CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Unaudited) - USD ($) shares in Millions, $ in Millions	Total	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock	Class A Stock	Common Stock
Beginning Balance (in shares) at Dec. 31, 2017					(3.8)	1.9	109.5
Beginning Balance at Dec. 31, 2017	$ 6,144.1	$ 3,512.9	$ 2,946.7	$ 0.6	$ (316.2)	$ 0.0	$ 0.1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of Common Stock in connection with exercise of stock options (in shares)							0.1
Issuance of Common Stock in connection with exercise of stock options	13.6	13.6					$ 0.0
Issuance of Common Stock in connection with Company 401(k) Savings Plan contribution (in shares)							0.1
Issuance of Common Stock in connection with Company 401(k) Savings Plan	(0.7)	(0.7)					$ 0.0
Stock-based compensation charges	85.8	85.8
Net income	478.0		478.0
Other comprehensive loss, net of tax	(9.7)			(9.7)
Ending Balance (in shares) at Mar. 31, 2018					(3.8)	1.9	109.7
Ending Balance at Mar. 31, 2018	6,567.6	3,611.6	3,287.8	(15.7)	$ (316.2)	$ 0.0	$ 0.1
Beginning Balance (in shares) at Dec. 31, 2018					(4.0)	1.9	111.1
Beginning Balance at Dec. 31, 2018	8,757.3	3,911.6	5,254.3	(12.3)	$ (396.4)	$ 0.0	$ 0.1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of Common Stock in connection with exercise of stock options (in shares)							0.6
Issuance of Common Stock in connection with exercise of stock options	140.9	140.9					$ 0.0
Common Stock tendered upon exercise of stock options and vesting of restricted stock in connection with employee tax obligations	(10.7)	(10.7)
Issuance of Common Stock in connection with Company 401(k) Savings Plan contribution (in shares)					0.1
Issuance of Common Stock in connection with Company 401(k) Savings Plan	10.5	4.3			$ 6.2
Repurchases of Common Stock from Sanofi (in shares)					(0.1)
Repurchases of Common Stock from Sanofi	(54.0)				$ (54.0)
Stock-based compensation charges	114.8	114.8
Net income	461.1		461.1
Other comprehensive loss, net of tax	15.1			15.1
Ending Balance (in shares) at Mar. 31, 2019					(4.0)	1.9	111.7
Ending Balance at Mar. 31, 2019	$ 9,444.7	$ 4,160.9	$ 5,725.1	$ 2.8	$ (444.2)	$ 0.0	$ 0.1