Net Income (Loss) Per Share - Narrative (Detail) - $ / shares shares in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Shares of common stock at prices ranging, lower limit	$ 9.83	$ 9.83	$ 7.14
Shares of common stock at prices ranging, upper limit	$ 19.95	$ 19.95	$ 19.95
Employee stock options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Share of common stock excluded from diluted net loss per share calculation (in shares)	0.6	0.7	1.1
Restricted Stock Unit And Performance Stock Unit And Deferred Stock Unit [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Share of common stock excluded from diluted net loss per share calculation (in shares)	1.0	0.9	0.7
Convertible Senior Debentures due 2018 [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Share of common stock excluded from diluted net loss per share calculation (in shares)	2.3	3.0	3.0