Stockholders' Equity - Narrative (Details) - USD ($)		1 Months Ended	3 Months Ended		9 Months Ended
	Dec. 31, 2020	May 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Jan. 31, 2021	Nov. 30, 2019
Class of Stock [Line Items]
Cost of treasury stock received			$ 190,500,000	$ 100,400,000	$ 802,700,000	$ 373,300,000
Number of shares repurchased (in shares)			300,000	200,000	1,600,000	900,000
Sanofi
Class of Stock [Line Items]
Cost of treasury stock received		$ 5,000,000,000
Number of shares repurchased (in shares)		9,806,805
Number of shares sold (in shares)		13,014,646
Common Stock, shares outstanding (in shares)		400,000
Sanofi | Libtayo development
Class of Stock [Line Items]
Cost of treasury stock received						$ 41,700,000
Number of shares repurchased (in shares)				0		77,677
Sanofi | Dupilumab/REGN3500 Eligible Investments
Class of Stock [Line Items]
Cost of treasury stock received						$ 93,300,000
Number of shares repurchased (in shares)						171,471
2019 Share Repurchase Program
Class of Stock [Line Items]
Share repurchase program, authorized amount								$ 1,000,000,000
Cost of treasury stock received	$ 1,000,000,000
January 2021 Share Repurchase Program
Class of Stock [Line Items]
Share repurchase program, authorized amount							$ 1,500,000,000
Remaining authorized repurchase amount			$ 697,300,000		$ 697,300,000