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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 6 Months Ended
Jun. 30, 2018
EquityUnit
$ / shares
$ / EquityUnit
shares
Jun. 30, 2017
EquityUnit
$ / shares
$ / EquityUnit
shares
Jun. 30, 2018
EquityUnit
$ / shares
$ / EquityUnit
shares
Jun. 30, 2017
EquityUnit
$ / shares
$ / EquityUnit
shares
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]        
Weighted average warrants for common stock | shares 116 433 167 511
Weighted average PSUs | EquityUnit 514 313 466 273
Weighted average grant date fair value per unit | $ / EquityUnit 13.83 17.12 13.83 17.12
Weighted average contingently convertible shares | shares 40,017 36,181 39,617 36,331
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 $ 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 $ 12.32 $ 12.32