Stockholders' Deficit - Share Repurchases (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Share Repurchases
Stock repurchased and retired, publicly announced authorizations (shares)	8.5	17.9
Stock repurchased and retired, publicly announced authorizations, value excluding commissions	$ 217.4	$ 335.5
Stock repurchased and retired, publicly announced authorizations, average cost per share excluding commissions (USD per share)	$ 25.45	$ 18.73
Authorized through December 31, 2021
Share Repurchases
Remaining amount available under share repurchase authorization through December 31, 2021	$ 782.6