Annual report pursuant to Section 13 and 15(d)

Share-based Compensation - Summary of Stock Option Transactions under Plans (Detail)

v3.8.0.1
Share-based Compensation - Summary of Stock Option Transactions under Plans (Detail) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]      
Number of Shares, options outstanding at beginning of period 387,901 510,148 603,872
Number of Shares, Options granted 0 0 28,000
Number of Shares, Options exercised (23,813) (94,997) (46,516)
Number of Shares, Options forfeited 0 (125) (71,608)
Number of Shares, Options expired 0 (27,125) (3,600)
Number of Shares, Options outstanding at end of period 364,088 387,901 510,148
Number of Shares, Options exercisable at end of period 354,753 341,561 333,460
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]      
Weighted-Average Exercise Price Per Share, Options outstanding at beginning of period (in usd per share) $ 17.49 $ 24.38 $ 23.78
Weighted-Average Exercise Price Per Share, Options granted (in usd per share) 0.00 0.00 23.47
Weighted-Average Exercise Price Per Share, Options exercised (in usd per share) 12.21 15.29 13.24
Weighted-Average Exercise Price Per Share, Options forfeited (in usd per share) 0.00 144.00 20.59
Weighted-Average Exercise Price Per Share, Options expired (in usd per share) 0.00 154.09 136.79
Weighted-Average Exercise Price Per Share, Options outstanding at end of period (in usd per share) 17.86 17.49 24.38
Weighted-Average Exercise Price Per Share, Options exercisable at end of period (in usd per share) $ 17.71 $ 16.85 $ 27.16