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Non-Vested Restricted Stock Awards and Changes Period (Detail) - Unvested restricted common stock
12 Months Ended
Dec. 31, 2016
$ / shares
shares
Number of shares  
Non-vested at December 31, 2015 | shares 16,413
Vested | shares (6,816)
Canceled or forfeited | shares (774)
Non-vested at December 31, 2016 | shares 8,823
Weighted-Average Grant-Date Fair Value  
Non-vested at December 31, 2015 | $ / shares $ 54.80
Vested | $ / shares 82.95
Canceled or forfeited | $ / shares 46.16
Non-vested at December 31, 2016 | $ / shares $ 47.51