CONSOLIDATED STATEMENTS OF STOCKHOLDERS' DEFICIT - USD ($) $ in Millions	Total	Common stock:	Additional paid-in capital:	Accumulated loss:	Treasury stock:	Accumulated other comprehensive loss:
Beginning balance at Dec. 31, 2014		$ 1.0	$ 743.2	$ (470.7)		$ (94.4)
Change in the number of shares of Class A common stock outstanding [Roll Forward]
Issuances and purchases of Class A common stock		0.0
Issuance of Class A common stock in connection with employee stock purchase plan			1.6
Net redemption of Class A common stock in connection with stock options and RSUs			(2.5)
Stock-based compensation			25.4
Tax effect from employee stock options and RSUs			(1.8)
Net loss				(1,394.3)
Other comprehensive income (loss)	$ (127.8)					(127.8)
Ending balance at Dec. 31, 2015	(1,495.5)	1.0	765.9	(1,865.0)	$ (175.2)	(222.2)
Change in the number of shares of Class A common stock outstanding [Roll Forward]
Issuances and purchases of Class A common stock		0.0
Issuance of Class A common stock in connection with employee stock purchase plan			0.0
Net redemption of Class A common stock in connection with stock options and RSUs			(6.1)
Stock-based compensation			33.7
Tax effect from employee stock options and RSUs			(2.7)
Net loss				(353.7)
Other comprehensive income (loss)	(111.4)					(111.4)
Ending balance at Dec. 31, 2016	(1,935.7)	1.0	790.8	(2,218.7)	(175.2)	(333.6)
Change in the number of shares of Class A common stock outstanding [Roll Forward]
Issuances and purchases of Class A common stock		0.1
Issuance of Class A common stock in connection with employee stock purchase plan			0.8
Net redemption of Class A common stock in connection with stock options and RSUs			(7.1)
Stock-based compensation			23.3
Tax effect from employee stock options and RSUs			0.0
Net loss				(242.3)
Other comprehensive income (loss)	133.9					133.9
Ending balance at Dec. 31, 2017	$ (2,027.0)	$ 1.1	$ 807.8	$ (2,461.0)	$ (175.2)	$ (199.7)