Stock-Based Compensation Plans and Arrangements - Restricted Stock and Performance Stock Unit Activity (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Weighted Average Grant Date Fair Value
Unrecognized compensation cost	$ 14.2
Weighted average period for total unrecognized compensation cost	1 year 4 months 15 days
Restricted Stock
Number of Shares
Beginning balance (in shares)	120,879
Granted (in shares)	81,103
Vested and converted to common stock (in shares)	(120,879)
Canceled (in shares)	0
Ending balance (in shares)	81,103	120,879
Weighted Average Grant Date Fair Value
Beginning balance (in dollars per share)	$ 6.37
Granted (in dollars per share)	10.85
Vested (in dollars per share)	6.37
Canceled (in dollars per share)	0.00
Ending balance (in dollars per share)	$ 10.85	$ 6.37
Fair value of shares vested	$ 0.8	$ 0.8	$ 0.5
Unrecognized compensation cost	$ 0.4
Weighted average period for total unrecognized compensation cost	6 months
Restricted Stock Units And Performance Stock Units
Number of Shares
Beginning balance (in shares)	7,486,610
Granted (in shares)	2,247,337
Vested and converted to common stock (in shares)	(3,765,803)
Canceled (in shares)	(22,854)
Ending balance (in shares)	5,945,290	7,486,610
Weighted Average Grant Date Fair Value
Beginning balance (in dollars per share)	$ 7.21
Granted (in dollars per share)	11.79
Vested (in dollars per share)	7.75
Canceled (in dollars per share)	8.82
Ending balance (in dollars per share)	$ 8.60	$ 7.21
Fair value of restricted stock units that vested and converted to common stock	$ 29.2	$ 22.2	$ 18.9
Unrecognized compensation cost	$ 13.4
Weighted average period for total unrecognized compensation cost	1 year 4 months 24 days