Stockholders' Deficit - Employee Stock Purchase Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock Compensation Plans
Number of shares issued	157,277	126,648	194,453
Stock compensation expense	$ 133.1	$ 96.2	$ 69.9
Employee Stock
Stock Compensation Plans
Employment period for eligibility to participate in the plan	1 month
Number of shares issued	157,277	126,648	194,453
Stock compensation expense	$ 3.2	$ 2.4	$ 2.4
Remaining shares available	927,233
Employee Stock | Minimum
Stock Compensation Plans
Number of employee working hours per week for eligibility to participate in the plan	20 hours