Stockholders' Equity - Sanofi Funding (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended					9 Months Ended
	May 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Equity, Class of Treasury Stock [Line Items]
Treasury stock, shares acquired (in shares)	9,806,805
Total cost of shares received	$ 5,000.0	$ 100.4	$ 5,071.8	$ 336.0	$ 48.7	$ 54.0
Libtayo development
Equity, Class of Treasury Stock [Line Items]
Treasury stock, shares acquired (in shares)		0			103,761		77,677	210,733
Total cost of shares received		$ 0.0			$ 29.2		$ 41.7	$ 73.3
Dupilumab/REGN3500 Eligible Investments
Equity, Class of Treasury Stock [Line Items]
Treasury stock, shares acquired (in shares)		0			69,143		171,471	93,286
Total cost of shares received		$ 0.0			$ 19.4		$ 93.3	$ 29.4