Summary of Significant Accounting Policies - Common Stock Stock Options and Convertible Stock Anti-Dilutive to Earnings Excluded from Weighted Average Shares Used in Computation of Earning Per Share (Detail) shares in Thousands, EquityUnit in Thousands	3 Months Ended
	Mar. 31, 2018 EquityUnit $ / shares $ / EquityUnit shares	Mar. 31, 2017 EquityUnit $ / shares $ / EquityUnit shares
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Weighted average warrants for common stock | shares	218	590
Weighted average PSUs | EquityUnit	418	233
Weighted average grant date fair value per unit | $ / EquityUnit	13.83	17.12
Weighted average contingently convertible shares | shares	39,205	36,409
Warrants for Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Weighted average exercise price per share | $ / shares	$ 0.01	$ 0.01
Contingently Convertible Shares
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Weighted average exercise price per share | $ / shares	$ 12.32	$ 12.32