Net Income Per Share - Schedule of Basic and Diluted Net Income Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net income - basic and diluted	$ 193.1	$ 461.1	$ 551.4	$ 478.0	$ 654.2	$ 1,029.4
Weighted average shares outstanding - basic (in shares)	109.2		107.8		109.1	107.7
Dilutive effect of stock options (in shares)	5.4		6.7		5.9	7.0
Weighted average shares - diluted (in shares)	114.6		114.5		115.0	114.7
Net income per share - basic (in dollars per share)	$ 1.77		$ 5.12		$ 6.00	$ 9.56
Net income per share - diluted (in dollars per share)	$ 1.68		$ 4.82		$ 5.69	$ 8.97