Stockholders' Deficit - Changes in Common Stock (Details) - shares shares in Millions		12 Months Ended
		Dec. 31, 2018		Dec. 31, 2017
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares outstanding as of beginning of period (in shares)		89.9	[1]	88.0
Shares issued as part of equity-based compensation plans and the ESPP, net of shares surrendered (in shares)		2.0		1.9
Shares outstanding as of end of period (in shares)	[1]	91.9		89.9

[1] Following the consummation of the reincorporation merger on January 10, 2018, each authorized, issued and outstanding share of Class A common stock of SGC, par value $0.01 per share, automatically converted into one share of common stock of the surviving corporation, par value $0.001 per share. The change in par value had no impact on total number of authorized, issued and outstanding shares.