Stockholders' Deficit - Share Repurchases (Details) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018 USD ($) shares	Sep. 30, 2018 USD ($) shares
Equity [Abstract]
Stock repurchased during period (in shares) | shares	107,814	383,615
Stock repurchased during period, value	$ 7.9	$ 27.9
Cumulative amount of shares repurchased (in shares) | shares	1,384,272	1,384,272
Cumulative payments for repurchase of common stock	$ 110.8	$ 110.8
Remaining dollar value of shares that may be repurchased	$ 39.2	$ 39.2