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Common Stock - Summary of Stock Options (Detail) - $ / shares
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]      
Number of shares under stock option, Beginning of the year 17,439,060 15,430,307 10,881,133
Number of Shares Granted 6,961,041 3,672,411 5,746,153
Number of Shares Forfeited (1,482,531) (1,517,065) (886,356)
Number of Shares Exercised (445,523) (146,593) (310,623)
Number of shares under stock option, End of the year 22,472,047 17,439,060 15,430,307
Number of stock options vested and expected to be vest, exercisable 14,309,944 9,828,897 7,498,414
Weighted Average Exercise Price, Beginning of year $ 54.08 $ 59.50 $ 61.22
Weighted Average Exercise Price, Stock Options Granted 36.51 28.26 54.74
Weighted Average Exercise Price, Stock Options Forfeited 55.22 49.95 62.73
Weighted Average Exercise Price, Stock Options Exercised 29.83 28.53 22.56
Weighted Average Exercise Price, End of the year 48.99 54.08 59.50
Weighted Average Exercise Price, Stock Options Exercisable $ 55.00 $ 61.56 $ 60.30