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
Issuance/distribution of Common Stock in connection with 401(k) Savings Plan (in shares)							0.1
Issuance/distribution of Common Stock in connection with 401(k) Savings Plan	(0.7)	(0.7)
Stock-based compensation charges	85.8	85.8
Net income	477.9		477.9
Other comprehensive gain (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	6,567.5	3,611.6	3,287.7	(15.7)	$ (316.2)	$ 0.0	$ 0.1
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]
Net income	1,624.0
Ending Balance (in shares) at Sep. 30, 2018					(4.0)	1.9	110.3
Ending Balance at Sep. 30, 2018	7,868.2	3,841.2	4,433.8	(10.5)	$ (396.4)	$ 0.0	$ 0.1
Beginning Balance (in shares) at Mar. 31, 2018					(3.8)	1.9	109.7
Beginning Balance at Mar. 31, 2018	6,567.5	3,611.6	3,287.7	(15.7)	$ (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)							0.4
Issuance of Common Stock in connection with exercise of stock options	19.8	19.8
Common Stock tendered upon exercise of stock options and vesting of restricted stock for employee tax obligations (in shares)							(0.2)
Common Stock tendered upon exercise of stock options and vesting of restricted stock for employee tax obligations	(31.9)	(31.9)
Repurchases of Common Stock from Sanofi (in shares)					(0.1)
Repurchases of Common Stock from Sanofi	(37.6)				$ (37.6)
Stock-based compensation charges	113.1	113.1
Net income	551.4		551.4
Other comprehensive gain (loss), net of tax	4.1			4.1
Ending Balance (in shares) at Jun. 30, 2018					(3.9)	1.9	109.9
Ending Balance at Jun. 30, 2018	7,186.4	3,712.6	3,839.1	(11.6)	$ (353.8)	$ 0.0	$ 0.1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of Common Stock in connection with exercise of stock options (in shares)							0.4
Issuance of Common Stock in connection with exercise of stock options	57.3	57.3
Common Stock tendered upon exercise of stock options and vesting of restricted stock for employee tax obligations	(45.2)	(45.2)
Repurchases of Common Stock from Sanofi (in shares)					(0.1)
Repurchases of Common Stock from Sanofi	(42.6)				$ (42.6)
Stock-based compensation charges	116.5	116.5
Net income	594.7		594.7
Other comprehensive gain (loss), net of tax	1.1			1.1
Ending Balance (in shares) at Sep. 30, 2018					(4.0)	1.9	110.3
Ending Balance at Sep. 30, 2018	7,868.2	3,841.2	4,433.8	(10.5)	$ (396.4)	$ 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
Common Stock tendered upon exercise of stock options and vesting of restricted stock for employee tax obligations	(10.7)	(10.7)
Issuance/distribution of Common Stock in connection with 401(k) Savings Plan (in shares)					0.1
Issuance/distribution of Common Stock in connection with 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 gain (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
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]
Net income	1,323.8
Ending Balance (in shares) at Sep. 30, 2019					(4.2)	1.8	112.1
Ending Balance at Sep. 30, 2019	10,504.4	4,388.4	6,587.8	16.5	$ (488.4)	$ 0.0	$ 0.1
Beginning Balance (in shares) at Mar. 31, 2019					(4.0)	1.9	111.7
Beginning Balance at Mar. 31, 2019	9,444.7	4,160.9	5,725.1	2.8	$ (444.2)	$ 0.0	$ 0.1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of Common Stock in connection with exercise of stock options (in shares)							0.3
Issuance of Common Stock in connection with exercise of stock options	13.9	13.9
Common Stock tendered upon exercise of stock options and vesting of restricted stock for employee tax obligations (in shares)							(0.1)
Common Stock tendered upon exercise of stock options and vesting of restricted stock for employee tax obligations	(29.7)	(29.7)
Issuance/distribution of Common Stock in connection with 401(k) Savings Plan	11.7	9.3			$ 2.4
Stock-based compensation charges	109.2	109.2
Net income	193.1		193.1
Other comprehensive gain (loss), net of tax	13.0			13.0
Ending Balance (in shares) at Jun. 30, 2019					(4.0)	1.9	111.9
Ending Balance at Jun. 30, 2019	9,755.9	4,263.6	5,918.2	15.8	$ (441.8)	$ 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	8.3	8.3
Common Stock tendered upon exercise of stock options and vesting of restricted stock for employee tax obligations	(0.1)	(0.1)
Issuance/distribution of Common Stock in connection with 401(k) Savings Plan	6.0	3.9			$ 2.1
Repurchases of Common Stock from Sanofi (in shares)					(0.2)
Repurchases of Common Stock from Sanofi	(48.7)				$ (48.7)
Conversion of Class A Stock to Common Stock (in shares)						0.1	0.1
Stock-based compensation charges	112.7	112.7
Net income	669.6		669.6
Other comprehensive gain (loss), net of tax	0.7			0.7
Ending Balance (in shares) at Sep. 30, 2019					(4.2)	1.8	112.1
Ending Balance at Sep. 30, 2019	$ 10,504.4	$ 4,388.4	$ 6,587.8	$ 16.5	$ (488.4)	$ 0.0	$ 0.1