000
Remaining authorized repurchase amount		$ 1,177,000,000