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								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net income - basic	$ 820.4	$ 594.7	$ 551.4	$ 478.0	$ 173.5	$ 388.3	$ 387.7	$ 248.9	$ 2,444.4	$ 1,198.5	$ 895.5
Convertible senior notes - interest expense and amortization of discount and note issuance costs									0.0	0.0	0.4
Net income - diluted									$ 2,444.4	$ 1,198.5	$ 895.9
Weighted average shares outstanding - basic (in shares)									107.9	106.3	104.7
Effect of dilutive securities:
Warrants (in shares)									0.0	0.0	0.9
Dilutive potential shares (in shares)									6.9	9.6	11.6
Weighted average shares - diluted (in shares)									114.8	115.9	116.3
Net income per share - basic (in dollars per share)	$ 7.58	$ 5.50	$ 5.12	$ 4.44	$ 1.62	$ 3.64	$ 3.66	$ 2.36	$ 22.65	$ 11.27	$ 8.55
Net income per share - diluted (in dollars per share)	$ 7.15	$ 5.17	$ 4.82	$ 4.16	$ 1.50	$ 3.32	$ 3.34	$ 2.16	$ 21.29	$ 10.34	$ 7.70
Stock options
Effect of dilutive securities:
Effect of dilutive securities (in shares)									6.9	9.1	10.2
Restricted stock
Effect of dilutive securities:
Effect of dilutive securities (in shares)									0.0	0.5	0.5