Stockholders' Equity - Sanofi Funding (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended				6 Months Ended
	May 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 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	$ 5,071.8	$ 336.0		$ 54.0
Libtayo development
Equity, Class of Treasury Stock [Line Items]
Treasury stock, shares acquired (in shares)		34,050		0		77,677	106,972
Total cost of shares received		$ 20.3		$ 0.0		$ 41.7	$ 44.0
Dupilumab/REGN3500 Eligible Investments
Equity, Class of Treasury Stock [Line Items]
Treasury stock, shares acquired (in shares)		86,184		0		171,471	24,143
Total cost of shares received		$ 51.5		$ 0.0		$ 93.3	$ 10.0