Stockholders' Deficit - Share Repurchases (Details) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2018 USD ($) shares	Jun. 30, 2018 USD ($) shares
Equity [Abstract]
Stock repurchased during period (in shares) | shares	137,163	275,801
Stock repurchased during period, value	$ 10.0	$ 20.0
Cumulative amount of shares repurchased (in shares) | shares	1,276,458	1,276,458
Cumulative payments for repurchase of common stock	$ 102.9	$ 102.9
Remaining dollar value of shares that may be repurchased	$ 47.1	$ 47.1