Net Income (Loss) Per Share - Narrative (Detail) - $ / shares shares in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Stock Options
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)	0.5	0.7
Shares of common stock at prices ranging, lower limit (in dollars per share)	$ 9.83	$ 9.83
Shares of common stock at prices ranging, upper limit (in dollars per share)	$ 19.95	$ 19.95
Deferred stock units, performance-based stock units and restricted stock
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)	0.8	0.9
Convertible Senior Debentures due 2018
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	2.4