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	12 Months Ended
	Dec. 31, 2017 EquityUnit $ / shares $ / EquityUnit shares	Dec. 31, 2016 EquityUnit $ / shares $ / EquityUnit shares	Dec. 31, 2015 EquityUnit $ / shares $ / EquityUnit shares
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Weighted average stock options | shares		11	20
Weighted average warrants for common stock | shares	463	337
Weighted average PSUs | EquityUnit	274	136	136
Weighted average grant date fair value per unit | $ / EquityUnit	17.12	22.17	35.35
Weighted average contingently convertible shares | shares	37,046	11,574
Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Weighted average exercise price per share | $ / shares		$ 166.10	$ 164.75
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