Debt - Conversion Value of Convertible Notes (Details) - Senior Notes - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Total by which the notes’ if-converted value exceeds their principal amount	$ 858.6	$ 125.4
Senior convertible notes due 2022
Debt Instrument [Line Items]
Total by which the notes’ if-converted value exceeds their principal amount	486.2	125.4
Senior convertible notes due 2023
Debt Instrument [Line Items]
Total by which the notes’ if-converted value exceeds their principal amount	$ 372.4	$ 0.0